Organic Weed Management

Jim Shrefler, Extension Horticulturistand

Merritt Taylor, Professor of Agricultural Economics

Oklahoma Cooperative Extension Service

Watermelon Trial - June 27

Watermelon Trial - July 10

Organics and Weed Management

Is there a quick guide to organic weed control?

Is it good to have weeds on your farm?

Do organic growers just need to learn to accept weeds?

Is learning to control weeds an ongoing process?

Will there ever be an “organic” Roundup?

Organics and Weed Management

Organic farming systems use vegetation management to:– Improve soil– Prevent soil loss and degradation– Enable production of field crops, forage

and produce

Organics and Weed Management

Organic farming systems use vegetation management to:– Improve soil– Prevent soil loss and degradation– Enable production of field crops, forage

and produce

Organics and Weed Management

Vegetation that is managed may include:– Intentionally planted annual and

perennial cover crops– Annual and perennial cash crops

Forages; Small Fruits

– Naturally occurring plant speciesMay include many species

Some may be considered “weeds”

Annuals and Perennials

Perennial Weeds

May or may not have abundant seed production

May survive the winter via roots or rhizomes

May or may not be woody plants

Include grasses, broadleaf and sedges

Annual Weeds

Generally produce abundant seeds

May be cool or warm season plants

Are adapted to disturbed environments

Have varied life cycle durations

Seed may persist for years

Organic Weed Management Practices

Various farm practices may contribute to weed management– Carefully timed planting of cover

crops– Prevention of the introduction of

undesirable plant species or cultivars

– Carefully timed planting of cash crops

– Prevention and removal of undesirable weeds in cash crops

Organic Vegetables in USA South Central Plains

Weeds pose a big challenge to all growersWith organics, normally manageable issues major concerns– e.g. annual grasses

Weed Control Methods for Organic Vegetables

PreventionCultural / Management Soil preparationExclusionMechanicalPruning / mowing / flamingChemicalAllelopathy

Prevention

Avoid Introducing new weeds– Organic mulches– Manures– Moving contaminated

equipment– Irrigation water– Moving soil

Prevention

Watch for new weeds that appear– Do not allow them to go to

seed– Seed may persist for many

years– If perennial, watch for

regrowth from vegetative propagules

Cultural / Management

Allelopathy

Organisms that release “chemicals” that inhibit growth of other organisms

May be while the plant is alive or during its decomposition

Certain varieties of cereal rye are an example

Application methods

Crop safety

Efficacy

Preemergence Weed ControlCorn Gluten Meal

Onions

Summer Squash

Soil Preparation

Use sound practices that do not harm soil

Eliminating all “green” can be a good starting point

Exclusion

Barriers that prevent weeds from establishing

Plastic mulch

Landscape fabrics

Carpet scraps

Organic mulches

Mechanical

Cultivation, hoeing, hand removal

Labor intensive

Proper equipment makes a big difference!

Cultivation for Weed Control in Organic Vegetables

For some crops, such as cucurbits, cultivation has limitations

Vining growth habitShallow roots

Wet soil conditions may preclude effective cultivation

Pruning / mowing / flaming

Anything the gets above ground parts of weeds– Grazing– Flaming– Mowing– String trimmers

We will see some examples later

ChemicalSynthetic chemicals are not allowed in organics– Glyphosate, 2,4-D, “Poast”

Natural weed control “chemicals” are being developed– Vinegar– Plant extracts, oils– These are “burn down” products– A natural “Roundup” is not yet available

Postemergence Herbicides for Organics

Integrated Management

Opportunities

Rye – a common winter cover crop

Rye has been shown to suppress germination / establishment of weeds

Various practices could be used to establish vegetables following a rye cover crop

Objectives

Compare field preparation / planting system practices for:– Impact on squash productivity– Weed incidence and removal costs

Compare treatments for production cost / economic benefit relationships

Materials and Methods2004

Soil – Bernow fine sandy loam

Field history – 2 year fallow– Abundant crabgrass and tumble pigweed

Soil test and apply nutrients with poultry litter in autumn

Beds – 6 foot centers

Sow ‘Elbon’ Rye

Materials and Methods2005

Divide field 3 planting dates– May, June and July

Mow cover crop, prepare plots and plant

Transplant Zucchini squash ‘Revenue’ – Spacings: Rows 1.8 m and plants 0.77 m

Drip irrigation

Weeds – count and hand hoe

Harvest squash over about 3 weeks

Treatment AdvanceProcedures

At Plant Procedures

Conventional Mow, Rework Rototill

Black plastic M+R; Plastic Nothing

Stale seedbed M+R Burn

Mow M M

Mow & Burn M M+Burn

Shallow till M Till (2-5 cm)

St & Burn M Till+Burn

Treatments

Response Variables

Squash Yields:– Impact on squash productivity

Number of marketable fruit

Weed incidence– Annual weeds

Weed removal requirements– Hoeing time

Squash Yields*First Planting Date

0

20

40

60

80

conven plastic stale seedbed mow mow & burn shallow shallow &burn

Fru

it pe

r p

lot

* Harvest period of 3 weeks

LSD (0.05)=15.4

Squash Yields*Second Planting Date

0

40

80

120

160

conven plastic stale seedbed mow mow & burn shallow shallow &burn

Fru

it pe

r p

lot

* Harvest period of 3 weeks

LSD (0.05)=23.6

Summary – Squash Yields

Plastic mulch best with May planting

Any tillage, especially at-planting tillage, was better than no tillage with June and July plantings– * conventional and shallow tillage were

comparable

CrabgrassFirst Planting Date

0

400

800

1200

1600

2000

conven plastic staleseedbed

mow mow & burn shallow shallow &burn

3 weeks 5 weeks

Pla

nts

/ m

2

CrabgrassSecond Planting Date

0

400

800

1200

conven plastic staleseedbed

mow mow & burn shallow shallow &burn

2 weeks 4 weeks

Pla

nts

/ m

2

CrabgrassThird Planting Date

0

100

200

300

400

conven plastic staleseedbed

mow mow & burn shallow shallow &burn

1 week 5 weeks

Pla

nts

/ m

2

Summary – Crabgrass

Crabgrass response to treatments varied with planting dates

For PD1 and PD2 – greatest early emergence with conventional and/or stale seedbed

PD3 – appreciable emergence in all except plastic and non tilled plots

Conclusions

The optimum system may depend on planting date– Plastic always produced greater or comparable

yields– Weed patterns will probably differ across dates

Shallow and conventional tillage yields were comparable– Are there benefits to shallow tillage?

Interesting results in 2005

Plastic mulch always best with early planting

Weed control results varied with dry (2006) and wet (2007) conditions

Predictability as a weed control tactic is questionable

Results Over 3 Years

In Summary

Learn your weedsStrive to prevent new weed infestationsLearn and apply good management practicesWhen all else fails – Keep on Hoeing!