Ornamental Plants
Annual Reports and Research Reviews
2001

Special Circular 186-02

Summary of Recommended Turfgrass Cultural Practices, Weed Control, and Disease Problems: 2001

Turf Weeds

Good cultural practices are the best defense against weed encroachment. Cultural practices, such as core aeration, proper fertilization, proper mowing practices, and mower maintenance, will produce a thicker stand of turfgrass and favor desirable turfgrass in its competition with weeds. However, where turfgrass is struggling, weeds may occur and require the use of herbicides to reduce and/or prevent heavy infestations.

Crabgrass (Digitaria spp.)

To prevent crabgrass and other annual grass weeds from germinating, apply a preemergent herbicide. Recommended dates for preemergent herbicide application are March 1 to April 1 for southern Ohio, March 15 to April 15 for central Ohio, and April 1 to April 25 for northern Ohio. Once soil temperatures at 2" below the surface reach 52F to 54F for several consecutive nights, crabgrass seed germinates. Since crabgrass usually begins to germinate in March or April, with the southern counties experiencing the earliest emergence, the crabgrass preventer should be applied before these dates, and may be applied as early as February.

The preemergent herbicides are primarily degraded by microbial activity. Frozen or cold soils have little microbial activity, so the product stays unaffected until the temperatures warm, and the microbes begin to digest the herbicide. Agronomists agree that it is a safe and practical approach to apply the preemergent earlier in the spring, rather than to wait and miss the target date of crabgrass germination. Irrigate the area within 48 hours after treatment to release the herbicide from the carrier. If some areas have already been seeded and a preemergent is still desired, use a product containing siduron (e.g., Tupersan.)

In early July of 2001, crabgrass grew luxuriously in the warm, wet weather and reached the three- to six-tiller stage. At that point, turf managers needed to consider whether to manage by mowing and physical removal or take a chance with herbicide application. When stressed by heat or drought, turfgrass is sensitive to herbicide injury. Similarly, absorption and uptake of herbicides decreases when temperatures are high and/or soil moisture is low. If the opportunity is present for good foliar absorption of an herbicide, quinclorac (e.g., Drive) is the herbicide of choice as it works on mature crabgrass, even in droughty situations. Quinclorac can be mixed with a preemergent herbicide such as dithiopyr (e.g., Dimension) for additional control. Both herbicides have a long list of precautions and restrictions; before using these or any other herbicides, READ THE LABEL.

Yellow Nutsedge (Cyperus esculentus)

Yellow nutsedge is a triangular-stemmed member of the Sedge Family (Cyperaceae). It prefers moist rich soil but will grow in most soils once established. This grasslike perennial weed spreads by means of tubers, rhizomes, and seeds. However, the main source for annual re-infestations is the tubers, which tolerate freezing temperatures and can remain viable for 10 years in the soil. Pulling up the plants simply breaks the roots from the tubers, generating additional roots, which then form new tubers. The optimal herbicide to control yellow nutsedge is halosulfuron (e.g., Manage), which can be used safely on perennial ryegrass lawns and fields, as well as on other turfgrass species. Bentazon (e.g., Basagran) and MSMA can also be used in areas infested with this weed, although turfgrass suffering from stress may become slightly discolored after an application of this herbicide. Follow label directions to avoid damaging turfgrass.

Dandelions (Taraxacum officinale)

During the week of April 12 in 2001, dandelions bloomed in central Ohio. Many people wanted to treat at that time. However, post-emergent herbicides would not have been effective unless conditions were conducive for the weeds to absorb and translocate the herbicide to their roots. Dandelions are best controlled when they enter the "puffball stage," when the seeds are formed. At that time, carbohydrates are sent back to the roots for storage, along with the herbicide. Although ester formulations of post-emergent herbicides work well at cooler temperatures, they are more likely than amine formulations to volatilize and injure adjacent desirable broadleaf plants at higher temperatures. If it rains within 24 hours after application, another application may be necessary if weeds show no change after two to three weeks. Delay mowing the treated area for at least three days after the treatment.

Canada Thistle (Cirsium arvense)

Of many herbicides labeled for Canada thistle control, clopyralid (e.g., Lontrel, Momentum) yields the best results. If possible, cut or wound the thistle, then apply the herbicide directly to the wound and leaves, being careful not to let it drift onto adjacent desirable broadleaf plants. Do not spray areas where tree/shrub roots or suckers are growing. A second option would be to use one or two applications of glyphosate (e.g., Roundup) or dicamba. Remember that dicamba is a selective herbicide, while glyphosate is non-selective and may kill the turfgrass as well as other plants contacted.

Violets (Viola spp.) and Other Broadleaf Weeds

Two new products containing the herbicide triclopyr are now on the market for homeowners to use on hard-to-control turfgrass weeds such as violets. This is significant because research has shown that triclopyr is the most effective chemical for controlling violets in turfgrass.

Clopyralid is very effective for controlling white clover and other hard to control weeds, such as ground ivy and buckthorn plantain. Several products for commercial use contain both clopyralid and triclopyr. These two chemicals in combination give better control of a number of weeds than either chemical by itself.