Crop and Pest Management Guidelines

4.1 Establishing a Weed Management Program

4.1.1 The Need for a Weed Management Program

Weeds compete with crop plants for water, fertilizer, light, carbon dioxide, and other resources essential for plant growth. Weeds also harbor insect pests; reduce air flow around desirable plants, resulting in a microclimate more conducive to disease; and in some instances, serve as alternate hosts to pathogenic organisms. In addition, the aesthetic quality of the landscape (and perceived quality of nursery stock) is reduced by weed growth. Consequently, weed management is an essential part of any nursery crop production or landscape management system.

4.1.2 Weed Identification

The first step in developing any pest management plan is to identify the pest, and a weed management program is no exception. The importance of proper identification cannot be overemphasized. Correct identification not only means knowing the proper name but also provides information about the weed, particularly its life cycle. Weeds that infest ornamental plantings have one of four life cycles. Becoming familiar with the life cycle allows us to determine at what time of year the most susceptible growth stage is occurring. Summer annuals emerge in the spring, flower, and set seed before the first frost in the fall. Winter annuals germinate at the end of summer and overwinter as small, dormant seedlings. Biennials are similar to winter annuals but germinate earlier in the summer. As days lengthen and temperatures rise in the spring, both winter annuals and biennials are stimulated to flower, set seed, and die before the end of the summer. Perennials, as the name suggests, survive more than two seasons and generally can propagate by seed or by vegetative means. Weeds can also be classified into broad categories based on their growth types-grasses, sedges, and broadleaves-that are often useful in determining herbicide selectivity. Several weed and wild flower identification guides are available to assist in this identification effort. For a list of such resources appropriate to your region or for help in identifying unknown species, contact your local Cornell Cooperative Extension office (cce.cornell.edu/localoffices).

4.1.3 Weed Scouting

Scouting fields for weeds should begin the year before planting, paying particular attention to species that will be difficult or impossible to control after planting. These species must be controlled before planting. Integrating crop planting maps with weed maps has provided optimal weed control and reduced crop injury from inappropriate herbicide use, excessive cultivation, or weed competition for crop and weed management groupings.

After planting, fields should be scouted at least twice a year: early summer and early autumn. In early summer, any summer annual weeds that escaped control are still small but identifiable and may be controlled with cultivation or selective postemergent herbicides. Also at this time, many winter annuals and biennials are flowering so there still may be time to control them before seeds ripen. Perennial weeds may be identified and mapped early in the season to allow optimal timing of control procedures. Some perennials, such as quackgrass, are best controlled early in the season, whereas others are best controlled at other times. (See Table 4.2.1 for specific guidelines for perennial weed control.) In early autumn, winter annual seedlings, perennial weeds, and summer annuals that escaped control procedures are identifiable. Winter annuals will be easier to control postemergently at this time, before they have overwintered. The results of the autumn scouting are also useful in evaluating the effectiveness of your overall weed management program.

The actual scouting process can be accomplished in a fairly simple manner. The first and most important aspect is to map the areas, noting the species and locations of weeds as well as the species of ornamentals present. Many nurseries have already developed planting maps that may be adapted to this purpose. Using this map, conduct a weed inventory of each growing area or block. Walking fields in a wide zigzag pattern is an efficient way to do this. Note the general weed population and record relative densities. Take particular note of heavy infestations of a single species, perennial weeds, species you do not know (could this be a serious weed in the future?), and weeds that may be new to the area. As this inventory of information builds, notice which species are not controlled by your current management program, for these species will become more numerous unless you alter your management program to compensate.

4.1.4 Weed Management Options

Plan your weed management strategy based on the scouting report. Perennial weeds and other difficult-to-control species should be controlled before planting. Also before planting, consider the postplanting weed management strategies to be employed. Doing this before planting may help avoid costly weed problems later on. After planting, weeds may be controlled with cultivation, mulches, cover cropping (or living mulches), or herbicides. A combination of these control strategies, coupled with cultural programs that minimize weed infestations and introductions, is generally the most practical and effective option. When selecting the most appropriate options, consider the economics, crop safety, efficacy, environmental stewardship, and "fit" within your overall crop management program.

4.1.5 A Few Terms Used in Weed Control

• An annual plant germinates and completes its life cycle by producing a new crop of seed during a growing season. A winter annual germinates in the fall and completes its growth by the next summer, e.g., Virginia pepperweed.
• A biennial plant, such as bull thistle or wild carrot, completes its life cycle in two growing seasons. It often has a fleshy taproot for storing plant food reserves. A systemic herbicide is needed for control.
• A perennial plant grows for two or more seasons; for example, quackgrass, mugwort, and Canada thistle. These are the most difficult to control because they propagate by seeds, rhizomes, bulbs, and other vegetative means, and they often have large food reserves.
• A preemergence application of an herbicide is made before plants emerge through the soil surface. The term can apply to either the crop plant or the weed plant, but here it refers to weed emergence.
• A postemergence application of an herbicide is made after seeds or bulbs germinate and the shoots have grown through the soil surface. The term 'emergence' is used here to indicate the weed status.
• Active ingredient (AI) is the amount of pure herbicide material in a compound or formulation. The expression AI/A refers to the amount of this active ingredient to be applied per acre. A recommendation of 2 lb. AI/A simazine means that a granular or liquid formulation of simazine should be applied at a rate equal to 2 pounds of actual simazine per acre. For example, Princep Caliber 90 is a commercial formulation containing 0.9 lb. of active ingredient simazine per pound of product. Therefore, to apply 2 lb. of simazine AI/A, it is necessary to apply 2.2 lb. Princep Caliber 90 per acre.

4.1.6 Herbicide Formulations and Abbreviations

Dry formulations to be applied in water solution (AI expressed as percentage by weight):

• DF: dry flowable; similar to the DG formulations.
• DG, WDG, or WG: water-dispersible granule; similar to wettable powders but compressed into small granules to reduce dust.
• W or WP: wettable powder.
• WSB or WSP: water soluble bag; water soluble packet; water soluble powder.

Liquid formulations to be applied in water solution (AI expressed as lb. per gallon):

• ACS: microencapsulated in aqueous suspension.
• CS: concentrate suspension.
• E, EC, EW: emulsifiable concentrate.
• FL or AS: flowable or aqueous suspension; these formulations require constant agitation.
• L: liquid.
• SC: soluble concentrate.
• W or WS: water soluble.

Formulations to be applied dry (AI expressed as percentage by weight):

• G or GR: granular.

3 Disease Management for Trees and Shrubs

4.2 Controlling Weeds before Planting