Crop and Pest Management Guidelines

4.3 Weed Management Strategies in Growing Crops

Growers must understand the growth characteristics of their crops and potential weed problems to develop an effective and safe weed control program. Some weeds of importance in nursery crops are listed in Table 4.3.1, along with their method of propagation and life cycles. The grower must also know the nature of the herbicides available.

4.3.1 Postplant Perennial Weed Control

Any herbicide application to blocks of established stock entails risk of crop plant injury. Therefore, growers should eliminate perennial weeds before planting and avoid planting weed-infested stock in clean blocks. If difficult-to-control perennial weeds are present, they must be eliminated, even at the risk of losing some valuable plants, to prevent serious weed problems in the future. Spot treatments with directed sprays (i.e., a spray directed at the weeds while desirable plants are shielded) or granular materials should be used. Plant foliage should be dry when applying granular herbicides and contact avoided entirely when using postemergence herbicide sprays. Roundup has given excellent results, but susceptibility to injury has not been determined for all plants. Do not treat nursery stock with 2,4-D or similar products; they cause distortion of young plant growth, tip necrosis, and sometimes death. Oxyfluorfen (Goal 2XL) as a spray at 0.25 to 0.5 lb. AI/A applied over conifers in beds or containers including juniper, yew, pine, and hemlock will kill seedling groundsel, bittercress, oxalis, and other weeds and give some preemergence protection. Do not spray on azaleas, rhododendrons, or other broadleaf plants.

Goal 2XL is labeled for use on conifers. Dichlobenil (Casoron 4G), applied as a granular in late fall or very early spring, will be absorbed by the plant roots and will kill many undesirable weeds.

Casoron (4G, 150 lb./A) will suppress or control quackgrass, mugwort, and several other rhizome-forming perennial weeds in many deciduous crops but should not be used on hemlock, spruce, or fir. Treatment should be applied in late fall or very early spring when precipitation is expected. Moisture activates the herbicide and prevents loss by volatilization. Casoron may also suppress nutsedge in established nursery stock. A late fall or winter application will have lost its effectiveness by late June.

4.3.2 Postplant Annual Weed Control

Once an area is free of growing weeds, every effort should be made to keep it that way. For many crops, cultivation may be the best method to accomplish this. If properly applied, preemergence herbicides will give excellent results at a fraction of the cost of cultivation.

A preemergence herbicide, granular or spray, applied on a weed-free, smooth, moist soil surface is the most effective treatment. The chemical will be concentrated in a shallow layer and thus give the best control. Moisture activates the chemical and hastens germination of weeds, making them more susceptible to the herbicide at its highest concentration.

Applications must be made before weeds get started because most preemergence herbicides will not affect growing weeds. All the preemergence herbicide sprays listed are safe when applied to dormant plants. Emulsifiable concentrate (EC) formulations may injure young foliage. Check the label for restrictions.

Plants should be cultivated and irrigated before treatment with preemergence herbicides. Cultivation will seal cracks in the soil surface so the herbicide cannot penetrate into the root zone and be picked up by the plant. Irrigation will smooth the surface, seal cracks, and assure that the chemical will be confined in the smallest possible area. However, granular pesticides should never be applied if the foliage is wet because granules will be caught on wet leaves and injury may occur.

Experiments on Long Island have shown safe, successful treatments from day of planting to 14 days later, if the planting was on newly prepared soil in the spring. With warmer temperatures later in the season, the effective interval may be zero to seven days. (See label for precautions.)

If you have not previously used herbicides or are trying a new material, use them on a trial area to gain familiarity and confidence before treating the entire nursery. Use recommended rates. Calibrate your spreader or sprayer. For details, ask your county Cooperative Extension educator. Measure the amount of herbicide required to treat any given area as a double check on equipment calibration and to assure that the proper amount of herbicide is applied.

4.3.3 Herbicide Combinations

Combinations of herbicides are used to broaden their spectrum of control without using excessive amounts of any one herbicide. Herbicides developed for both grass and broadleaf weed control include OH2 (a combination of Goal and Pendulum) and others. These have been used successfully by growers. (See label for directions.)

4.3.4 Seedling Weeds in Established Plantings

A directed spray of *^FGramoxone, Diquat SPC or Roundup and a preemergent herbicide will give knockdown and residual control of many weeds. Do not spray *Gramoxone on foliage of desirable plants or on green-barked trees. Roundup may be applied as a directed spray in some circumstances. See label for details. Roundup is safe for most shade trees when applied as a basal spray unless foliage low on the trunk is contacted. Exceptions include young pear trees and birch, which can be injured when Roundup comes in contact with tree bark.

4.3.5 Seedling Weeds in Containers

The three most troublesome container weeds are groundsel, oxalis (woodsorrel), and bittercress. Many of the commercial preemergence granular products will give satisfactory control of these weeds if the growing medium is free of germinated weeds at the time of application. Removing the whole weed prior to preemergence herbicide use is important because regrowth from rhizomes, stolons, and crowns is a common cause of herbicide ineffectiveness.

4.3.6 Weeds Around Container Plants

Postemergence herbicides (Roundup Pro, Scythe) are labeled for use around container production areas. Chemicals should not be sprayed on foliage of ornamental plants; use extreme caution. Research shows Roundup is most damaging to the foliage and bark of deciduous plants but somewhat less injurious to evergreen species. Symptoms include chlorosis, defoliation, twig dieback, malformed growth, and death.

4.3.7 Controlling Weeds in Adjacent Uncropped Areas

The effectiveness of a weed control program can be reduced if fringe areas around the nursery are allowed to grow wild with weeds. Their seeds quickly invade the crop and reduce the quality of preventive efforts. Use a postemergence herbicideor mowing to keep weed pressure from blown-in seeds to a minimum. These materials, combined with a preemergence herbicide, will provide total kill and residual control.

Be cautious of herbicides used on sloping areas adjacent to nursery stock and of residual herbicides sold for other uses, such as highway or railroad maintenance. These can move down slopes to contaminate nursery soils, seriously injure nursery stock, and sterilize the soil for long periods.

4.3.8 After Treatments are Applied

If a few weeds start to appear, hand weed or hoe as needed. Shallow cultivation (no deeper than 1 inch) is less likely to cause root injury and dilute the herbicide than is deeper cultivation. If the soil is cultivated too deeply, the protective herbicide layer is broken and new seeds are exposed. Spot treat for perennial weeds before cultivating.

Preemergence treatments in most nurseries can be expected to last for 8 to 12 weeks in the field but only 6 to 8 weeks in containers. The residual effect depends on the chemical, soil temperature, moisture, and amount and source of organic matter. As the effectiveness of the herbicide decreases, weed or cultivate, irrigate, and re-treat as needed with any of the labeled preemergence materials discussed earlier.

4.2 Controlling Weeds before Planting

4.4 Herbicides and Cover Crop Establishment