Crop and Pest Management Guidelines

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.

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.

4.1 Establishing a Weed Management Program

4.3 Weed Management Strategies in Growing Crops