Gardening in the Global Greenhouse The Impacts of Climate Change on Gardens in the UK Summary Report Research was carried out by: Gardening in the Global Greenhouse was funded by the following organisations: The UK Climate Impacts Programme The UK Climate Impacts Programme (UKCIP) is based at the University of Oxford and funded by DEFRA to coordinate an assessment of how climate change will affect the UK, and help organisations assess how they might be affected. UKCIP, Union House, 12–16 St. Michael’s Street, Oxford, OX1 2DU, UK; [email protected]. Tel: +44(0)1865 432076; Fax: +44(0)1865 432077. Summary report written by Dr Phil Gates, University of Durham. Cover images (from top): Protea longiflora (NT); Hardy’s Cottage (Eric Chrichton, NTPL); Nymans (Patsy Fagan, NTPL). Printed on 50% recycled paper. Published in November 2002 under the UK Climate Impacts Programme.
Transcript
Gardening in theGlobal Greenhouse The Impacts of Climate Change on Gardens in the UK
Summary Report
Research was carried out by:
Gardening in the Global Greenhouse was funded by the following organisations:
The UK Climate Impacts Programme
The UK Climate Impacts Programme (UKCIP) is based at the University of Oxford and funded by DEFRA to coordinate an assessment ofhow climate change will affect the UK, and help organisations assess how they might be affected.
UKCIP, Union House, 12–16 St. Michael’s Street, Oxford, OX1 2DU, UK; [email protected]. Tel: +44(0)1865 432076; Fax: +44(0)1865 432077.
Summary report written by Dr Phil Gates, University of Durham.
Published in November 2002 under the UK Climate Impacts Programme.
Gardening in the Global Greenhouse: Summary Report Introduction
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The British are noted for their love of gardens. As an art form, gardening is an essentialpart of our culture. It is one of the leading hobbies in Britain, with an estimated 27 milliongardeners (41 per cent of the population) participating in some way. Gardeners sustain a multi-million pound horticultural retail industry. Our heritage gardens and their plantcollections, representing 500 years of garden history, attract 24 million visitors each year,contributing an estimated £300 million to the tourism industry.
Introduction: The British and their gardens
A wake up call from the weather
Gardeners are more aware than many howthe weather affects them and their interests.Over the last four decades, extreme weatherevents have severely damaged many gardensand resulted in major economic losses. Theseevents include:
• severe winter weather in 1962/63 thatkilled many hardy plants;
• drought in 1976, which weakened treesand dried out lakes;
• storms in 1987 and 1990 that felledmillions of trees;
• torrential and prolonged rain leading tosoil erosion, flooding and drowning ofplant roots in 2000 and 2001.
These events highlight the vulnerability ofgardens to the vagaries of weather andclimate. A workshop was held in London in April 2000, attended by gardening
organisations, commercial horticulturists,landscape consultants, the horticultural press,and representatives from local and nationalgovernment, universities and researchorganisations, to consider how gardens wouldbe affected by climate change.
The primary outcome of the meeting was the commissioning of a technical report,entitled ‘The Impacts of Climate Change on Gardens in the UK’, by Richard Bisgroveand Professor Paul Hadley, of the Universityof Reading (2002). The authors assembledand evaluated evidence on how plants andgarden components respond to alteredclimatic conditions, and assessed the likelyimpact of the UKCIP1 climate changescenarios on gardens in the United Kingdom.
This document summarises their report. Itdescribes effects on garden plants, domesticgardens, nationally important heritagegardens, the landscape industry and the retailhorticultural industry. It discusses aspects ofgardening that might benefit from climatechange and identifies techniques andpractices that might reduce undesirable effectsof climate change on gardens. It alsohighlights the important role that gardens canplay in raising awareness of environmentallysustainable practices which can minimise theeffects of climate change, and identifies areasfor further research.
Drought dried out lakes and reservoirs in 1976 (Mike Sleigh, RHS)
1 The UKCIP02 scenarios describe how the climate ofthe UK is likely to change over the course of the 21st century. They were produced for the UK ClimateImpacts Programme by the Hadley Centre at the Met.Office and by the Tyndall Centre for Climate ChangeResearch with funding from the Government’sDepartment for Environment, Food and Rural Affairs.
Trees at Chartwell, Kent felled by the storm in 1987 (Peter Baistow, NTPL)
Flooding at Westbury Court, Glos. in 2000/01 (NT)
Gardening in the Global Greenhouse: Summary Report What is happening to our climate?
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Gardening in the Global Greenhouse: Summary Report What is happening to our climate?
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What is happening to our climate?
The current phase of climate change is part of a continuous warming of the climaterecorded over the last 150 years and is being caused primarily by industrial activities that have raised levels of greenhouse gases which retain heat within the atmosphere.
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There is overwhelming evidence that the rateof warming is increasing, althoughprojections for the rate and extent ofwarming vary, depending on predictions ofeconomic growth, human population levelsand compliance with internationalagreements to regulate emissions of carbondioxide, the most important greenhouse gas.
In the UKCIP02 high emissions scenario, withrapid economic growth in developed and lessdeveloped countries and a market drivenreliance on fossil fuels, by the 2080s average temperatures are likely to rise by 2°- 4.5°C, at a rate of between 0.3°- 0.5°Cper decade. This is between two and threetimes faster than the current rate oftemperature increase.
In the low emissions scenario, at the currentrate of 0.1°- 0.3°C rise per decade annualaverage temperatures will increase by1° - 2.5°C by the 2080s. This lower rate ofincrease would depend on the use of cleanand efficient technologies, reduced use ofnatural resources that generate carbondioxide, and global solutions to problemsof social and economic stability andequity between developed anddeveloping countries.
Climate change is already occurring andfurther change is inevitable, with commonconsequences for climate in the UK. These are:
• higher mean annual temperatures, whichwill increase the length of the growingseason for many plants. A 1°C increase inmean temperature will increase thegrowing season by three weeks in southeast England and by 10 days in thenorth west;
• greater warming in summer and autumnthan in winter, with summer maximumsrising faster than summer minimums,leading to increased frequency of hotsummer days. By the 2080s temperaturesmay exceed 42°C about once per decadein lowland England;
• winter minimum temperatures rising fasterthan winter maximums, leading to milderwinters with a reduced temperature rangeand fewer frosts. In many parts of the UK,especially in the south west, frosts will berare and might occur in Cornwall onlyabout once every 10 years by the 2080s,although local variations will occur.Snowfall will decrease everywhere inlowland and coastal regions by as much as90 per cent by the 2080s, and by 60 percent in high snowfall areas such as theScottish Highlands;
• mean annual rainfall may decrease by10-20 per cent, but with 10-30 per centmore falling in winter and 20-50 per centless in summer by the 2080s. Rain willtend to fall with greater intensity;
• by the 2080s summer droughts will bemore frequent, as will very wet winters,but autumns will be drier. Highertemperatures and less cloud cover insummer will lead to greater evaporativeloss from soils and leaves,worsening drought conditions;
• although predictions for extreme weatherevents are less certain than fortemperature, weather patterns are likely tobecome more erratic, with greaterfrequency of torrential rain, temperatureextremes and storms.
Within these overall trends there will bepronounced regional differences, with thelowest rainfall and highest temperaturesoccurring in south east England. The reductionof soil moisture will therefore be greatest inareas where water supplies are already low.
By the 2050s sea levels are expected to riseby 14-18 cm, and by the 2080s by 23-36 cm.The effects will be greatest in the south east,where the land mass is naturally subsiding.Coastal flooding could damage somedomestic and major heritage gardens.
These climate changes will have majoreffects on plant growth and on theways in which we design, plant andmaintain gardens.
Source: UKCIP02 Climate Change Scenarios
Traditional scenes such as this at Bodnant, Gwynedd will become a rarity (Christopher Gallagher, NTPL)
Droughts will become more frequent (NT)
Gardening in the Global Greenhouse: Summary Report Plants in a warmer world
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Gardening in the Global Greenhouse: Summary Report Plants in a warmer world
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Plants in a warmer world: how climate affects plant growth and development
“.. doubling carbon dioxidelevels can increase plant growthby as much as 50 per cent ..“ Bisgrove and Hadley (2002)
Effects of individual elements of climatechange have been tested in controlledenvironments for some plant species ofagricultural importance and there is alreadyevidence that rises in temperature andatmospheric carbon dioxide levels haveincreased growth of forest trees. Furtherresearch is required for the majority ofgarden plants.
Higher carbon dioxide levels
Atmospheric carbon dioxide, the primaryagent of climate change, is used by plants inphotosynthesis to produce chemical energy,sugars and other organic molecules essentialfor plant growth and development. Increasesin carbon dioxide levels, which are expectedto rise from the current 350 parts per million(ppm) to between 525 and 810 ppm by the2080s under the low and high emissionsscenarios respectively, will increase the rateof photosynthesis and so enhance plantgrowth. Experiments show that doublingcarbon dioxide levels can increase plantgrowth by as much as 50 per cent, althoughthis varies depending on plant species,temperature, water supply and mineralnutrient availability.
The way in which plants use the additionalproducts of photosynthesis varies betweenspecies and is affected by otherenvironmental factors. Under conditions ofwater stress, some plants divert resourcesinto root production, improving wateruptake. Plants use water more efficiently as carbon dioxide levels rise, because areduction in the numbers and opening ofstomatal pores on leaf surfaces slows waterloss. Nitrogen is also used more efficiently,
so although plants will grow more rapidly,fertiliser requirements do not necessarilyincrease. Increases in carbon dioxide levelscan hasten bud burst, flowering and fruitingand lead to earlier cessation of growth,depending on species investigated. In roses,higher carbon dioxide levels increase thenumber of flower buds, which bloom earlierand accelerate flowering.
Higher temperatures
Higher temperatures increase growth rates,provided that no other factors are limiting. Itis unlikely that the higher temperaturesexpected by the 2080s will physicallydamage plants in the open ground, exceptfor brief, exceptional periods, but damagemay occur in glasshouses unless these arewell shaded and ventilated.
Higher temperatures produce faster seedgermination, bud burst, leaf expansion andflowering. Advances in flowering date canbe predicted for species whose temperatureresponse characteristics are known. Plantslike Mexican orange (Choisya ternata) andRhododendron ‘Praecox’ are likely to
Plants are influenced by extreme weather events and by gradual climate changes, the effects ofwhich are more insidious but potentially dramatic. A small change in average annual temperatureof, say, 0.1°C has a cumulative effect through the growing season, with a large net impact. Ananalogy would be a small change in bank interest rate, which might have little immediate effecton the daily economy but would have a profound impact on annual accounts.
Higher carbon dioxide levels can hasten bud burst (Stephen Robson, NTPL)
“Grape cultivation could extendas far north as Scotland duringthe second half of the21st century.“Bisgrove and Hadley (2002)
flower earlier, but others such as honesty(Lunaria annua) and bleeding heart (Dicentraformosa) will be largely unaffected.
Temperature increase is likely to extend thenorthern limit for growing some vegetablesand fruit. Grape bud burst and maturitydates may advance by up to 50 days andyield may increase by up to 25 per cent bythe 2050s.
Temperature levels play a key role inregulating the annual growth cycle ofperennial plants. Normal bud burst oftendepends on accumulated experience ofwinter chilling, which breaks dormancy andprepares buds to open as soon as favourablespring weather arrives. At the end of thegrowing season, falling temperatures haltgrowth and trigger leaf fall. The interveningperiod constitutes the growing season, whichwill extend as climate changes. Currentlyspring is arriving 2 - 6 days earlier perdecade and autumn two days later eachdecade. Precocious spring flowering, delayedleaf fall, extended lawn growth andunseasonal winter flowering are alreadycommonplace and will increase in extent andfrequency as climate changes.
Earlier springs lengthen the growing seasonand allow earlier planting dates. Bud burst ofmost plants will be advanced, but the likelihoodof frost damage to precocious growth will beno greater than at present, because frosts willbecome less frequent. Frost damage in autumnmay become more serious, as higher averagetemperatures delay dormancy and clearer skiesincrease the possibility of frost.
Failure of plants to shut down metabolicactivity in winter might increase frost damage,but higher sugar levels generated in tissuesof evergreen species by winter growth mighthave the reverse effect. Species-specific effectsare likely, and further research is needed.
Interactions between higher carbondioxide levels and higher temperatures
As temperatures rise, plants respond morevigorously to higher carbon dioxide levels.Doubling atmospheric carbon dioxide levelsand raising temperature by 3°C can increasegrowth by over 50 per cent, although thismay not be reflected in higher yields. Thebenefits are greater in vegetables harvestedearly in development, such as carrots, than inthose that are cropped closer to maturity,such as onion and cauliflower. Warmertemperatures shorten the growing period ofvegetables like onions, lowering ultimateyields. In potato, higher carbon dioxide levelsincrease rates of photosynthesis and soslightly increase overall yields, but highertemperatures hasten foliage death.
Grapes could be grown in Scotland in future (RHS)
Autumn is arriving later and later (Michael Caldwell, NTPL)
Gardening in the Global Greenhouse: Summary Report Pandora’s greenhouse
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Gardening in the Global Greenhouse: Summary Report Soils
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The UKCIP02 scenarios point to major changes in soil water content and hence nutrientavailability. Managing these changes represents one of the biggest challenges for gardeners in the 21st century.
Soils: winter waterlogging, summer drought
Seasonal temperature increases will enhancebiological activity of soil micro-organisms,leading to more rapid breakdown of soilorganic matter and faster nutrient release.Increased availability of soluble soil nutrientswill bring about faster and more vigorousplant growth, but will also increase the lossof soil nutrients through leaching. Heavier,intense rain, especially in winter when thereis no vegetation cover, may wash away soilminerals, leading to pollution of streams,ponds and lakes.
Strategies for conserving soil fertility andmoisture will depend largely on soilconditions and local effects of climatechange. Peak garden demand for water will be greatest in the denselypopulated south east where it is leastavailable. Free draining sandy soils will loseorganic matter faster, further reducingwater-holding capacity during summerdroughts, and will be more prone to watererosion in winter. Addition of organic matterand mulching will be required. Heavy soilsshould retain sufficient water for plantgrowth in summer but winter waterloggingmay kill plants unless grit is incorporated orraised beds are constructed.
Higher summer temperatures will increasewater evaporation from leaves and the soil,lowering soil moisture content. A 3°Cincrease in soil temperature, of the kind thatmight be experienced by the 2080s underthe high scenario, can increase such waterloss by 30 per cent and decrease soilmoisture by 25 per cent. When this effect iscombined with substantially lower summerrainfall, plants will suffer from severe droughtstress more frequently.
Sudden heavy downpours, coupled with soilcompaction, poor drainage and landscapingwith impervious materials, will lead to rapidsurface run-off and localised flooding.Prolonged waterlogging weakens and killstree roots and will increase the risk ofdamage to property from large moribundtrees growing close to houses. Cedars at theNational Trust garden at Osterley Park inMiddlesex and hedges at Westbury Court inGloucestershire have already been affectedby flooding. This can be avoided by improvingdrainage, using gullies, soakaways andditches in large gardens but is much moredifficult to control in closely spaced, smallsuburban gardens.
Garden pests, diseases and weeds tend to have short, rapid life cycles and reproduce in largenumbers – ideal credentials for organisms that can benefit from the predicted changes inclimate. Increased temperatures shorten pest life cycles so that populations increase faster,while longer growing seasons allow additional reproductive cycles at a time of the year whenpest levels are already high.
Pandora’s greenhouse: more pests, more diseases, more weeds
Pests
Pests that undergo many breeding cycles in a growing season, such as thrips, aphids andspider mites, will complete their life cyclesfaster. Milder winters will benefit many insectpests that already overwinter as adults,while others that currently overwinter aseggs may also adopt this strategy. Earliersprings will allow them to disperse ontogarden plants sooner, provided that theseplant hosts also begin growth earlier. Thecabbage aphid (Brevicoryne brassicae)already benefits from mild winters and forevery 1°C increase in temperature, aphidattacks begin two weeks earlier. An increasein spring temperature of 2°C will allowcabbage root fly (Delia radicum) to becomeactive a month earlier than at present.Faster plant growth reduces leaf nutritivevalue and can increase pest feeding activityby 20-40 per cent.
Beneficial or benign insects will be equallyaffected by temperature change, soincreased natural predation of pests is likely.The nematode currently used as a biologicalcontrol for vine weevil could become morewidely used in future, as warmer conditionswill enable it to be effective for a longerseason. There is already evidence thatbutterflies are emerging earlier and thattheir ranges are extending, but increasedactivity of bees in mild winters reduces theirchances of survival.
Many insect pests transmit virus diseases,which will become more prevalent as pestinfestations intensify and occur earlier in theyear, when plants are more severelyvulnerable. Insects that are currentlyglasshouse pests, such as western flowerthrips, may move into the open garden,carrying virus diseases such as tomatospotted wilt virus and Impatiens necroticspot viruses, which can infect a range ofvegetables. There is good reason to supposethat other pest species currently confined tothe sheltered environment of glasshouseswill move into the open garden. This hasalready happened with red spider mite.
Warmer temperatures in the UK are alsolikely to favour a northwards advance ofnative pest species and an influx of pestsfrom continental Europe, either by naturalmigration or accidental introduction.Termites and a distinct strain of the Asiangypsy moth (Lymantria dispar ssp.) havealready been recorded in southern Britain.
The compost heap: addition of organic matter willbecome increasingly important (Stephen Robson, NTPL)
Hedges at Westbury Court, Gloucestershire, have been affected by flooding (John Millar, NTPL)
“Evidence suggests that nitratelosses from soils may double.Maintaining soil fertility, byreplacing organic matter, willbe essential.”Bisgrove and Hadley (2002)
“Managing rainwater run-off ..will be of crucial importance.”Bisgrove and Hadley (2002)
Warmer conditions will allow greater use of biological control of the vine weevil (Chris Prior, RHS)
Gardening in the Global Greenhouse: Summary Report Pandora’s greenhouse
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Gardening in the Global Greenhouse: Summary Report Pandora’s greenhouse
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There is great potential for severe fungaldamage to plants that form key structuralelements in the formal planting of heritagegardens, including yew, box and holly. Boxblight (Cylindrocladium buxicola) is spreadby water splash and will be favoured bywarmer, wetter conditions. Yew root rot(Phytophthora cinnamomi) will become agreater problem in wetter, warmer wintersand will spread northwards and eastwards.Holly leaf blight (Phytophthora ilicis) arrivedfrom the USA in the 1980s and its recentspread may be due to warmer, wetterwinters. It has already caused damage in thegarden at Nymans in Sussex. Other speciesat increasing risk from Phytophthora speciesinclude Lawson cypress and alders.
Weeds
More favourable conditions for germinationand growth will increase the need forweeding, while slow growth in dry summersmay reduce the effectiveness of glyphosateand hormone-based herbicides, which workbest on plants that are growing rapidly.Herbicide spraying will need to beconducted earlier in the year.
There is evidence that bracken (Pteridiumaquilinum), which is a serious weed on largeestates, will benefit significantly from climatechange, colonising at higher altitudes and penetrating further into lightly shaded woodlands.
Fears have been expressed about thepossibility of the uncontrolled spread ofalien plant introductions. Himalayan balsam(Impatiens glandulifera), Rhododendronponticum and Japanese knotweed (Fallopiajaponica) have all escaped from gardens and pose threats to natural ecosystems. Asthe UK climate continues to warm it will benecessary to monitor carefully the potentialand observed threats from invading exoticplants, but the threat that they representshould not be overstated.
Horse chestnut leaf miner (Camerariaohridella) has devastated trees in Spain,become established in Northern Italy andrecently appeared in Wimbledon. An influxof exotic pests into British gardens is a likelyconsequence of climate change.
Larger pests, like roe deer and grey squirrels,will be favoured by warmer winters, whichwill increase their survival rates. Grey squirrelshave a predilection for beech (Fagus sylvatica),which is also very sensitive to drought, andthe combined effects of climate and squirrelson this species could be particularly severe.
Diseases
Plant diseases often have complex life cycles and a range of hosts, so predictingconsequences of climate change for diseaseinfestations is difficult. Wetter, warmerwinters will favour the spread of water-transmitted diseases such as Phytophthora,or species with higher temperaturerequirements like red-thread (Laetisariafuciformis) disease of lawns. Drier, warmersummers will increase the incidence ofaerially dispersed spores of diseases likepowdery mildew.
Continued plant growth during mild winterswill enhance winter survival of diseases andlead to their rapid re-establishment in earliersprings. Rates of disease will depend ontheir tolerance of drier, warmer summers butin general higher temperatures would beexpected to increase the severity of diseaseattacks. Drought also tends to increaseplants’ susceptibility to diseases, as in thecase of sooty bark disease (Cryptostromacorticale) of sycamore. Warmer winters willfavour bark and wood invading fungi suchas honey fungus (Armillaria spp.) and applecanker (Nectria galligena).
As with insect pests, climate change can beexpected to bring an influx of new diseasesfrom overseas, posing major threats togarden plants. Soil borne Athelia (Corticium)rolfsii, which would be a serious threat togarden plants, has already been introducedinto Britain and could become establishedhere if warmer winters enable it to survive.Olive scab (Spilocaea oleagina) has alsobeen introduced and is an example of howgardeners may inadvertently introduce newdiseases as they exploit climate changes togrow new plants.
Quarantine regulations, currently designed toprotect plants of major economic importance,may need to be revised and extended to garden plants. Camellia petal blight(Ciborinia camelliae), originated in Japan andhas spread around the world, unimpeded byplant health authorities that did not considerit necessary to impose import restrictions.The disease has become established inDevon and Cornwall, at the northern limit ofits climatic tolerance, but warmer conditionsin future will favour its spread.
Oak wilt (Ceratocystis fagacearum), a majordisease in North America, is transmitted byan insect that cannot survive under currentUK climate conditions. When these change,accidental introduction of this insect couldbe a serious threat to oaks in Britain.
Carmeraria ohridella is now affecting horse chesnuts in England (J. Metzer, LWF)
Camellia petal blight will spread from Devon and Cornwall (Chris Prior, RHS)
Warmer conditions will favour honey fungus (Chris Prior, RHS)
Holly leaf blight is already causing damage in the UK (Béatrice Henricot)
Gardening in the Global Greenhouse: Summary Report Garden management in a warmer world
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Gardening in the Global Greenhouse: Summary Report Garden management in a warmer world
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Climate change and the character of gardens
The extent to which the overall style andcharacter of gardens is affected by climatedepends on how much we wish to preservetheir character for cultural reasons.
Domestic garden design is subject to thewhims of horticultural fashion, promoted bymagazines and television programmes.Historically important gardens are maintainedas cultural assets – to perpetuate significantlayouts, period styles, plant collections andthe accumulation of past owners’ influences.In such gardens, maintaining their characterand current combinations of species willrequire special attention and will becomeincreasingly difficult and expensive, if not unsustainable.
Destruction of gardens because of climate change in the medium term is only likely in those few coastal gardens that could be directly affected by sea level rise, orperhaps in some that are subject toprolonged and persistent flooding inland.In such cases, documentary recording isprobably the only option.
Domestic and contemporary gardens
Climate change could bring about rapidchanges in garden style. Adventurousgardeners have always tried to explore thelimits for cultivating unusual and demandingspecies, so climate change could providenew variations on familiar challenges. Theirsuccess will depend on careful soilpreparation and plant maintenance duringperiods of drought or excessive rain.Currently, between 10,000-15,000 plantspecies and varieties are cultivated in Britishgardens; climate change is likely to extendthe range of species that can be cultivatedoutdoors throughout the year, especially bygardeners in northern Britain.
The retail horticultural trade can respond tothese changes in gardening fashion, and isalready meeting consumer demand for suchexotic species as phormiums, bamboos,cannas, palms, tree ferns and bananas whichwere, until recently, only grown in large,specialist gardens.
Heritage gardens
Most plants that are currently cultivated inheritage gardens are likely to be maintainedover this century by the use of suitable soilmoisture conservation techniques andirrigation in summer, but at increasing cost.
Difficult decisions over suitable replacementplanting will arise when extreme weatherevents, such as storms, lead to loss ofmature trees. When individual plants areunique or of particular historical, botanicalor biological importance, then strenuous andexpensive efforts will need to be made tomaintain them in the face of climate change,as will be the case for archaeologycontained within historic parks or gardens.Balancing consideration of future climaticconditions with issues of historicalauthenticity will become an increasinglyimportant factor in the maintenance ofheritage gardens. Garden conservationmanagement plans offer a tool toplan ahead.
The horticultural trade is already meeting consumerdemand for exotic species (Pardee Tree Nursery)
Historic collection of plants, such as this fernery atBiddulph Grange, will become increasingly difficult to maintain (Nick Meers, NTPL)
The character of our gardens and the plant collections found in them are influenced by thephysical constraints of what may be grown, plant hardiness, temperature and water requirements,and what is desirable from the perspective of historical precedent, culture or fashion.
Garden management in a warmer world
The craft and science of horticulture allowsus to grow species that are poorly adaptedto the climate in the United Kingdom, suchas alpine plants and mediterranean species,and also to manage the garden environmentto allow cultivation of highly bred cultivarswhich could not thrive unassisted. Currenthorticultural skills will need to be refined toensure the survival of existing garden plantsand allow the cultivation of new species.
Hardiness
Some 85 per cent of garden plants originatefrom areas with warmer climates than ourown. Higher average winter temperatureswill reduce frost damage to marginally hardy species, such as Abutilon vitifolium,Bottlebrush (Callistemon spp.), Carpentariacalifornica and Jerusalem sage (Phlomisfruticosa), and will extend their cultivationnorthwards. In general, higher averagewinter temperatures should enhance thewinter survival prospects for the majority of species grown in our gardens.
Frost will remain a risk – albeit adiminishing one – throughout much of theUK, even in a warmer climate, at least untilthe end of the 21st century. By judicious soil preparation and recognition of localmicroclimates within large gardens,gardeners already extend the limits forcultivation of tender species. This skill willbecome even more important with climatechange, to minimise the risk of frost damageto precocious buds in spring.
Temperature and water
Maintaining plants is particularly difficultduring periods of higher summer temperaturesand drought conditions, especially in parts of southern England where water resourcesare already limited. Even short periods ofextreme stress can kill many annuals andperennials, and may weaken perennials andtrees, making them vulnerable to stress insubsequent years.
Gardeners in areas with summer watershortages can plant drought tolerant species,but these will require skilled cultivation.Such species do not tolerate saturated soilaround their roots and will be vulnerable todisease in the warmer, wetter winters thatcan be expected in the 21st century.
“Tresco has spread to TunbridgeWells and is on its way to Teesside.”Bisgrove and Hadley (2002)
Difficult decisions will have to be made when mature trees in heritage gardens are lost throughextreme weather (George Wright, NTPL)
“.. in the contemporary garden,climate change offers excitingopportunities ..”Bisgrove and Hadley (2002)
Climate change is likely to extend the range of speciesthat can be cultivated (Derek Croucher, NPTL)
Gardening in the Global Greenhouse: Summary Report Rewriting the garden manual
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Gardening in the Global Greenhouse: Summary Report Rewriting the garden manual
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All gardeners will need to plan for new planting opportunities, a longer growing season,
improvements in garden drainage and watering facilities, continuous grass mowing and
difficult growing conditions under glass.
Rewriting the garden manual: climate change and key garden components
Some of the challenges gardeners will face, and some possible solutions, are set out below:
Trees
It should be possible to select treeprovenances that will be better adapted tofuture climates than present cultivars by, forexample, choosing disease-resistant species.Many existing long-lived trees will suffer stressfrom climate change and will require carefulmanagement programmes to deal with:
• summer drought, minimised by skilled soil management;
• waterlogging, avoided by planned drainage measures;
• damage and loss from high winds,requiring planned long term replacementprogrammes, planting in suitably shelteredsite, and perhaps judicious crownreduction of vulnerable trees;
• large scale storm damage in woodland,minimised by avoiding uniformmonocultures and planting a diversity of tree species in new woodlands;
• risk of damage to conifer hedges by fire in periods of extreme drought, reduced by limiting access and establishing fireprevention measures.
Fruit trees and bushes
Many existing cultivars have a minimumwinter chilling requirement for bud breakand flowering, which may not be met infuture mild winters, so maintaining historicallyimportant cultivars will become more difficult.
Breeding new cultivars with a lower chillingrequirement, or complete substitution of thesecrops may be needed. Pears, cherries andpeaches, for example, could replace applesin some locations.
Shrubs
Most problems that afflict trees will alsoapply to shrubs, but to a lesser extent, andthe same management procedures apply.Shade-adapted shrubs can be damaged byintense sunlight and weed competition afterstorm damage to the tree canopy, requiringtemporary shading during a period of acclimatisation.
Sub-shrubs
Higher temperatures and fewer frosts willallow wider use of sub-shrubs such asFuchsia, Indigofera and Penstemon, whichare marginally hardy at present in manylocations. In northern locations such specieswill still require careful management, toguard against late damage from frosts,which are unlikely to be totally eliminatedduring the next century.
Peaches could replace apples(Neil Campbell-Sharp, NTPL)
Herbaceous perennials
These are so diverse in their climate optimathat it is difficult to generalise on theirperformance. Traditional herbaceous borderspecies such as aster (e.g. Aster novi-belgii),delphinium (Delphinium x cultorum), lupin(Lupinus x regalis spp.) and phlox (Phloxpaniculata), requiring deeply cultivated,fertile, moisture retentive soil, will not adaptwell to drier summers and will need moreintensive care, as will old cultivars of iris,which tolerate dry summers but are killed bywinter waterlogging. Selecting species andcultivars that are drought resistant andtolerant of waterlogged soils in winter willbe a challenge for plant breeders andnurseries. More thorough staking ofherbaceous plants will be required, to withstandperiods of intensive rainfall in summer.
Bulbs
Species propagated from bulbs andunderground storage organs typically requirelow winter temperatures to stimulate rootdevelopment and, in some cases, highsummer temperatures to initiate flower buddevelopment. Requirements of differentspecies vary widely, but if warmer winterslead to failure of root initiation, lifting andrefrigeration of bulbs would be needed toensure normal development. Higher soil andair temperatures in spring may alter thebalance between flower and foliagedevelopment in species such as hyacinth(Hyacinthus orientalis).
Warmer summers will benefit summer andautumn flowering subjects and will permitcultivation of a wider range of species in theopen border. Tulips and some crocus cultivarsdo not tolerate waterlogging, so will eitherrequire excellent soil drainage regimes, or willneed to be lifted and stored over the winter.
Annuals
Higher temperatures in spring will accelerategermination, development and flowering ofhardy annuals, and lead to their earlier death.This trend might be minimised by better soil
preparation and irrigation. Higher temperatureswill favour use of a much wider range ofhalf-hardy bedding plants that will no longerbe subject to frosts. Separate spring, summerand autumn bedding schemes will bedesirable when the length of the growingseason increases.
Lawns
Fine lawns – the icon of British gardens – arehigh-maintenance features particularly suitedto our current climate. Hot summers andwetter winters will increase browning duringdroughts, promote soil compaction during wetweather and increase risk of diseases. A longergrowing season will demand year-roundmowing and lawn care. More extensive mossgrowth in winter is likely, although summerdrought will minimise its impact on the lawn.These changes will raise maintenance costsin large gardens, making conversion to morenatural meadow areas an attractive propositionin less formal areas. The cost of lawn irrigation,if permitted at all in areas of water shortage,will be an important consideration.
Lawns will need year-round maintenance (Mike Sleigh, RHS)
Trees that are resistant to storm damageSource: White (1994)
Acer pseudoplatanus Sycamore
x Cupressocyparis leylandii Leyland cypress
Magnolia (tree species) Magnolia
Ilex aquifolium Holly
Metasequoia glyptostroboides Dawn redwood
Robinia pseudoacacia Black locust
Sequoiadendron giganteum Wellingtonia
Taxus baccata Yew
The plants grown in traditional cottage gardens willrequire more care (Neil Chrichton, NTPL)
“Whilst the challenge for manygardeners may be the introductionof new and exciting species intotheir gardens, as the climateprogressively changes a muchgreater challenge to gardeners..will be to create the traditionalEnglish cottage garden.”
Bisgrove and Hadley (2002)
Faster-growing foliage may obscure hyacinth flowers (David Sellman, NTPL)
Careful management will help avoid large scale stormdamage in woodland (NT)
Many major gardens depend on revenue from visitors. Garden managers will need to plan forthe financial impacts of climate change. They will need to meet the cost of storm and flood repairsand rising insurance premiums, and to adapt to the effects of climate change by installing waterstorage facilities and flood protection. They will also need to prepare for a longer visitor seasonand greater visitor impact on the garden infrastructure. There is scope to develop environmentallysustainable gardening techniques further.
The bottom line: the commercial impact of climate change on gardens
Visits to heritage gardens
Climate, or more precisely prevailingweather, is just one of many factors that willinfluence future trends in garden visits.Many social, cultural and economic factorsare involved, including population dynamics,disposable income, competing attractionsand access to gardens by public transport.
Improved weather, especially early in theyear, may attract more visitors to gardens,while prolonged autumns with 10-20 percent less rainfall and enhanced autumnfoliage colour are likely to extend the visitor season. Exceptionally high summertemperatures would be a deterrent, unlessgardens incorporate design features such as shady woodland and lakeside walks toincrease visitor comfort.
Gardens will benefit from investment invisitor facilities, such as glasshouses, sheltersand information centres, if summer weatherbecomes even more unpredictable.
Garden managers need to be aware thatclimate change will favour increases in thenumber and variety of biting insects thatbreed in water, which may deter visitors.
The major impact of more garden visitorswill be increased wear and tear, especiallyafter heavy rain, which could be minimisedby contingency planning for managingvisitor movements. Gardens can draw onexperience from the sports turf industry incoping with wear and compaction of lawns.
Gardeners need to be proactive in managinggardens in ways that make minimumdemands on scarce water resources, and indemonstrating sound environmentalpractices to their visitors. This is important,both in terms of managing immediateproblems caused by the current phase ofclimate change, and in modifying behaviourand attitudes to prevent further undesirableclimate changes in the future. Gardens opento the public could help demonstratesustainable horticulture techniques.
Garden industries
Nurseries and garden centres can benefitcommercially from climate change, but willexperience increased operational difficulties.
Most plants sold at horticultural retailoutlets are grown in containers, with rootsconfined in small volumes of growingmedium. Increased labour costs for wateringwill favour the installation of automaticwatering systems. Contingency planning willbe required to ensure reliable suppliesduring drought periods.
Increased incidence of stormy weather willrequire better staking and support of largecontainer-grown plants, which are inherently unstable.
Working conditions under glass during hotsummer weather will deteriorate, although a milder climate in general will make year-round outdoor working more amenable.
Gardening in the Global Greenhouse: Summary Report The bottom line
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Gardening in the Global Greenhouse: Summary Report Rewriting the garden manual
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In gardens open to the public, managingvisitor access to lawns to minimise soilcompaction will be important. Unpredictablesummer rain and prolonged grass growththrough winter will force greater flexibility inmowing regimes to avoid lawn damage,raising costs when contracted labouris employed.
In the longer term, conversion to lawnsconstituted from coarser, more droughtresistant grasses that tend to be less tolerant of close mowing may be needed,to reduce costs of irrigation and sward maintenance.
Garden paths and wooden structures
Paths act as conduits for storm water run-off, and so are prone to rapid erosion. Pathdesign and choice of surfaces can minimisethis hazard. In winter and early springslippery algal growth on paths is likelyto increase.
In addition to unpredictable storm damage,cycles of summer drying and intensive andprolonged rain will lead to more rapiddeterioration of wooden structures. Morerobust construction and use of more durabletimbers from sustainable sources will berequired. Shading and better ventilation ingreenhouses will be needed in summer, forthe benefit of plants and gardeners whowork under glass and, as winters becomewarmer and wetter, good ventilation will beessential to deter fungal diseases.
Ponds, lakes and water supplies
High summer temperatures will lead toincreased frequency of algal blooms inponds, promoting stagnant conditions. Waterfeatures in gardens will need more intensivemaintenance, with frequent topping up ofsmall ponds in summer. Seasonal fluctuationof water levels in large lakes and ponds willrequire the use of spillways and sluices inwinter and improved marginal planting insummer, to minimise the visual impact offalling water levels.
Supply of water for irrigation during summercould be problematic across the UK.Rationing for all but essential uses is likely incritical areas. Water charges may alsoincrease, encouraging installation of moreefficient irrigation systems and the use of‘grey’ water from domestic activities. Waterconservation measures, either via soilmulching or collecting of rainwater in waterbutts, will become a summer priority andlarge gardens may need to constructreservoirs for irrigation purposes.
Hot work
Extreme summer temperatures represent ahealth hazard. Those working on energetictasks will need to guard against dehydrationand sunburn and will require better protectiveclothing when working with chemicals in hot conditions. Coping with effects of climatechange will entail more work and stress,resulting from increased routine maintenancetasks and the need for greater flexibility inworking practices in large gardens.
Brown lawns will become a more common sight (English Heritage)
The cost of lawn irrigation will be a consideration (David Sellman, NTPL)
Warmer weather will give rise to more algae in ponds (NT)
Working in extreme heat represents a health hazard (RHS)
Enhanced autumn colour will extend the visitor season (Mike Sleigh, RHS)
“The gardening community has the potential to set anexample of good practicewhich will further increasepublic appreciation andsupport for gardens ..”Bisgrove and Hadley (2002)
Gardening in the Global Greenhouse: Summary Report Next Steps
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Gardening in the Global Greenhouse: Summary Report The bottom line
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Next steps: research and action There are serious limitations in our knowledge of the likely impact of climate change on gardens.
This study has highlighted challenges and opportunities for the gardening industry to consider, as well as a number of areas for future research and recommendations for action (see below).
The partners will explore what research and practical action they can take together,as well as using this study to inform their own policy and decision-making.
It is difficult to plan for devastating extremeweather events, such as storms and hail, butbusinesses in areas with increasing risk offlooding may need to improve water run-offmeasures, face rising insurance premiumsand consider relocation. For garden centresand nurseries, contingency planning forextreme weather damage will becomeincreasingly important. Extreme weatherconditions, such as drought and flooding,are also indirectly damaging to garden salesbusinesses, since public interest in gardeningrapidly declines during such periods.
As undesirable effects of climate changebecome obvious to gardeners, there will begreater demand for solutions to gardeningproblems. There will be new marketingopportunities for environmentally benignsolutions such as composting and organictechniques, water storage equipment andbiological control methods for dealing withpests. Greater interest in irrigation systemsis also likely.
Climate change will lengthen the growingseason and extend peak sales periods forgarden plants and sundries such as gardenleisure equipment. Demand for replacementplants for those lost through drought orwaterlogged soils in winter will increase.
There could be opportunities for marketing awider range of perennial exotic species andplants that are currently half-hardy annuals,which will survive in the open garden inmilder winters. Faster growth and floweringwill encourage gardeners to replant beddingand containers more frequently.
Exploiting opportunities for marketing awider range of plants requires advancedplanning, since basic research is required toidentify optimum cultivation techniques,especially for perennials, shrubs and treeswhich will need to tolerate summer droughtand waterlogging in winter. Research willalso be required in ‘hardening off’procedures for species that are marginallyhardy, although this will become lessproblematic by the 2080s.
Extreme weather conditions are damaging to gardensales businesses (Notcutts)
Many plants, such as cannas, willperform better in a warmer climate (Mark Bolton, NTPL)
Plants likely to perform better in a warmer climateSource: Emmett (pers. comm.)
• establish the degree of exposure of Britain’s gardening heritage toclimate change
• identify resources and level of investmentrequired to maintain the integrity of thosegardens of greatest historical significance
• establish the criteria for themanagement of significant plantcollections on a national basis
• develop a greater understanding of howgarden plants respond to climate change,specifically in relation to:– frost sensitivity and hardiness – winter dormancy – flowering and autumn colour – susceptibility to pests and diseases – interactions with symbiotic soil fungi
and beneficial micro-organisms
• identification and monitoring ofpotentially invasive species
• minimise soil nutrient loss and reduce the impact of water logging and drought on plants
• develop methods for mowing grass thatminimise compaction of wet soils
• improve methods for water managementin the garden and heat management in glasshouses
• refine models for regional impacts,allowing more accurate prediction of future local climates
• investigate garden visitor response to climate change
Recommendations:
• encourage networking between majorgardening organisations and institutionsto exchange and coordinate observations,ideas and innovations, disseminateinformation and inform policy decisions
• initiate a Hortus Europaeus, to map thedistribution of garden species in Europe,allowing large scale monitoring of effectsof climate change on garden plant species
• produce a list of indicator plant specieslikely to be sensitive to climate change,allowing nationwide monitoring of itsprogress and impacts
• implement practical actions to enablegardens to adapt to climate change.Forward plan for flooding, drought,soil changes and structural impacts on buildings, features and facilities
The season for visiting gardens will lengthen (Chris King, NTPL)
Key challenges and opportunities Domestic gardens
Challenges include:
• managing drier soils in summer and wettersoils in winter
• maintaining soil fertility
• intensification of pest, disease and weed problems
• maintaining a smooth, green lawn
• meeting the needs of drought-adaptedperennials and bulbous species that do not tolerate waterlogging in winter
