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Weed ecology and management. Martha Rosemeyer Ecological Agriculture November 4, 2003. Sustainable Agriculture. Three attributes: Economically viable, environmentally sound, socially just An integrated system including: natural resources: land/soil, crops, animals, climate

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Weed ecology and management

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Weed ecology and management

Weed ecology and management

Martha Rosemeyer

Ecological Agriculture

November 4, 2003

Sustainable agriculture

Sustainable Agriculture

  • Three attributes: Economically viable, environmentally sound, socially just

  • An integrated system including:

    • natural resources: land/soil, crops, animals, climate

    • socio-economic resources: capital, labor and management

That are organized to satisfy the following goals

That are organized to satisfy the following goals:

  • Provide food and fiber

  • Enhance environmental quality and natural resource base

  • Make the most efficient use of non-renewable and on-farm resources

  • Sustain the economic viability of farm operations

  • Enhance the quality of life for farmers, their families, communities and society as a whole



  • Definition

  • Weed ecology and succession

  • Weed problems and benefits

  • Weed management: systems modifications vs. input subsitution

Weed ecology and management


Labor 40-70% labor in traditional systems or herbicides to remove weeds

Pesticide use in major crops herbicides major quantity

Pesticide use in major crops: herbicides major quantity

What is driving the system why are weeds so important

What is driving the system? Why are weeds so important?

  • Disturbance and Recovery via Succession

  • Succession is the process by which one community gives way to another

    • Here referring to plants

  • Primary vs. secondary succession

Two approaches to agriculture

Two approaches to agriculture

  • Don’t let succession proceed

    • Invest much energy in the form of human, animal or fossil fuel

    • Herbicides are used to prevent succession from occurring

  • Try to mimic successional stages of the natural ecosystem, the “analogue model”

    • plant a corresponding successional stage, filling that niche. If the niche is filled a “weed” can’t compete

Analogue approach

Analogue Approach

  • Commonly used in tropics where forest is climax

  • Disturbance can be introduced at any stage to bring back to beginning stage of succession

Why we go to all the effort to bring it back to early stages

Why we go to all the effort to bring it back to early stages!

Annual npp addition decreases while overall biomass increases

Annual NPP addition decreases while overall biomass increases



  • Defined as “practices that intentionally retain trees on land used for crop production or grazing”

  • Over space or time (rotations that use fallows with trees in it)

  • fallow:

  • Trees can be spatially arranged for different effects

Roles of trees

Roles of trees

Good foundation for developing the

“emergent properties”

of more complex ecosystems”

Other roles:

Gliessman p259

  • microsites for beneficials

  • reduction of wind

Optimizing the positive impacts

Optimizing the positive impacts

  • Potential negative effects:

  • shading, allelopathic influence

  • microclimate effects that favor disease

  • branches etc. can fall on harvestable portion

  • Can be mitigated by planting plan

Fallow= field not planted to a crop

What is a weed a plant out of place

What is a weed? “A plant out of place”

  • A weed is any plant, native or non-native, that interferes with crop production by doing more harm than good and encroaches where it is not wanted

  • A successful weed has closely adapted to the life-cycle of the crop and farming practices

    • Most of our crops and farming practices came via Europe with European weeds

  • Learn to work with succession instead of preventing it

When working against succession weed problems competition

When working against successionWeed problems: competition

  • Nutrients

  • Light

  • Water

  • Results in lower yields and poor crop quality

Interference with harvesting operations

Interference with harvesting operations

  • Weed roots such as bindweed wrap around sub-soil blades used for undercutting root crops during harvest

  • European, extensive problem, difficult to eradicate, root system to 20’, seeds viable for 50 years

  • Reproduction by seed or rhizomes

Field bindweed, (Convulvulus

arvensis), Morningglory Family

Build up of soil seedbank vs crop seed bank for storing crops

Build up of soil seedbank vs crop seed bank (for storing crops)

  • Weed seeds present in soil

  • Seedbank can increase rapidly in one season with fertilization and irrigation and ineffective control

  • One weed plant can produce thousands of seeds

  • Affected by management

Redroot pigweed amaranthus retroflexus amaranthaceae fam

Redroot Pigweed Amaranthus retroflexus Amaranthaceae Fam.

  • Common throughout the West

  • One plant can produce thousands of seed

  • Relative’s seed eaten by Andean populations (Grain amaranth, A. hypochondiacus)

  • North American indigenous crop (A. blitum)

Hairy galisoga galinsoga ciliata asteraceae fam

Hairy Galisoga, Galinsoga ciliataAsteraceae Fam.

  • One of the most difficult-to-control weeds in vegetable production on fertile soil, esp. in NE US

  • Continually produces seeds throughout growing season

  • Can produced 7500 seeds/plant

Allelopathic affect on crop plant

Allelopathic affect on crop plant

  • Black walnut and tomatoes

  • Lamb’s quarters roots secret oxalic acid

  • Velvetleaf, quackgrass, Canada thistle, giant foxtail, black mustard and yellow nutsedge

  • Mechanism: root secretion, decomposition of residues, effects microbial symbionts

Weeds can harbor diseases and pathogenic fungi esp crop relatives

Weeds can harbor diseases and pathogenic fungi, esp. crop relatives

  • Classic case of wheat rust disease on wheat with alternative host European barberry

  • 1970-1990 caused $100 million annually

  • Barberry eradication project saved farmers $30 million per year

Can be directly parasitic

Can be directly parasitic

  • Dodder (Cuscuta sp.) Convolulaceae (Morningglory) Family

  • Major problem in West US with alfalfa, clover, potatoes, safflower

  • First germinates root then when finds host becomes parasite

Striga is serious problem of corn and sorghum in africa

Striga is serious problem of corn and sorghum in Africa

  • Striga in corn, Witchweed, Scrophulariaceae Fam.

Benefits of weeds

Benefits of weeds

  • Enhance soil structure and water penetration

  • Improve soil tilth

  • Capture nutrients that would otherwise be lost

  • Provide habitat for beneficial insects

Indicate soil characteristics and suitability for crops

Indicate soil characteristics and suitability for crops

  • Creeping buttercup, Ranunculus repens, Ranunculaceae Fam.

  • Introduced from Europe as ornamental

  • Reproduces from seed or rhizomes

  • Toxic to cattle

  • Could indicate moist soil conditions

Weed ecology and management

From: Beeby, J. 1997.

May be edible good weed bad weed concept

May be edible: good weed/bad weed concept

  • Common purselane Portulaca oleracea Portulacaceae Fam.

  • Introduced from Europe

  • Seeds can remain dormant for years

  • Edible fresh or cooked, esp. used in Mexican cooking

Know your weed biology critical to control life habit classification

Know your weed biology: Critical to controlLife habit classification

  • Annual: A plant that completes its life cycle in one year

  • Summer annual

    • Purslane, galinsoga, pigweed, lambsquarters, pineappleweed

Winter annuals common chickweed black mustard annual bluegrass

Winter annuals: common chickweed, black mustard, annual bluegrass

Brassica nigraStellaria media



A plant that completes its life cycle in two years above ground

Examples are wild carrot (Daucus spp.), bull thistle (Circium vulgare) and poison hemlock (Conium maculatum)



Wild carrot is a host of leafhopper-vectored

aster yellows phytoplasma



  • A plant that lives a number of years, often producing seed once each year

  • Simple perrenials reproduce by seed

  • Examples are dandelion ((Taraxacum officinale), curly dock (Rumex crispus) and plantain (Plantago sp.)

Broadleaf plantain, (Plantago


Creeping perennials

Creeping perennials

  • Reproduce by seed and asexually through rhizomes, stolons (a horizontal above ground stem that roots at the nodes =runners) tubers and rootstalk

  • Examples are Johnson grass, Bermuda grass, Nutsedge (Cyperus sp.), Field bindweed

Management goal balance with crop not complete eradication

Management Goal: Balance with crop, not complete eradication

  • Retain diverse community

  • Economic threshold concept

    • Do not start to modify agricultural practices unless it reaches a point where it pays to deal with it in long and short term

  • Can include ecological concepts as well

    • If ecological benefit then may want to leave

Weed control systems approach vs input substitution

Weed control: Systems approach vs. Input substitution

  • Systems approach: Direct intervention is last resort!

  • First modify:

    • soil conditions

    • change crop rotation

    • cultivation and sowing practices

    • increase competitiveness of crop

    • introduction of animals as grazers

Farming practices affect weed community composition

Farming practices affect weed community composition

  • No-till increases percentage of perennial weeds (annuals shaded out by residue)

  • Decreased reliance on grass “leys” (2-3 yr rotation into grass (European) increases perennial weeds

    • grass shades and cutting reduces thistle

  • Organic production may have worse problems with perennials

  • Annuals increased by crop nutrition

Weed ecology and management

  • Prevention because of system effect

    • Crop rotation

    • Tillage

    • Fallow

    • Mulches

      • black plastic, organic, living

  • Direct effects

    • Pesticides

    • Flame and soil sterilization

    • Chemicals- vinegar

Systems approach vs input substitution

Systems approach vs input substitution

  • Systems approach modifies system so that weeds are not a problem

  • Prevention of weed problems

  • Alternate summer and winter crops (where possible)

  • Rotate crops- different crops favor different weed., pest and disease populations

Weed ecology and management

Crop Rotation effects on weeds

Warn season weeds

Cool season weeds

From: Harwood

Weed suppressive cover crops

Weed-suppressive cover crops

  • Sudan grass, buckwheat, sesbania (hot summer areas)

  • Perennial rygrass (Lolium perenne) (PNW)- dense growth and allelochemicals suppress weeds (at organic farm)

  • Optimize planting time to ensure uniform cover



  • Pre-plant tillage to allow weed seeds to sprout-- removing part of weed seed bank

    • may till several times depending on weed pressure

  • One of main weed control methods of organic farmers

  • Can also use moldboard plow to bury the weed seeds



  • For control of perennial weeds like

    • Johnson grass, crab grass and Bermuda grass

  • Tillage can actually increase infestation



  • Black plastic mulch in strawberry (also warms soil)

  • Organic mulches- straw, sawdust, tree leaves, secondary succession vegetation

  • Living mulches

    • intercrop with a

      cover crop

I direct intervention pesticide what is a pesticide

I. Direct intervention: pesticideWhat is a pesticide?

  • An umbrella term used to describe any chemical that controls or kills a pest

  • Classified by the type of pest they kill

    • insecticide

    • fungicide

    • bactericide

    • herbicide- kills plants

    • rodenticide- kills rodents

    • nematicide- kills nematodes

    • acaricide- kills mites

Weed ecology and management

  • Contact herbicides

    • Paraquat: persistent, high acute mam. Toxicity, slightly to high toxic aquatic

    • Triazine herbicides: Atrazine- persistent, low acute mam. toxicity, carcinogen,

      endo. disruptor, ground water

  • Systemic herbicides

    • glyphosate (OP)- not persistant, low acute mam. tox., slightly toxic aquatic, hormones

    • (2,4-D (auxin); 2,4,5-T) - not persistent, moderate acute mam. tox.,

      poss. carcinogen; sus. endo. disruptor,

      potential ground water contaminant

Not only is there stimulation of cancer but new story endocrine disruptors

Not only is there stimulation of cancer but new story: Endocrine disruptors

Theo Colborne, 1987. Our Stolen Future.

  • PhD UW, observations of birds of Great Lakes

  • Led to endocrine disruptor hypothesis

  • Data still emerging

  • National EPA still not released list of suspected chemicals, though Illinois EPA, Keith, Colborn and Benbrook list and Canada

Function of endocrine disruptors

Function of endocrine disruptors

  • They can act like a natural hormone and bind to a receptor. This causes a similar response by the cell, known as an agonist response.

  • They can bind to a receptor and prevent a normal response, known as an antagonistic response.

  • A substance can interfere with the way natural hormones and receptors are synthesized or controlled.

Source of agricultural endocrine disruptors

Source of Agricultural Endocrine Disruptors

  • Agricultural runoff /Atmospheric transport

    • Organochlorine Pesticides (found in insecticides, many now phased out)

      • DDT, dieldrin, lindane

    • Carbamate insecticides

      • **Aldicarb (PAN-pesticide database)

  • Agricultural runoff

    • Pesticides currently in use

      • **Atrazine, trifluralin, permethrin

Atrazine triazine herbicide

Atrazine (triazine herbicide)

  • #1 herbicide in corn in the MidWest

  • Now banned in EU because of human health and environmental concerns

  • Just re-approved in US

  • Discrediting of US scientists by Syngenta (e.g. Hayes), where Atrazine is 1/4 of total revenues

  • Proven endocrine disruptor-- in frogs changes testes to ovaries (Lumenstyk,2003 Chronicle of Higher Educ 50:A26)

Weed ecology and management

Atrazine detected

in groundwater

in Wisconsin

Other direct interventions used by organic farmers

Other “direct” interventionsUsed by organic farmers

  • Flaming

  • Solarization

  • Biological control

  • Vinegar (salt not recommended)



Impt to have

small seeds

Can use propane

gas tank on small scale

Solarization with clear or black plastic

Solarization with clear or black plastic

  • Useful to kill annual weed seeds

  • Can kill certain perennials, e.g. creeping buttercup in the PNW

  • Can be combined with

    cabbage or cole crop

    decomposition for


Biocontrol of weeds using insects or plant pathogens a pathogen causes a disease

Biocontrol of weeds using insects or plant pathogens (a pathogen causes a disease)

  • One of major natural occurrences- Only 25% of weed seeds do germinate- thought to be due to insects and pathogens in the soil

  • Used only in specific circumstances-

    ie not widely used,

    unlike diseases of insects

Carabid beetle



  • 5% (household white vinegar)-10% concentrations of organic vinegar had 85-100% kill rate

  • Canada thistle- one of most susceptible with 5% concentration with 100% kill rate

  • Spot spraying in corn cost $20-30 per acre

    • ARS researchers Radhakrishnan, Teasdale and Coffman

  • Not tested for effects on soil that cannot be replace by lime

  • Salt not recommended

The systems approach

The systems approach

  • How to avoid the use of direct intervention methods?

    • Organic farmers growing annuals use long (4-7 yr) rotations and a combination of other methods

  • How can we mimic the natural ecosystem to avoid the effects of weeds?

    • How do does letting succession take place avoid problems with weeds?



  • Weed photos identification and management: www.ipm.ucdavis.edu

  • Whitson et al. 2000. Weeds of the West (9th Edn).Western Society of Weed Science.

  • Taylor, R. 1990. Northwest Weeds. Mountain Press, Missoula

  • Uva, R., J. Neal, J. DiTomaso. 1997. Weeds of the Northeast. Comstock Press.

  • Beeby, John. 1997. Test Your Soil with Plants. Ecology Action Self-teaching Mini-Series Booklet #29



  • Would you consider the lack of weeds an “emergent property” of an agroecosystem? Explain.

  • Ch 17 Question #1, 2 (spend some time on this),

  • Upper division Ch 17 # 4, 6

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