Post on 03-Apr-2018
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Weed Management
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TopicsinThischapTer
By Chip Bubl, Extension Agent, Columbia
County, Oregon State University.
Adapted by Thomas R. Jahns, Extension Faculty,
Agriculture and Horticulture, Cooperative
Extension Service, University of Alaska
Fairbanks.
Why are weeds a problem?
Why do we all have weeds?
Weed identication
Weed life cycles
Shade trees
Weed competition
Invasive plants
Managing weeds
E
very garden has weeds, and every gardener wonders
what to do about them. This chapter will explore the
origin of weeds, their adaptations and impacts, andthe techniques you can use to manage weeds in your land-
scape.
Why are weeds a problem?The plants we call weeds are aggressive, pushy, in-your-
face plants. There are no shrinking violets in this group.
This is not to say that they dont have some redeeming
qualities, because many weeds have a substantial set of vir-
tues. However, these virtues often are overwhelmed by their
weedy attributes.The most useful denition of a weed is a plant that is a
hazard or a nuisance or one that causes injury to people,
animals or a desired crop. A plant may be dened as a weed
in some cases and not in others. Clover can be a valuable
addition to a pasture and is viewed with tolerance in many
lawns. But if you are a park manager faced with reducing li-
ability due to bee stings, you may feel compelled to remove
clover in some parts of the park. Tree seeds such as aspen,
cottonwood or birch that sprout in an herbaceous peren-
nial ower bed are weeds in that context but may well have
come from trees that are part of the planted landscape.Weeds can cause a range of problems in your garden or
community. They:
Compete with desirable crops
Reduce the aesthetic qualities of a landscape
Obstruct sight lines on roadways
Interfere with water drainage from roads
Create a re hazard along railways and next to power
substations
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Chapter 20Weed Management 397
plants are categorized as noxious with a
legal designation.
It should be noted that not all non-nativeplants are considered harmful or invasive.
Of the 4,000 species of exotic plants that
have become established in the United
States, only about 10 percent have been
identied as a threat. Examples of non-
native plants that are not invasive include
apple, potatoes and peonies.
While Alaska is fortunate to not have
many noxious weeds or invasive plants, we
need to be vigilant as more species become
introduced. Once weedy species are es-tablished in a region, it is virtually impos-
sible to remove them completely. Instead,
gardeners must live with them and work to
lessen their negative impacts. Fortunately,
new techniques offer a range of options to
achieve that goal.
Weed identication
If you can accurately identify a plant thatis causing problems in your landscape, you
have taken the rst step toward a good man-
agement plan. Fortunately, the list of weeds
that cause most problems is surprisingly
short. Often, a little time spent with a good
reference (see For more information) will
help you put a name to some of the chal-
lenging plants you encounter. You may nd
it helpful to give some of your worst weeds
specic nonsense names until you get a
more accurate identication. Then it is easyto mesh your new knowledge with your old
identication.
Weed books usually classify plants rst
into two groups and a miscellaneous cat-
egory:
Figure 1.Reed canarygrass
(Phalaris arundinacea), an
example of a monocot.
Figure 2.Garlic mustard
(Alliaria periolara), an example
of a dicot.
Figure 3.Field horsetail (Equisetum
arvense).
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Monocots (Figure 1) include grasses,
lilies and the like.
Dicots (Figure 2) are broadleaf species.
Other weeds include mosses, horse-
tails and ferns (Figure 3).
Plants are further subdivided within each
major category byfamily for example,
the buckwheat, rose and borage families.
Within these family groupings are thegen-
era, and within the genera,species.
Scientic (botanical) names are in Latin
and list the genus name followed by the
species name. For example, common dan-
delion is known as Taraxacumofcinale.
The scientic name is used worldwide and
eliminates the confusion caused when the
same common name refers to several plants.
You dont have to be a botanical expert
to work through a weed identication book,
Common name Botanical name
Annual weeds
Grass weedsAnnual bluegrass Poa annuaCheatgrass Bromus tectorum
Broadleaf weedsCommon chickweed Stellaria mediaCommon groundsel Senecio vulgarisLambsquarters Chenopodium albumPineapple weed Matricaria matricarioidesProstrate knotweed Polygonum aviculareShepherds purse Capsella bursa-pastoris
Hempnettle GaleopsisbidaCorn spurry Spergula arvensis
Biennial weeds
Grass weeds
None
Broadleaf weedsCommon mullein Verbascum thapsusTansy ragwort Senecio jacobaeaWhite sweet clover Melilotusalba(M.ofcinalis)
Common name Botanical name
Perennial weeds
Grass weedsQuackgrass Elymus repensReed canarygrass Phalaris arundinaceaFoxtail barley Hordeum jubatum
Broadleaf weedsCanada thistle Cirsium arvenseCreeping buttercup Ranunculus repensCommon toadax Linaria vulgarisCommon plantain Plantago majorDandelion Taraxacumofcinale
Red sorrel Rumex acetosellaOrange hawkweed Hieracium aurantiacum
Other weedsHorsetail Equisetum arvense
Table 2.Weeds classied by life cycle.
Note: This list is not exhaustive. It does include many of the most common weeds that gardeners should recognize.
although it does help to learn some of the
key characteristics of the major families.
Once you know a plant, you can gather
important details about its life cycle and
how it spreads within the landscape or
garden. With practice, you can learn to dis-
tinguish weed seedlings from your planted
vegetables and owers.
Weed life cyclesMost gardens have a mix of annual and
perennial weeds, with a few biennials
thrown in to keep things interesting. When
you understand the life cycle of trouble-
some weeds, you can begin to make intel-
ligent decisions about control strategies.
Later sections of this chapter examine some
of the management strategies that can make
your gardening life easier.
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Table 2 lists some of our worst weeds by
common and botanical name. It also shows
their life cycles.
Annuals
Annuals go from seed to seed in less than
1 year, often in periods as short as 45 days.
Once they have thrown off their usually
prodigious quantities of seed, the plants
die. As you might suspect, there are lots
of annual weeds since their growth habits
parallel our agricultural cropping patterns.
Annual weeds can be found throughout
the gardening season, as germination andowering often start early and end late to
maximize reproduction.
Many annuals germinate from late fall
through early spring and go to seed in
spring and early summer. Some common
examples include little bittercress, several
mustards, downy brome, annual bluegrass,
shepherds-purse and chickweed (Figure 4).
Clearly, the cooler temperatures and lower
light intensity are not an obstacle to them. A
few annuals, especially common groundsel,
seem to germinate in ushes throughout the
year.
Most annuals get started in the spring and
summer and go to seed in the summer and
fall. Many of our most annoying weeds are
in this group, including chickweed, shep-
herds purse, pineapple weed, hempnettle
and lambsquarter. Most annuals germinate
quickly, ahead of many ower and veg-
etable seeds. Others do not germinate until
May or June and then grow aggressively
and complete their life cycle without delay.
While it often appears that weeds are con-
stantly germinating, in reality an established
seed bank is always ready to take advantage
of good growing conditions.
As discussed below, annuals have sophis-
ticated mechanisms to ensure good year-to-
year seed survival.
Biennials
Biennial species are less common. They
take more than 1 year but less than 2 years
to complete their life cycle. Most start from
seed in the spring and grow through the
summer, fall and the next spring. Bienneals
overwinter as rosettes. In the second sum-mer, they ower, set seed and die. Examples
include white sweet clover, bull thistle and
the livestock-poisoning weed tansy ragwort
(Figure 5).
Perennials
These weeds often are the most
difcult to manage. Woody species
generally go dormant in the winter
and begin growth in spring from
aboveground stems. Aboveground
parts ofherbaceous perennials may
die back, but their underground stor-
age organs survive the winter. Many
are deep rooted and survive summer
droughts as well. The life span of
perennials varies. They spread from
seed and often from roots, tubers,
bulbs, stolons and rhizomes as well.
Figure 4.Shepherds purse(Capsella bursa-pastoris), an
example of an annual weed.
Figure 5.Tansy ragwort(Senecio jacobaea), an
example of a biennial weed.
Figure 6.Field
bindweed (Convolvulus
arvensis), an example of
a perennial weed.
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Tilling perennial weeds often spreads them,
and mulches may have little impact. Weeds
in this group include blackberries, quack-grass, hawkweeds, Canada thistle, horsetail
and bindweed (Figure 6).
Weed competitionGardeners work to create landscapes that
provide food and are a source of beauty.
Weeds can disturb the appearance of a land-
scape and injure desirable plants. A veg-
etable garden is supposed to produce lots of
good vegetables. This can be difcult whenweeds gain the upper hand.
Most garden soils contain a lot more
weed seeds than the vegetable or ower
seeds you plant. Studies on commercial
vegetable farms show that viable weed seed
populations in the top 6 inches of soil aver-
age 900 per square foot, with some elds
having in excess of 7,000 viable seeds per
square foot!
What is worse, weed seeds germinate
very quickly when the ground is tilled. Twoweeks after planting, it may be very difcult
to nd your owers and vegetables amid
all the weeds. Some poorly tended gar-
dens may end up with 10 to 20 times more
weight in weeds than in desired plants.
Weeds compete directly with garden
plants for light, nutrients and water. A suc-
cessful weed grows aggressively to capture
whichever resource is in the shortest supply.Crop plants may end up stunted and unable
to produce a normal product. Paired test
plots of vegetables that were either weeded
or left untouched give some eye-opening
results (Table 3).
Research in England showed that a delay
in weeding could reduce nal yield by 3
percent per day, depending on the crop,
weed species and weed density. Some
plants (onions, for example) need at least
a certain leaf area to produce a marketablecrop, while others (e.g., potatoes and arti-
chokes) can send up new leaves and recover
to produce a near-normal yield if weed
competition is removed early enough.
Some characteristics that help weeds
compete include:
Aggressive vegetative growth from seeds
Abundant and rapid reproduction
Good means of dispersing seeds
Long-lived seeds and other plant parts
Wide adaptability to soil types andclimates
Ability to time germination to coincide
with favorable conditions
Ability to thrive in disturbed or bare soil
Rapid root and top growth
Weeds grow quickly to capture sunlight,
water, space and nutrients. They often can
alter their branching pattern, leaf size and
leaf orientation to win the resource battle.
Weed root growth can stunt crop roots in
moisture-short soil.
Sophisticated reproductive strategies
Gardeners know that weeds can produce
tremendous numbers of seeds. Weeds also
have ways to prevent all of their seeds from
germinating during years with less favor-
Table 3.Vegetable yields in weeded and
unweeded plots.*
Yield
Crop Weedy NonweedyCarrots 27.9 lb 503.3 lbBeets 45.9 lb 240.3 lbCabbage 129.1 lb 233.6 lbOnions 3.6 lb 67.7 lbPotatoes 52.7 lb 148.3 lb
*Plot sizes not specied, but weedy and nonweedy plots
were equal in size. With the exception of weed manage-
ment, both plots were treated the same.
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able weather. Seeds can be buried in undis-turbed soil for an amazing length of time
and still be able to germinate. Table 4 shows
weed seed production and survival rates.
Just so you dont give up, these survival
rates are under optimal conditions, and most
weed seeds are long gone before they reach
these ages. Nevertheless, as the saying goes,
1 year of weeds leads to 7 years of hoeing.
Chemical warfare between plants
Some weeds, especially some herbaceousperennials, produce compounds in their
leaves and/or roots that can kill or harm other
plants. This phenomenon is called allelopa-
thy. The compounds arent effective on all
the species with which a particular weed
might be competing, but they at least can
improve the weeds odds. Some weeds that
have allelopathic qualities include quack-
grass, Canada thistle, garlic mustard, purs-
lane and some hawkweeds. As if these plants
didnt have enough going for them already!
Invasive plantsInvasive plants are weeds that, in addition
to being a nuisance in the garden, degrade
the economic and ecological value of the
areas they invade. There are many terms
used in the discussion of invasive plants.
Native or indigenous plants
These plants have evolved with the eco-system within which they grow. Although
native, some species can dominate natural
areas, especially after a disturbance. In
Alaska, reweed (Epilobium angustifolium)
is a good example of a native plant that
dominates natural areas following distur-
bances, typically re.
Non-native or exotic plants
Non-native plants are plants that have been
introduced from one geographically isolatedarea to another with the help of a human
vector. Not all non-native plants are invasive,
but some have the potential to invade natural
areas and cause ecological damage.
Invasive plants
These aggressive plants eventually
dominate an area, displacing native species,
altering ecosystems and degrading the eco-
nomic value of the land. Spotted knapweed
(Centaurea stoebe) is an invasive plant thathas begun to establish in Alaska. In areas of
the Lower 48 where is has invaded, it has
been found to degrade wildlife habitat and
reduce the agricultural value of land.
Noxious weeds
Noxious weeds are plants that have been
legally designated by a federal, state or coun-
ty government as injurious to public health,
agriculture, recreation, wildlife or property.
There are 72 terrestrial, 19 aquatic/wetland
and 5 parasitic weeds listed on the current
Federal Noxious Weed List. In Alaska, there
are 12 prohibited noxious weeds (see Table
5).
Table 4.Weed seed production and seed
survival in soil.
No. of seeds Seed survivalWeed species per plant (years)Lambsquarter 72,450 40Purslane 52,300 40Dandelion 15,000 6Canada thistle (per stem) 680 21Crabgrass 25,000 3
Note: Seed survival means that some viable seed remains.
Generally, however, most seeds germinate or lose viability
within 3 to 10 years or less, depending on soil conditions. A
few, however, will hang on to aggravate future gardeners.
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Invasive Species Laws
Federal
In 1999, President Bill Clinton signed an
Executive Order (13112) directing Federal
agencies to use their authority to prevent
the introduction of invasive species, control
their spread and monitor and restore native
plant communities affected by invasive spe-
cies. The National Invasive Species Coun-cil, which includes 20 federal agencies,
developed a National Management Plant
(NMP), that outlines how federal agen-
cies will work with other groups to prevent
infestation, educate the public, manage
information and conduct research. The
NMP does not dene how invasive spe-
cies should be managed and controlled;
that responsibility is left to the states.
State
The Alaska Division of
Agriculture is authorized to
prevent the importation and
spread of pests that are injuri-
ous to the public interest and
to protect the agricultural
industry. Regulations
relating to noxious
weed control are found in Title 11 Chap-
ter 34 of the Alaska Administrative Code.
The rules for establishment of quarantines,inspections, noxious weeds lists and con-
trol measures are all provided for in current
state regulations.
Private and public landowner control of
noxious weed infestations is encouraged.
Public education rather than enforcement
is the preferred method for dealing with
noxious weed problems in Alaska.
Prevention
Due to the difculty involved with re-moving/eradicating invasive plants, pre-
venting their establishment is crucial, which
is why education and early detection are
key components of a successful invasive
plant management plan.
Learn how to identify invasive plants
of concern by visiting the Alaska Com-
mittee for Noxious and Invasive Plant
Management website (www.cnipm.org).
A plethora of resources are available on
this site, including photos of invasive
plants, a downloadable pocket guide of
invasive plants, an events calendar with
invasive plant identication workshops
and much more.
Reduce the opportunity for invasion by
not planting known invasive plants.
What can homeowners do?
Remove invasive plants on their property
and dispose of them properly. Cuttingsand/or whole plants should not be thrown
into the woods or compost, where they
may survive and spread. Instead, place
invasive plants into garbage bags, securely
tying the top to ensure that the plants do
not break through the bag.
Volunteer at weed removal projects in
your area.
Table 5.Prohibited noxious weeds in the state
of Alaska
Quackgrass (Agropyron repens)Whitetop and its varieties (Cardaria drabe,
C. pubescens, Lepidium latifolium).Russian knapweed (Centaurea repens)Canada thistle (Cirsium arvense)Field bindweed (Convolvulus arvensis)Leafy spurge (Euphorbia esula)Galensoga (Galensogaparviora)Hempnettle (Galeopsis tetrahit)Blue-owering lettuce (Lactuca puchella)Austrian eldcress (Rorippa austriaca)Horsenettle (Solanum carolinense)Perennial sowthistle (Sonchus arvensis)
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Report establishing invasive plant popu-
lations you observe at www.cnipm.org.
Invasive plants do not recognize propertyboundaries; successfully battling invasive
plants requires partnerships between public
and private landowners, government agen-
cies, industries, academia and non-govern-
ment organizations. As a Master Gardener
you can play a crucial role in limiting the
impact of invasive plants in Alaska by edu-
cating others about invasive plant issues and
controlling invasive plants on your property
(see chart below for a list of invasive orna-
mentals and alternatives).
Managing weedsWeeds are part of the dynamic and shift-
ing garden landscape. As discussed earlier,
many weeds are especially adapted to a
cultivated environment.
A gardener should plan for weeds. It is
possible to develop a fairly comprehensive
weed management strategy that takes intoaccount landscape objectives, weeds al-
ready present, available tools and personal
gardening philosophy.
In the broadest sense, weed management
strategies have three objectives:
To prevent the introduction of new weeds
To discourage weeds so they cant com-
pete with desired plants
To stop weeds from going to seed, thus
reducing, over time, the weed seed bur-
den in the soil
Prevention
While most garden soils already have
many weeds present, and other weed seeds
can be blown in, the alert gardener can take
some steps to prevent the introduction of
new weeds. First, examine any soil you plan
Invasive ornamentals Alternative
Dont Plant in Anywhere in AlaskaRampion bellower (Campanula rapunculoides) Peach-leaved bellower (Campanula persicifolia), Milky
bellower (Campanulalactiora)Creeping Charlie, Ground Ivy (Glechoma hederacea) Bugleweed (Ajuga reptans), Hermann's Pride (Lamiastrum
galeobdolon)Butter and Eggs, Toadax (Linaria vulgaris) Annual snapdragonsPurple loosestrife (Lythrum salicaria, L. virgatum) Blazing star (Liatris spicata), Salvia,Delphinium
or native lupinesOrnamental ribbongrass (Phalaris arundinaceae, Picta) Japanese silver grass (Miscanthus sinensis, Variegatus)Japanese knotweed (Polygonum cuspidatum),Bohemian knotweed (Polygonum x bohemicum)Common tansy (Tanacetum vulgare)Common mullein (Verbascum thapsis) Verbascum bombyciferum, V. chaxii
Oxeye daisy (Leucanthemum vulgare) Shasta daisy (Leucanthemum Xsupurbum), Arctic daisy(Dendranthema arcticum)Ornamental jewelweed, Washington orchid, Impatiens nolitangere
Policemans helmet (Impatiens glandulifera)
Don't Plant in Southeast AlaskaScotchbroom (Cytisus scoparius)St. John's wort (Hypericum perforatum)Foxglove (Digitalis purpurea)Creeping buttercup (Ranunculus repens)Sweet rocket (Hesperis matronalis)
Species list compiled from CES publication FGV-00146
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to bring to your lot. Pay particular attention
to the presence of horsetail, which will
cause years of agony if it nds yourlandscape.
Second, watch container plants
from nurseries, garden exchanges and
plant sales. They can be a source of several
tough plants, particularly chickweed, grass-
es, creeping charlie, buttercups
and even Canada thistle. Remove
any sign of these plants before
placing new trees, shrubs or
owers in your landscape.
Finally, cultivate a relation-ship with your neighbors that allows
for a coordinated approach to creep-
ing perennial weeds such as orange
hawkweed, creeping buttercup, eld bind-
weed, rampion bellower and oxeye daisy.
Cultural and mechanical control
The alert gardener looks for opportunities
to reduce the impact and spread of weeds.
This section will look at ways to achieve
those goals without the use of herbicides.
Gardening involves lots of small deci-
sions that can have a cumulative effect on
weed problems. Working from the premise
that weeds thrive in disturbed ground, you
can manipulate the landscape in ways that
reduce weed success. These techniques gen-
erally are called cultural control methods.
Several are discussed below.
Rotation
Rotating the vegetable garden can reduceweed infestations. Noncompetitive crops
such as carrots and onions should follow
more aggressive vegetables such as potatoes.
Fallowing(keeping part of the garden clean-
tilled or in a summer cover crop) can help
reduce weed problems for the next season.
Weeding
Cultivating soil to control weeds has been
practiced for thousands of years. Agriculturalyields improved tremendously when cultivat-
ing equipment became widely available.
Advantages: A germinating weed is very
vulnerable to root disturbance from hoe-
ing, hand pulling or rototilling. A secondary
benet of tilling is that it helps break up soil
crusts that may limit vegetable or ower
seedling growth.
Disadvantages: Mechanical weed re-
moval must be done early and often to be
effective. Also, tilling brings new weedseeds to the surface. In addition, excessive
cultivation destroys soil tilth and causes
compaction.
When tilling, take care not to damage
roots of desirable plants. For example, it
generally is unwise to hoe close to shallow-
rooted perennials, rhododendrons and
blueberries.
Bed planting
In this method of garden design,planted areas are designed with un-
tilled areas between them. Soil work-
ing is reduced and mulches are used
extensively. Flower or vegetable
crops often are planted close together
to compete against weeds. Shading
is effective in preventing some weed
species from germinating. Trans-
plants are used extensively.
Advantages: The reduction in
tillage reduces the number of weedseeds brought to the surface. Leav-
ing untilled areas (either in sod or
heavily mulched) reduces the area
that needs to be managed for weeds.
Disadvantages: Most
of the work (tillage and
weeding) must be done by
See Chapter 8,Vegetable
Gardening.
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hand. Some gardeners plant too closely and
stunt their crops much as weed competition
would have done.
Using transplants
Vegetables or owers can be started in a
cold frame or greenhouse and set out into
the freshly worked garden.
Advantages: The transplant has a head
start against germinating weed seeds and, if
managed well in the transplanting process,
should effectively compete against many
weeds. This can be one of the best weed
management strategies.Disadvantages: It takes time and plan-
ning to produce transplants, or money to
purchase them. Also, not all plants trans-
plant well.
Mulches
Mulching should be part of any land-
scape planting for the weed suppression it
provides. Mulch materials may be organic
(such as shredded bark or leaves, spruce
needles, compost or newspapers) or syn-
thetic (such as plastic or landscape fabrics).
They can be applied in the spring, summer
and fall for year-round weed control. Mulch
materials may be home generated or pur-
chased; however, purchased materials can
be somewhat expensive.
Advantages: If used effectively, mulches
reduce weed-pulling time signicantly.
When bare soil is covered, many weed
seeds either wont germinate or cant grow
through mulch. Also, when mulching is
used instead of hoeing or rototilling, fewer
weed seeds are brought to the surface. Done
correctly, mulching also can cool the soil
and help manage soil moisture.
Disadvantages: Mulches are not without
problems. If organic mulches are applied
too thickly they may produce acids that
can hurt plants and cool soil temperatures.
Around some deep-rooted trees
and shrubs, and on well-drained
and aerated soils, plastic mulchescan work well. However, around
shallow-rooted species such as
rhododendrons, the inability of
plastic mulches to breathe can
create a zone of lifeless soil that
often leads to root disease prob-
lems. In general, plastic mulch
is not the best choice for woody
landscape beds.
Synthetic mulches generally are covered
with an organic mulch to improve theirappearance. However, over time weed
seeds blow in and often germinate in the
organic material on top of the weed barrier,
thus reducing weed control. In addition,
both plastic and landscape fabrics can give
shelter to mice or voles and run the risk of
signicantly reducing soil temperatures.
Mulches for woody landscapes
Yearly applications of organic mulches
such as shredded plant materials go a longway toward reducing weed problems. As
the material decays, it improves soil tilth
and aeration around landscape plants. Fall
applications of mulch can reduce spring
weed problems.
Landscape fabrics are better than plastic
mulches in a woody landscape bed, since
water and air can pass through them. Al-
though more expensive, laminated fab-
rics such as Typar are porous, wick water
through the ber and prevent weed rootsfrom penetrating and becoming established.
Mulches for annual gardens
Plastic mulch does have a place in annual
vegetable and ower gardens. It usually is
used with drip irrigation and bed planting.
Make slits in the plastic and place trans-
plants next to the water source. Add fertil-
izer to the soil prior to transplanting or later
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through the irrigation system. Dis-
pose of the plastic after the grow-
ing season.Be sure to stretch the
plastic tightly over the soil
surface to get the benets
of soil warming as well as
weed control. Black plastic
is preferred for weed control
because it reduces light to the
soil, thus preventing weed
growth. Clear plastic mulch
warms the soil faster than black
plastic but does not control weeds.Infrared-transmitting mulch warms the
soil and helps control weeds.
Landscape fabrics can be used in the
same way, but since they allow moisture to
pass through, they dont require a drip sys-
tem. However, they are thicker than plastic
and tend to cool the soil instead of warm-
ing it. This can be a problem for crops that
require warm soils.
Organic mulchesOrganic mulches cool the soil but also
improve the soil as they break down. Ex-
amples of organic mulch include grass clip-
pings, leaves and straw that is not weedy.
Be careful with woodchip mulches and
sawdust; they contribute to nitrogen de-
ciency as they decompose. Woody mulches
work best in landscape situations, not in
vegetable gardens or ower gardens that
include annuals.
Water managementWeeds need water to germinate and grow.
When a garden is irrigated with sprinklers,
the entire area usually is watered and must
be weeded. When water is directed only to
desired plants, such as with drip irrigation,
much of the garden stays dry. In those drier
areas, weed problems are much reduced.
Drip systems often are combined with bed
planting and/or plastic mulches in vegetable
and annual ower plots.Advantages: Drip systems are excellent
tools for reducing weed growth. They are
fairly easy to set up and have come down in
cost.
Disadvantages: Time must be spent man-
aging clogged noozles and xing leaks.
Cover crops
Cover crops generally are
grown in the late summer on
annual vegetable and owerbeds. The cover crop may be a
grain, a legume or a combina-
tion of the two.
Advantages: The cover crop smothers
much late weed growth. Fast-growing crops
such as buckwheat are the most com-
petitive. Cover crops also capture excess
fertilizer and improve soil tilth and increase
organic matter when turned under. They
generally are very inexpensive. In warm
areas, some cover crops can be cut andleft on the surface as a weed-suppressing
mulch, and an early crop such as lettuce can
be transplanted through the residue. Crop
residue left on the soil surface enhances the
environment for benecial soil insects.
Disadvantages: The only major dis-
advantage is that gardeners in most areas
have to till in the cover crop before they
can plant a spring/summer garden. In a wet
spring, this can delay planting. Cover crops
are by no means 100 percent successful inweed suppression, so there may be some
weeds to contend with. Also, slugs may
prosper if the residue isnt turned under.
Chemical control (herbicides)
Herbicides are another tool for managing
weeds. The extent to which you choose to
use them depends on personal philosophy,
See Chapter 3,Soils and
Fertilizers.
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garden objectives and the particular weed
problems in your garden. Herbicides may
have a place in your garden, but alwaysunderstand what you are trying to achieve,
what the alternatives are and what decisions
make the most sense given your situation.
All herbicides have detailed label instruc-
tions on mixing, application timing, weeds
controlled, plants around which they can be
used and other signicant issues concerning
their safe and effective use. It is crucial that
you read these instructions before you pur-
chase a product and follow them as you mix
and apply it. The label is the legal documentthat denes the use of a product and your
responsibilities as a consumer.
Herbicides control weeds by interfering
with critical plant functions, thus resulting
in the death of the plant. Not all herbicides
act in the same manner.
You need to understand some important
terms and concepts before purchasing and
applying herbicides.
Aselective herbicide controls certain
plants and not others. For example, mostlawn herbicides control broadleaf plants,
such as dandelions, without damaging
lawn grasses. A few products control some
(though not all) annual weed grasses with-
out harming turf. Other selective herbicides
affect germinating seeds and sprouting her-
baceous plants but not established woody
trees and shrubs. A handful of products
control grasses without damaging broadleaf
species.
If an herbicide is selective, the label willgive extensive information about which
plants it is safe to use around and which
weeds and plants it is likely to affect. In ad-
dition, there will be instructions on when to
use the herbicide to get the desired results
and avoid problems.
Nonselective herbicides potentially can
damage any type of plant. Some last a long
time (have some residual effect); others
do not. Again, the label will detail how the
herbicide acts.It is important to remember that some
products may be selective at certain rates
but lose that selectivity as the amount ap-
plied increases.
Most herbicides aresystemic, which
means they move from the point of initial
absorption to other parts of the plant. They
may be sprayed on the leaves and move to
the roots or vice versa. These products cir-
culate through the plant to have their effect.
Nonsystemic orcontactherbicides affectonly the part of the plant they touch. When
a contact product is sprayed on leaves, it
kills those leaves but does not travel to the
root system. Thus, contact herbicides may
control young annual or biennial weeds, but
established perennial weeds that lose leaves
due to a contact herbicide application will
resprout. Products containing soap, clove
oil and vinegar act as contact herbicides.
They are most effective on annual weeds
when they are young.Herbicides also are classied by the way
they move into plants. Many common prod-
ucts arefoliar-active, meaning they enter
through leaves. For example, glyphosate
(e.g., Roundup) must enter through green
tissue. If this product is applied to bare soil,
it has no effect on germinating seeds. Also,
glyphosate cannot be picked up by roots in
most soils. Soil-active products, on
the other hand, are ab-
sorbed through roots orthrough the growing
tips of germinating
seeds.
A number of
products are both
soil-active and foliar-
active, although one
See Chapter 22,Understanding
Pesticides.
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408 Weed ManagementChapter 20
absorption route usually is more important.
Again, the label will describe how to apply
the herbicide.Finally, it is important to know when
to use an herbicide relative to the growth
of the weeds and the crop. Apreemergent
herbicide is put on before weeds sprout.
The desirable plants (landscape trees and
shrubs, vegetables, owers, bulbs, etc.) may
or may not be present. Careful label reading
is important!
Apostemergentproduct is used after
weeds are up. Selectivity of postemergent
herbicides may depend on crop age, pres-ence of bark, rate applied or other factors.
Its important to understand application
timing as it relates to both weed and crop
growth. Again, read and follow the label.
Herbicide interactions
Herbicide results are inuenced by a
number of factors. A plant with a waxy leaf
(such as Japanese knotweed) or a hairy leaf
(such as hempnettle) may not absorb an her-
bicide as well as does a plant with a smoothleaf. Likewise, a plant with narrow, upright
leaves may be hard to cover adequately
with spray mixture. Sometimes, a weeds
growing point is protected under the soil
surface.
Environmental condi-
tions can affect an herbi-
cide, rendering it useless
against the target plant or
causing unintended dam-
age to nontarget plants.In general, as tempera-
ture increases, herbicides
work faster. However,
some herbicides can be-
come volatile (gaseous)
at certain temperatures.
Dichlobenil (Casoron) tends
to become volatile when temperatures
exceed 55F. While this particular chemi-
cal does little harm as it volatilizes, it also
doesnt provide much weed control.On the other hand, some formulations of
2,4-D and some of its relatives can come off
leaves shortly after application if tempera-
tures climb into the mid 80s. Since small
amounts of this herbicide can visibly distort
sensitive plants, high temperature combined
with a little wind can cause serious prob-
lems in your own garden or your neighbors
garden.
Wind drift by itself can be a problem when
using herbicides. As you pump a sprayer, thepressure increases, which in turn creates a
smaller spray droplet. The smaller the drop-
let, the more likely it is to become airborne
and move away from where you want it to
land. Again, this drift can cause unintended
consequences and neighborhood problems.
Do not spray if conditions do not permit, and
always use safety precautions when calculat-
ing wind speed to avoid spray drift.
Moisture in modest quantities is needed
to move soil-applied herbicides into thesoil. However, if a downpour occurs shortly
after the material is applied, the chemical
may wash downslope instead of entering
the soil. This runoff can damage lawns or
other plants in its path.
When you use foliar chemicals, there
must be enough time between application
and rain to allow the plant to absorb the
herbicide. Normally, 8 hours of dry weather
is enough. However, there are many stories
of gardeners misapplying herbicides andthen trying to wash them off only to nd
that the plant already was damaged. The
take-home message is that while 8 hours is
ideal, signicant plant damage can occur in
considerably less time.
Sometimes, herbicides work poorly in
dry weather. Plants that are drought stressed
conserve water by reducing transpiration
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Chapter 20Weed Management 409
through leaves. In this condition, they are
less able to absorb herbicides. Several sys-
temic products have specic statements onthe label about their reduced effectiveness
when plants are moisture stressed.
Specic herbicides
Note:Trade-name products are men-
tioned as illustrations only. This does
not mean that the University of Alaska
Fairbanks Cooperative Extension Service
endorses these products or that they intend
to discriminate against products not men-
tioned.2,4-D
This systemic, foliar, post emergent her-
bicide affects broadleaves, especially her-
baceous annuals, biennials and perennials.
It is a common component in lawn products
since it doesnt damage established grass.
It is sold under numerous trade names and
often is combined with other closely related
chemicals (meco-prop and dichlorprop) to
broaden the spectrum of weed control.
Dicamba
Dicamba has the same effect on plants
as 2,4-D. It also is systemic, postemergent
and foliage applied. Dicamba is very ac-
tive against broadleaves, especially some
of the harder-to-control lawn weeds. It is
used in some lawn herbicide mixtures. It is
more dangerous to woody plants than 2,4-D
and can be absorbed through roots. It is not
uncommon to see landscape tree and shrub
injury when dicamba is used extensively onlawns surrounded by woody plants. Di-
camba also is sold under many trade names.
Normally, it is mixed with other herbicides
at very low rates.
Dichlobenil
A systemic, preemergent, soil-applied
herbicide, dichlobenil is available as gran-
ules. It controls annual, biennial and peren-
nial broadleaves and grasses but generally
does not damage woody plants. It is used in
landscape beds where there are no under-plantings of herbaceous owers or bulbs.
Dichlobenil is best used before weeds
emerge. It needs light rain and cold tem-
peratures to move it into the soil. If tem-
peratures are too warm when it is applied,
it becomes volatile and does not provide
much weed control. If it rains too much just
after it is applied, it may travel downslope
and damage nontarget plants. Generally, it
is best applied as a pre-emergent. Dichlo-
benil persists in soil for 6 to 9 months ormore, depending on rainfall, soil conditions
and application rate. Its use may limit later
planting options. It is sold under the trade
names Casoron and Noxall for home use.
Glyphosate
Glyphosate is a systemic, nonselective,
foliar-applied herbicide. It potentially af-
fects any plant with which it comes in con-
tact. Glyphosate is absorbed through leaves,
green bark (usually a very young tree orshrub) or freshly cut stems. The chemistry
of glyphosate is such that it becomes tied up
on soil particles. There is little chance that
it can be picked up by roots unless they are
exposed in some manner.
Glyphosate tends to follow the ow of
sugars in plants. If it is applied to an actively
growing woody plant, the chemical tends to
move to the new leaves (where the sugars are
going) and stunt the terminal growth. In that
circumstance, it may not move to the rootsystem in sufcient quantities, and the plant
survives. Thus, it is best used on perennial
plants as they begin owering or in the fall
as they start storing sugars for winter.
Glyphosate can be used on some grasses
almost any time they are green and on annu-
als when they are actively growing. It works
more quickly (7 to 10 days) in warm weath-
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410 Weed ManagementChapter 20
er. It also can be effective in colder weather,
although the results might not be evident for
a month or more. It is sold under the nameRoundup and several other brands.
Triclopyr
This product is systemic and predomi-
nately foliage absorbed. In some cases, it
can be absorbed through bark. Triclopyr is
active against broadleaf plants, especially
woody species. It has no effect on estab-
lished grass. It is mixed with 2,4-D in the
product Crossbow and is sold alone in other
brush killer and lawn formulations. Read
the label carefully to see whether the formu-
lation is an ester or an amine. Avoid apply-
ing esters in hot weather to reduce potential
volatility and drift.
Oryzalin and trifuralin
These preemergent compounds act on
germinating seeds. There is a fairly com-
plex list of weeds (some, but not all, grasses
and broadleaves) that they control and
plants around which they can be used. They
are most effective on annual weeds.It is very important to read the labels
carefully when purchasing and using these
products. They differ in their need for
incorporation into the soil, the weeds they
control and the plants around which they
can be used. Triuralin is sold as Preen,
Trean and several other names. Oryzalin is
marketed as Suran and other prod-
uct names. Plants sensitive to these
herbicides cannot be planted until at
least a year after use.
Fluazifop
A fairly new herbicide, uazifop
controls many grass species but does
not harm most broadleaf plants. It is
applied after grasses emerge and are
rapidly growing. It takes 7 to 14 days
or more to see results. It is sold under
the name of Ortho Grass-B-Gone and many
other brand names.
Sethoxydim
Sethoxydim controls many grass species
but does not harm most broadleaf plants.
It is applied after grass has emerged and is
growing rapidly. The most common trade
name is Poast.
Corn gluten meal
Corn gluten meal (CGM) is a natural by-
product of the corn milling process. It has
been used as livestock feed for many years.
CGM has preemergent herbicidal activity;it inhibits root growth of susceptible germi-
nating seeds. It affects a variety of annual
and perennial broadleaf and grass seedlings.
It is not effective on established perennial
weeds. CGM has been used on existing turf
and around established plants, transplants
and deeply seeded vegetables in vegetable
and ower gardens. It usually is surface
applied and lightly worked in. If worked
in too deeply, it might damage nontarget
plants. Read the label cautions and use rec-ommended rates and application techniques.
Weed management for specicsituations
Lawns
Choose grass species suited
for your area. A vigorous lawn
reduces weed invasion. Good
fertility, watering, mowing
and aeration programs will keep your turf
in top condition. Plan an annual program of
overseeding weak areas, especially those in
some shade.
If the grass you plant can tolerate a mow-
ing height of 2 inches or more, the shade
cast by the grass will inhibit many broadleaf
weeds. Use a fertilizer low in phosphorus
to avoid stimulating clover (assuming you
See Chapter 14,Lawns.
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Chapter 20Weed Management 411
dont want clover in your lawn). Some
broadleaf weeds can be managed by hand
pulling, but clover and dandelions mayprove difcult to control this way.
Weedy perennial grasses such as quack-
grass can be very invasive. There are few
options short of complete renovation once
these plants get established. Some garden-
ers spot spray the patches with glyphosate
and then overseed them.
Annual bluegrass is common in lawns.
Some herbicides are available to help man-
age this species.
Moss grows where grass is weak. Usu-ally, lawns are weakest in shady areas and
where drainage is a problem. Control moss
either with products containing iron or by
dethatching. In shady areas, overseed lawns
with red fescue to maintain a viable turf.
Many homeowners use broadleaf herbi-
cides, either alone or in combination with
fertilizer (weed-and-feed). To reduce
the amount of herbicide used, it generally
is much better to spot spray weedy areas
rather than to apply herbicides over theentire lawn each time you fertilize.
If young children or pets use the lawn,
be very cautious in the use of herbicides!
Renovation
Lawn renovation often is done to reestab-
lish turf grasses where weedy grass species
have taken over. The weedy species must be
killed (especially perennials) before a new
lawn is planted.
A single rototilling generally spreadsrather than controls problem grasses. Re-
peated tillage over 3 to 4 weeks can give
acceptable control in dry weather.
Some homeowners use glyphosate to
kill an existing weedy lawn. They then
dethatch, aerate and overseed. Rototill-
ing is not used unless the lawn needs to be
reshaped.
With good temperatures and water, a
new lawn can be up and growing in 3 to 4
weeks.
Woody landscape areas
Weeds in landscape beds can be man-
aged with a mix of techniques that include
mulching, water placement, competitive
planting, hand pulling and herbicides ap-
plied as both spot and broadcast treatments.
Mulches should be your rst line of
defense against weeds. They reduce the
germination of weed seeds and protect the
soil. Organic mulches such as bark probablyare the most effective and cause the least
problems. Hoeing in mulch is very easy
and disrupts most annual weeds. Landscape
fabrics are ne for use on pathways, but
in landscape plantings are only useful for
deterring weeds for a few years. Do not use
black plastic around trees and shrubs. Pe-
rennial weeds generally are not deterred by
organic mulches and may defeat landscape
fabrics as well.
Drip irrigation places water around desir-able plants but doesnt water everything.
The potential for weed growth in the dry
areas is greatly reduced.
When bare areas are planted with robust
plants, weeds struggle to compete. Complex
landscapes that cover most of the ground
generally have fewer weed problems as the
plants get established. This can be a great
reason to buy more plants! It does help to
have a plan and to know which species work
best in this role. In some cases, competitiveplantings may limit your herbicide choices.
The herbicides most commonly used in
woody landscapes are oryzalin, triuralin
and spot application of glyphosate. Be sure
you understand how these products work to
avoid damaging desirable plants.
See Chapter 11,Woody Landscap
Plants.
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412 Weed ManagementChapter 20
Annual ower and vegetable gardens
Annual vegetable and ower gardens can
be weed nightmares. Working the garden inthe spring offers an opening for weed seeds.
Their aggressive growth can quickly domi-
nate a garden.
Weeds are best managed in ower and
vegetable gardens by a combination of hoe-
ing, hand pulling, vigorous plants (includ-
ing extensive use of transplants) that shade
the ground as they mature, drip irrigation,
mulches and relentless attention that keeps
weeds from going to seed.
Some gardeners use glyphosate beforethe rst spring cultivation to control persis-
tent perennial weeds, especially quackgrass.
Triuralin (Preen or Trean) is labeled for
use aroundsome (but not all!) owers and
vegetables. Preen with corn gluten meal as
an active ingredient is also available as a
preemergent. Oryzalin has some home-gar-
den labels for owers. Read the labels very
carefully and follow instructions if you use
these products.
For more information
UAF Cooperative Extensionpublications
Dont Plant a Problem, FGV-00146.
Lawn Establishment, HGA-00036.
Lawn Maintenance, HGA-00334.
Lawn Maintenance and Pesticides, HGA-
00236.ManagingAlaskanLawns:WeedIdentica-
tion, Prevention and Control,
HGA-00239.
Moss Control in Lawns, HGA-00133.
NoxiousWeedIdenticationandPreven-
tion, FGV-00144.
Reducing the Spread of Non-Native Inva-
sive Plants in Alaska: Voluntary Codes
of Conduct for the Gardening Public,FGV-00142.
Reducing the Spread of Non-Native Inva-
sive Plants in Alaska: Voluntary Codes
of Conduct for Nursery Professionals,
FGV-00147.
Other references
Invasive Thistles in Alaska, USDA Forest
Service Leaet R-10-TP-132.
Japanese Knotweed, USDA Forest Service
Leaet R-10-TP-133.Orange Hawkweed, USDA Forest Service
Leaet R-10-TP-129.
PacicNorthwestWeedManagement
Handbook,WEED.
Perennial Sowthistle, USDA Forest Service
Leaet R-10-PR-17.
PlantsofthePacicNorthwestCoast, Pojar
& Mackinnon; Lone Pine Press.
Plants of the Western Boreal Forest, John-
son et al. Lone Pine Press.
Rice, P.M., J.C. Tonye, D.J. Bedunah and
C.E. Carlson. 1997. Plant community
diversity and growth form responses to
herbicide control of Centaurea macu-
losa.Journal of Applied Ecology: 34:
1397-1412.
Selected Invasive Plants of Alaska, USDA
Forest Service Leaet R10-TP-130B.
Spotted Knapweed, USDA Forest Service
Leaet R-10-TP-125.
Weeds of the Northern U.S. and Canada,
Royer & Dickinson; Lone Pine Press.
Weeds of the West, Western Society of Weed
Science.
See Chapter 8,Vegetable
Gardening, and
See Chapter 9,Annual and
Perennial Flowers.
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