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Crop and Pest Management Guidelines

A Cornell Cooperative Extension Publication
44743

4.2 Controlling Weeds before Planting

Eliminating perennial weeds before planting is the safest, least expensive, and most effective weed control method. Perennial weeds can be eliminated mechanically or chemically. Frequent mechanical cultivation (disking or rotary tilling) throughout a dry growing season will exhaust weed food reserves and result in weed death. This may be the best treatment in many situations but could be expensive in terms of fuel, labor, and soil erosion and may break down the soil structure.

Chemical treatments applied to actively growing weeds in moist soil before planting can be less expensive and will maintain soil structure while killing perennial weeds. The most effective treatments are those applied in late summer or early fall. (See Table 4.2.1 for herbicides to control specific weeds.)

Systemic chemical treatment is most effective when applied to actively growing or mature weeds. Spring treatment is ineffective because most perennial weeds do not start growth early enough to allow time for herbicide translocation to the roots or to allow time for the chemical to dissipate from the soil before planting.

When controlling weeds that can be propagated vegetatively, soil should not be cultivated before treatment with a systemic herbicide because disking or rotary tilling will break up the plants and their roots into smaller sections and distribute them deeper over a wider area. Each piece is capable of forming a new plant. The pieces do not grow at the same rate, however, and those deeper in the soil take longer to emerge. Thus, foliar treatment will affect only those that have emerged and are contacted. The newly emerging plants may not be killed and others will continue to sprout over time.

After allowing sufficient time for the systemic herbicide to completely penetrate the weeds, the field can be plowed. A cover crop of sudan-sorghum or oats, or rye if late fall, will deter annual weed regrowth, build up soil organic matter levels, prevent erosion, and improve soil structure. The field will be ready for planting the following spring.

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4.2.1 Preplant Treatment with Glyphosate

Excellent results have been obtained using glyphosate (Roundup, Roundup Pro) for preplant weed control. Treatments on well-developed weeds in late summer or early fall, made when the soil is moist and plants are actively growing, are most effective. Roundup is a nonselective systemic herbicide that is absorbed through foliage and translocated to all plant parts. The value of Roundup is that it kills plant parts below ground as well as above, thus enabling control of stems and rhizomes that are not directly contacted by the spray. Roundup requires 3 to 10 days to kill effectively, depending on how fast the herbicide moves within each weed species. As such, it is much slower than a treatment like paraquat (*FGramoxone), which kills only aboveground plant parts and is not translocated below the surface. Repeat treatments may be required for complete cleanup of certain weeds or heavily infested areas.

Excellent control was obtained with Roundup after one treatment on poison ivy and quackgrass. Repeat treatments may be required for complete kill on Canada thistle, horsenettle, nutsedge, milkweed, field bindweed, and mugwort (see Table 4.2.1).

For information about alternatives to glyphosate in the landscape, see the factsheet "Glyphosate Alternatives in Landscapes" (under Selected Publications) at: ccesuffolk.org/agriculture/weed-science.

Table 4.2.1. Preplant control of perennial weeds

Weed

Material

Time of Application

Rate (AI/A)

Canada thistle

glyphosate

Plant actively growing at or beyond flower bud stage

2-3 lb.

 

Remarks: Minimum of seven days before cultivation.

clopyralid

Plants 6-8 in. to bud stage

0.25 lb.

 

dichlobenil

Late fall or early spring

6 lb.

Field bindweed

glyphosate

Plant actively growing at or beyond flower bud stage

3-5 lb.

 

Remarks: Minimum of seven days before cultivation.

Goldenrod

glyphosate

Summer or early fall

2-4 lb.

 

Remarks: Spot treat as necessary. Cultivation is also effective.

Horsenettle

glyphosate

Plant actively growing at or beyond flower bud stage

3-4 lb.

 

Remarks: Minimum of seven days before cultivation.

Japanese knotweed

glyphosate

Early fall

2-4 lb.

 

Remarks: Minimum of seven days before cultivation.

Mugwort

glyphosate

Plant actively growing

2-3 lb.

 

Remarks: Minimum of seven days before cultivation.

dichlobenil

Late fall or early spring

6 lb.

 

Remarks: Do not use in warm weather or around fir, spruce, or hemlock.

Yellow nutsedge

glyphosate

Late July when all sprouts are up, before nutlets form

2-3 lb.

 

Remarks: Minimum of seven days before cultivation.

 

dichlobenil

Late fall or early spring

8-10 lb.

 

Remarks: See mugwort.

halosulfuron

3- to 8-leaf growth stage

0.031-0.062 lb.

 

Remarks: Do not mow two weeks before or after application.

Poison ivy

glyphosate

Mature foliage, late season (August or September)

2-3 lb.

 

Remarks: Minimum of seven days before cultivation.

Quackgrass

pronamide

Late fall prior to freeze

2 lb.

 

Remarks: See label for plant-back restrictions.

glyphosate

At least 8 in. high and actively growing

2-3 lb.

 

Remarks: Minimum of seven days before cultivation.

clethodim

4-8 in. high and actively growing

0.125-0.25 lb.

dichlobenil

Late fall or early spring

6-8 lb.

 

Remarks: Do not use in warm weather or around fir, spruce, or hemlock.



Table 4.3.1. Thirty common weeds

Common name

Scientific name

Growth habit

Propagation

Barnyardgrass

Echinochloa crus-galli

Annual

Seed

Bindweed, field

Convolvulus arvensis

Perennial

Seed, rhizome

Brome, downy

Bromus tectorum

Annual

Seed

Carpetweed

Mollugo verticillata

Annual

Seed

Chickweed, common

Stellaria media

Annual

Seed

Crabgrass, smooth

Digitaria ischaemum

Annual

Seed

Dandelion

Taraxacum officinale

Perennial

Seed

Deadnettle or Henbit

Lamium spp.

Winter Annual

Seed

Dock, curly

Rumex crispus

Perennial

Seed

Foxtails

Setaria spp.

Annual

Seed

Galinsoga

Galinsoga spp.

Annual

Seed

Goldenrod

Solidago spp.

Perennial

Seed, rhizome

Groundsel, common

Senecio vulgaris

Annual

Seed

Horsetail, field

Equisetum arvense

Perennial

Spores, rhizome

Lambsquarters, common

Chenopodium album

Annual

Seed

Milkweed, common

Asclepias syriaca

Perennial

Seed, rhizome

Mugwort

Artemisia vulgaris

Perennial

Rhizome, rarely seed

Mustard

Various genera and species

Annual

Seed

Nutsedge, yellow

Cyperus esculentus

Perennial

Seed, tubers

Pigweed

Amaranthus spp.

Annual

Seed

Plantain

Plantago spp.

Perennial

Seed

Poison ivy

Toxicodendron radicans

Woody

Seed

Purslane, common

Portulaca oleracea

Annual

Seed

Quackgrass

Elytrigia repens

Perennial

Seed, rhizome

Ragweed, common

Ambrosia artemisiifolia

Annual

Seed

Smartweed

Polygonum spp.

Annual

Seed

Sorrel, red

Rumex acetosella

Perennial

Seed, stolon

Thistle, Canada

Cirsium arvense

Perennial

Seed, rhizome

Witchgrass

Panicum capillare

Annual

Seed

Yarrow

Achillea millefolium

Perennial

Seed, rhizome