Probably the most common weed in turf is crabgrass. Actually, the term crabgrass encompasses several species, including smooth crabgrass (Digitaria ischaemum), large crabgrass (D. sanguinalis) and southern crabgrass (D. ciliaris). Fortunately, you can use the same management program for all three plants, so separating out the three species is not critical for control. Smooth crabgrass lacks hairs on the sheath. Large and southern crabgrass have hairy sheaths and are separated based on floral parts. All three species have membranous ligules, are rolled in the bud and can root at the nodes. Goosegrass (Eleusine indica), which is sometimes called hard crabgrass or silver crabgrass, is in a different plant genus than crabgrass and responds differently to certain herbicides. Goosegrass is folded in the bud, has a whitish base and does not root at the nodes.
Crabgrass is a summer annual that begins to germinate in early spring, and germination continues through summer. Because of this long germination period, especially in southern states, obtaining full-season control can be difficult. Crabgrass flowers in summer and early fall and dies with the first hard frost. The cycle repeats itself the following spring with new germination.
PRE VS. POST
Grounds care professionals have a variety of tools to control crabgrass. Generally, people choose pre-emergence herbicides coupled with cultural practices to favor the competitiveness of the desired turf. However, there are some disadvantages to pre-emergence herbicide use.
You must apply pre-emergence herbicides prior to crabgrass germination. The onset of crabgrass germination varies from year to year based on weather conditions, so you could be too early with an application if you use a calendar-year approach, the blooming of forsythia or some other method for predicting germination. As pre-emergence herbicides break down in soil, late-season crabgrass germination can occur, especially in wet years. Since most of the pre-emergence herbicides lack post-emergence effects, late application can result in poor crabgrass control.
Because pre-emergence herbicides are applied prior to weed germination, you must treat the entire site, even though some parts of a lawn may not require treatment.
Pre-emergence herbicides require activation through rain or irrigation. If these chemicals sit on the soil surface for several weeks without rain, it can result in reduced control.
- Waiting to seed
After treating a site with a pre-emergence herbicide, you may have to wait for months before you can seed it.
As an alternative, there are several post-emergence options for crabgrass control. One of the benefits of a post-emergence herbicide is that you only need to apply it if a crabgrass stand develops, so there is potential for spot treatment if only parts of a lawn are infested. Therefore, post-emergence herbicides fit Integrated Pest Management (IPM) programs better. You can obtain full-season crabgrass control if the optimum timing is achieved and a pre-emergence herbicide is added for residual control. Disadvantages of post-emergence crabgrass control are the greater risk of turf injury posed by certain of the available products, short soil residual for most choices and reduced control at later growth stages of crabgrass.
TREATING IT RIGHT
Do not mow sites to be treated with a post-emergence herbicide for several days before and after application. This allows for sufficient weed foliage to absorb the chemical and sufficient time for translocation in the crabgrass plants following leaf uptake. A rain-free period of one day after application allows for optimum leaf uptake in crabgrass. Only treat actively-growing crabgrass that is not under drought stress. If crabgrass is stressed by low soil moisture, it will not sufficiently absorb and translocate these herbicides, resulting in poor control.
Crabgrass propagates by seed, first sending up a cotyledon and then forming true leaves. After a crabgrass plant has formed three or four leaves, it will start to produce side shoots, called tillers. Crabgrass is easier to control when plants are in the pre-tillering stage. As a crabgrass plant grows larger, the more tillers it gets and the harder it will be to control.
It may be difficult obtaining suitable coverage when crabgrass is in the cotyledon to one-leaf stage. Coverage is especially important for chemicals primarily absorbed by foliage. Also, you should wait until the majority of the crabgrass has germinated, so a good time to treat is when crabgrass is in the two- to four-leaf stage, when little-to-no tillering has occurred. Scouting is important to identify this growth stage.
Dimension (dithiopyr) provides pre-emergence and early post-emergence crabgrass control. Dimension may stunt growth of crabgrass when applied after tillering but generally will not provide acceptable control. Therefore, you should apply this compound prior to tillering. An advantage of Dimension is that can provide three or more months of residual crabgrass control. Crabgrass is one of the few weeds Dimension will control post-emergence. You could control crabgrass for most of the growing season with one Dimension application appropriately timed after emergence.
Dimension is a root inhibitor, and works by affecting cell division in grass roots. You should only apply it to established turf, because established turf has a well-developed root system. You can apply this herbicide to a wide range of cool and warm-season turfgrass species. Check the label for sensitive cultivars, especially bentgrass and fine fescue. Many ornamentals tolerate Dimension, and you can use this compound for weed control in ornamental beds as a directed spray.
Drive (quinclorac) is a unique herbicide. When looking at its weed control spectrum and the injury symptoms it causes, it appears to be a hybrid between a grass herbicide and a broadleaf herbicide. It controls crabgrass both post-emergence and pre-emergence, with a month or more residual control after treatment. Drive will not control all broadleaves, so combinations with 2,4-D and other broadleaf herbicides will increase the number of broadleaf weeds controlled. Drive can be less effective when applied during crabgrass tillering, so pretillering applications would be the preferred time to apply the chemical. Adding a methylated seed oil as an adjuvant improves leaf uptake and, thus, weed control.
Drive inhibits cellulose biosynthesis in plants, and may also have other sites of action. Drive can injure susceptible broadleaf plants through leaf or root uptake. Do not use lawn clippings to mulch vegetables or other broadleaf crops because Drive can damage crops like tomatoes. It can cause injury in certain broadleaf ornamentals, so avoid spray drift when treating lawns near flower and shrub beds. Depending upon the turf species, there is flexibility in regards to seeding of turfgrass after a Drive application, and applications can be made to relatively young turf. Drive cannot be applied to certain turfgrass species, such as St. Augustine and centipedegrass.
The organic arsenicals MSMA and DSMA (various trade names) also control emerged crabgrass in established turfgrass. Generally, multiple applications are required, especially for well-tillered crabgrass. The organic arsenicals can cause yellowing or burning of turf, such as in tall fescue. This injury is temporary and turfgrass will outgrow the damage in a week or two. An advantage to using organic arsenicals is control of sedge species like yellow nutsedge. The organic arsenicals are also sold commercially in combination with broadleaf herbicides like 2,4-D. This is one-stop shopping, if you will, because these mixtures will suppress or control a wide range of broadleaf, grass and sedge weeds. A disadvantage to these combinations is that a given site may only have broadleaf weeds, so the MSMA or DSMA is not required, while other sites may have only crabgrass and, thus, not require the 2,4-D component. MSMA and DMSA do not provide residual crabgrass control, so you must either repeat applications for later-germinating plants or apply a pre-emergence herbicide. Follow label directions in regard to addition of a nonionic surfactant.
Acclaim Extra (fenoxaprop) is a post-emergence herbicide with little-to-no soil residual effects. It is primarily used in cool-season turfgrass, and can be used to suppress bermudagrass that has invaded turfgrasses like Kentucky bluegrass and tall fescue. Acclaim Extra will not control any broadleaf weed or sedge species. Post-emergence broadleaf herbicides like 2,4-D, dicamba or triclopyr can antagonize crabgrass control with Acclaim Extra if you do tank mixes. You can avoid this by applying Acclaim Extra first, then waiting a week or two before applying the broadleaf herbicide. Acclaim will control pre-tillered and tillered crabgrass, but higher application rates are required for larger crabgrass plants. You can combine Acclaim Extra with pre-emergence crabgrass herbicides to obtain residual control.
Acclaim Extra inhibits fatty acid synthesis in sensitive weed species. The old formulation of fenoxaprop, Acclaim 1EC, contained two forms of the herbicide: one that controls weeds and another form that is essentially inactive. The manufacturer was able to synthesize only the active formulation of the herbicide, resulting in lower rates of application for the active ingredient. Acclaim Extra only contains the active form of fenoxaprop. Acclaim Extra can cause temporary yellowing in some grasses like Kentucky bluegrass, but the turf will quickly outgrow any adverse effects. Broadleaf ornamentals are tolerant of Acclaim Extra, so you could also use this chemical for crabgrass control in landscape beds.
MIX IT UP
Why did I mention the mechanism of action for these herbicides? One reason is the occurrence of weed resistance. Some parts of the country have crabgrass biotypes that are resistant to certain post-emergence crabgrass herbicides. To avoid development of herbicide resistance, rotate herbicides with different modes of action. Do not rely on the same herbicide year after year. It would be a good idea to rotate herbicides every three or four years. Because Dimension, Drive, the organic arsenicals and Acclaim Extra all have a different mode of action, any rotation among these chemicals will reduce the potential for crabgrass resistance.
Lawn care companies have a range of options for crabgrass control, including strictly pre-emergence herbicides, strictly post-emergence herbicides or a program utilizing both options. You should base your choice of a post-emergence crabgrass herbicide on the turfgrass species you will treat, the crabgrass size and the presence of other weed species. Combinations with pre-emergence herbicides will control both emerged crabgrass and provide residual control.
Jeffrey Derr is a professor of weed science at Virginia Polytechnic Institute and State University (Blacksburg, Va.).
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