Post-emergence Weed Weaponry

Crabgrass (Digitaria spp.) and goosegrass (Eleusine indica) are among the most common and troublesome turf weeds. These grasses germinate in the spring and summer, and are killed by frost in the autumn. As they mature, they produce seedheads that in turn, yield seeds providing the next year's crop of seedlings. You can control these weedy grasses with two types of herbicides: pre-emergents and post-emergents. The first type, pre-emergents, is applied before weed seeds germinate and prevent crabgrass, goosegrass and many other weed seedlings from emerging. The second type, post-emergents, removes emerged and actively growing plants. In most situations, it is easier and less expensive to manage these annual grassy weeds with pre-emergence herbicides.

In some years, such as 2003, pre-emergence herbicides fail to provide acceptable control. The major reason for last year's failures in many regions was due to excessive rainfall, which had multiple impacts on herbicide performance and weed seed emergence. For example, the wet weather resulted in a longer period of crabgrass and goosegrass emergence. Furthermore, the rain promoted the chemical and microbial degradation of pre-emergence herbicides and probably diluted or leached product from the upper surface of the turf where most weed seeds are found.

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Post-emergence control of annual grass weeds is an art and not a science. Success largely depends on selecting the proper rate, application interval, optimum timing and other nuances. Each of the aforementioned factors may vary from one herbicide to the next. In general, crabgrass and goosegrass are easiest to control with post-emergence herbicides when plants are small and in the 2- to 4-leaf stage of growth. This normally occurs between mid-June and mid-July in most areas. The most common post-emergence herbicides used for annual grass control in cool-season turf are the organic arsenicals, dithiopyr (early post-emergence only), fenoxaprop-ethyl and quinclorac. Only fenoxaprop-ethyl effectively controls goosegrass.

POST-EMERGENCE HERBICIDES FOR COOL-SEASON TURFGRASSES

For post-emergence control of weeds in cool-season turfgrasses, there are two compounds in the organic arsenical group: disodium methanearsonate (DSMA; Methar 30) and monosodium methanearsonate (MSMA; Daconate 6). These post-emergence herbicides are most effective when applied to young crabgrass plants. The organic arsenicals are generally not very effective in controlling goosegrass, but they are the only herbicides that effectively and safely control dallisgrass (a warm-season, perennial grass weed) in cool-season turf. You need to make at least two, and sometimes three, treatments with these herbicides on a 7- to 10-day interval. It is essential that you maintain the spray interval within the 7- to 10-day window or partial and inadequate control will result. Soil should be moist before you apply these herbicides. Desirable turfgrasses may be temporarily discolored after treatment, especially if they are under stress from heat or drought. Applying these herbicides in 100 or more gallons of water per acre, however, greatly reduces potential turf injury problems. Rates vary among formulations, but MSMA and DSMA generally are applied at 1.0 and 2.0 lb ai/A, respectively. As with all herbicides, you should be sure to follow container directions for the proper rate to apply, taking into consideration weed maturity, turf species tolerance and other important information. Do not irrigate for 24 hours after you apply the herbicides and do not apply if rain is forecast within 24 hours of the day you intend to apply an organic arsenical.

Dimension (dithiopyr) primarily is used as a pre-emergence herbicide, but it can be effective post-emergently for a short period after crabgrass emergence. That is, Dimension only provides early (i.e. 1- to 4-leaf stage) post-emergence control of crabgrass, and will provide pre-emergence control of crabgrass seedlings emerging thereafter. Tillered crabgrass will not be effectively controlled. Little information exists regarding its ability to post-emergently control other annual grasses. For post-emergence control, apply Dimension to 1- to 4-leaf-stage crabgrass at 0.25 to 0.38 lb ai/acre for granular formulations and at 0.5 lb ai/acre for the sprayable formulation. You should not use Dimension on creeping bentgrass as a post-emergence herbicide, but it is safe to apply post-emergently to most other turfgrass species. Dimension use after mid-June generally is not recommended in most regions because its soil residual may interfere with late summer overseeding operations and because it is ineffective after weeds have tillered.

Acclaim Extra (fenoxaprop-ethyl) controls crabgrass, goosegrass and some other annual grasses, such as foxtail. This herbicide may stunt or discolor Kentucky bluegrass if applied during cool and moist periods. Acclaim Extra also is safe to use on perennial ryegrass, tall fescue and fine-leaf fescues. Acclaim may yellow low-cut bentgrass turfs, such as those grown on fairways and tees. Do not apply Acclaim Extra to bermudagrass, and refer to the label regarding low rate usage on bentgrass. Acclaim Extra is relatively safe (may cause yellowing) to apply to tall fescue and perennial ryegrass seedlings, but it may severely injure seedling Kentucky bluegrass and other turfgrass species less than one year old. Apply Acclaim Extra when crabgrass or goosegrass plants are just above the turf canopy (usually early July), but prior to the time when weeds have developed three or more tillers. There must be good soil moisture and actively growing weeds for Acclaim Extra to be effective in a single application. Due to antagonism within the weed, you should not tank-mix Acclaim Extra with a broadleaf weed herbicide or apply it within 14 days following a broadleaf weed herbicide application. You may, however, apply a broadleaf weed herbicide 24 hours after applying Acclaim Extra without diminishing the ability of the herbicide to control annual grass weeds. Make sure not to mow treated turf within 24 hours of application to allow time for translocation of the herbicide to stems. Acclaim Extra is more effective when applied through flat-fan nozzles, though effectiveness may be reduced by nozzles (i.e., flood jet nozzles) that produce large droplet sizes. The recommended rate of Acclaim Extra will vary (0.09 to 0.18 lb ai/acre) with weed size, vigor and other factors noted on the label. Multiple applications are generally needed for goosegrass control because this weed germinates throughout the summer and Acclaim Extra has no pre-emergence activity.

Drive (quinclorac) effectively controls crabgrass and barnyardgrass, but not goosegrass. You must tank-mix Drive with 1% v/v with either methylated seed oil or crop oil concentrate to be effective. Drive most effectively controls young crabgrass plants that have not tillered. It is best to begin applications about mid-June to early-July, before crabgrass plants have tillered. Generally, two Drive applications (0.50 + 0.50 lb ai/acre applied on a two week interval) are required to control crabgrass in situations where crabgrass levels are severe (i.e., dense populations). Where crabgrass populations are low, a single application of 0.75 lb ai/acre between mid-June and early-July can be very effective. Drive will “burn-down” multi-tillered crabgrass plants later in the season, but many plants are likely to recover from a single application. It is safe to apply to Kentucky bluegrass and perennial ryegrass. Drive may cause slight yellowing in tall fescue, but it can be phytotoxic to fine-leaf fescues (creeping red, Chewings, hard, sheep) or cause a severe yellowing or mottling in creeping bentgrass maintained on fairways. The discoloration caused by Drive applied to fairway creeping bentgrass can persist from two to 10 weeks or longer. The discoloration can be partially masked by tank-mixing Drive with liquid iron plus nitrogen products like Lesco's 12-0-0 with iron plus micronutrients, Ferremec and other similar products. It also effectively controls clovers, violets and some speedwells. However, it is weak or ineffective against foxtails, dandelion, plantains, oxalis and other broadleaf weeds.

FINAL CONSIDERATIONS

A weed control program must be accompanied by a good turf management program. When weeds are removed, you should sod or re-seed large bare areas in the autumn as these open spaces will be invaded by new weeds. Proper fertilization, irrigation and mowing greatly reduce or delay significant annual grass weed problems from developing in turf. Herbicides can be used safely if you handle them with care. Make sure to use only the recommended rates, and follow directions and precautions stated on the label.

Peter Dernoeden, Ph.D., is professor of turfgrass science at the University of Maryland (College Park, Md.).

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