THE WEED STORY

Prepared and presented by

Carla Bucknor

and

Timon Williamson

What is Good or Bad about Weeds?

• Good– Some indicate soil

nutrient status– Increase plant

diversity– Provide good wild

life habitat

• Bad– Compete with

cultivated crops causing yield losses

– Out compete native plants

– Habitat for insect pests and vectors

Principles of Weed Management

• Principles – fundamental truths

• The question is ‘What are the fundamental truths we need to know about weeds so that we can effectively manage them?’

• PROACTIVE APPROACH– Manage vs. Control– Key aspects

• Why weeds grow• How weeds grow• Which weeds species are present

WEED MANAGEMENT

Principle 1 – What is a weed?What is a Weed ?

•A plant growing out of place?

•An undesirable plant?

•Plant with a negative value?

•A pest?

•A plant that competes with other plants and man for soil?

Life span of weeds

• Annuals• Produce a seed crop in one year; • Competitive - succeed in highly unstable

and unpredictable environments (frequent tillage, drought etc.) as they must make a seed crop before the next disturbance;

• Seed dormancy• Long lived seed• Yield more seed than perennials

Life span of weeds contd.

• Perennials• Produce seed crop in excess of one year;

focus is to preserve the parent plant and produce seed for future generations

• Competitive - more stable environs• Perennating parts – stolons, bulbs, tubers,

rhizomes• Storage parts – Perennating parts serve as

stored food reserves for rapid re growth

Morphology of weeds

• Grasses– Family Gramineae

• Sedges– Family Cyperaceae; thin triangular stem,

absence of a ligule

• Broadleaves– Other families of Monocotyledonae and

Dicotyledonae; fully expanded broad leaves

Grass and Grass-like Weed ID• Key features to look for on

grasses– ligule– auricles– blade/sheath hairs

• Not all features will be present• Most ID keys begin with the

type of ligule

No ligule present

Auricles and liguleAuricles and ligule

Membranous liguleMembranous ligule

Hair-like ligule

Collar region

Ligule

Broadleaf Weed IDKey features to look

for:• Leaf arrangement on

stem– alternate– opposite

• Leaf and/or cotyledon shape

• Other features to consider:– leaf margin traits– leaf texture/hairs– odor

• Most seedling keys begin by asking for leaf arrangement, then leaf shape

Broadleaf Weed ID contd.

• Leaf arrangement on stem

– alternate

– oppositeLeaf arrangementLeaf arrangement

Alternate – leaves emerge at different points or times on the stem; older ones are larger.

Alternate – leaves emerge at different points or times on the stem; older ones are larger.

Opposite – leaves emerge at the same point and time and are the same size.

Opposite – leaves emerge at the same point and time and are the same size.

Broadleaf Weed ID contd.•Leaf and/or cotyledon shape

•Cotyledons or “seed leaves” emerge first and are always opposite. Do not confuse them with the first true leaves, which are used to determine leaf shape on most ID keys.CotyledonsCotyledons

First true leavesFirst true leaves

Broadleaf Weed ID contd.

Various leaf

margins and

shapes

Note toothed leaf margin

Pere

nnia

lJohnson grass(Sorghum halepense)

• Key features– Membranous ligule– Prominent midrib– Reddish-brown seed– Rhizomes

• Lifecycle– Perennial

Rhizome with new shootRhizome with new shoot

Pere

nnia

lNutsedge (Cyperus)

• Key features– No ligule– Triangular stem– Shiny blades– Nutlets

• Lifecycle– Perennial

Annu

alJimsonweed

(Datura stramonium)• Key features

– Long lanceolate cotyledons with prominent midrib

– Smooth, lanceolate /elliptic true leaves

– Purple stems– Odor when crushed– Large white/purple

trumpet-shaped flowers– Egg-shaped seed pod

with spines

• Lifecycle– Summer annual

Why and where weeds grow?

• ‘Nature’s means of restoring stability by protecting bare soils and increasing biodiversity’

• Succession –‘The cycle of a natural progression of different plant and animal species over time’.

Why and Where? contd.

• Year 1 – Annual weeds

• Year 2 – Briars and bush

• Years 3 to 4 – Perennial vegetation

• Year 5 – Young trees and/or Perennial grasses

How and when weeds grow?

• How do they survive ?

• Why are they so prolific?

Harper, J.L. 1977. PopulationBiology of Plants. AcademicPress, London

WEED SEED BANK CONCEPTDiagrammatic Model of Plant Population

Behavior

WEED SEED BANK CONCEPTDiagrammatic Model of Plant Population

Behavior• I Weed seed

bank

• II Environmental sieve

• III Seedlings, reproductive adults

• IV Seed production

I Weed seed bank

II Environmental sieve:Weed managers and nature manipulate the number of sieves and the ‘leakiness’ of the sieve. For example: dormancy, the influence of the previous year’s weed control

Weed Seed Bank Concept

III Seedlings and reproductive adults: These have survived II and have germinated. They also have en effect on I due to spectral quality because they can now act as a cover crop and so smother or prevent the germination of other weed seeds in I.

IV Seed production: The adults from III have now produced seed which go back into the seed bank.

Weed Seed Bank Concept contd.

Deposition

Germinatio

n

Activation – several days to 50-80 years

Established seedlin

g

5-10 days

Infestation

Infestation develops over a period of several years

Seed Pro

duction

Dispersal

120 days 30-60 days

Timeline for a typical weed infestation

This shows that weed management CANNOT be done in ONE YEAR

SUMMARY

WHAT?

WHERE and WHY?

HOW?

Answers to the above questions will help us to know more about the weeds we need to manage.

With this information a suitable weed management plan can be created.

The End