January 1997

--Spring Ahead-- Pre-Emergence for Summer Weeds

David M. Kopec, Extension Turfgrass Specialist

The month of February usually means pre-emergence application time for spring weed control of summer annual weeds. Summer annuals complete their life cycle in one year. They germinate in the spring and persist through the warm season and then die out in the fall. Summer annual weeds persistent on turfs include the following partial list. Southwest cupgrass, crabgrass, ricegrass, goosegrass and stinkgrass. These weeds produce large amounts of seed, which will last several years in the soil. The cupgrass, goosegrass and crabgrass will withstand almost any mowing height, while ricegrass and stinkgrass are more of a problem on low maintenance roughs on larger acreage and older courses. Pre-emergence herbicides for the control of cupgrass and crabgrass are essentially the same. We have far more cupgrass than crabgrass in Arizona, and they are easily mistaken for each other.

Broadleaf weed control for summer annuals deals with mostly the control of the spurges and seeds of khakiweed. (More about khakiweed in a later issue).

Note that cupgrass and crabgrass will germinate first, followed by spurge and goosegrass about a month later. This is due to soil temperature requirements for the seed to germinate.

Some of the most commonly used herbicides for pre-emergence of cupgrass and goosegrass include Balan, Dacthal, Tupersan, Ronstar, Surflan, Pendimethalin, Prodiamine and Dimension. Commercial mixtures are available such as Balan XL (mixture of Benefin and Surflan) and Team (mixture of Benefin and Treflan).

If spurge is a problem as well, then note that the literature states that Dacthal, Pendimethalin and Ronstar provide good to excellent control of spurge.

Goosegrass is controlled to a great extent by Ronstar. It will also control crabgrass/cupgrass.

Dimension will also control cupgrass if the plants are very young and in the 1-2 tiller stage. Don't wait to apply it if you already see lots of the cupgrass or crabgrass already up. By then it may be too late. Rather, it's safe to know that young emerged plants will usually be controlled by it. If you miss the application window (you can see very small weeds) than tank mix MSMA with Dimension for better post emergence control of very young plants.

The last two considerations include the choice of split applications and herbicide selection if you have overseed ryegrass. Split applications may be beneficial in situations where you are trying to control both cupgrass/crabgrass, followed by goosegrass. Here, the goosegrass will germinate about a month later, so you may want to apply the herbicide at the full 1x rate up-front (one application), if the herbicide is something other than Ronstar. This is because more of the chemical is usually needed to inhibit the goosegrass seed, than that of the cupgrass/crabgrass.

If ryegrass has been overseeded, than Surflan or Surflan mixes should not be used. It will damage the ryegrass and reduce its growth tremendously. This information is on the label!

For other annual broadleaf weeds, Gallery (active ingredient = Isoxaben) is available. It will control most broadleaf weeds pre-emergence including henbit, knotweed, spurges, oxalis, pigweed, etc. Prostrate pigweed is becoming more of a problem on roughs and fairways. By convention, this chemical should control it.

For Phoenix and Tucson applications should be made by March 1-10 for cupgrass/crabgrass. If you have a known history of goosegrass, you may need to apply Ronstar March 25-April 5. For Yuma, decrease two weeks from the above schedule for the above listed weeds.

Always check the label for:

1) Weeds controlled.

2) Tolerance of turfgrasses.

3) Choice of split or single applications.

4) Re-seeding period. (How many weeks, or months until re-seeding of turf can take place).

5) "Water in" requirement and irrigation amount after herbicide application.

Control of summer annual weeds or greens is risky, and hopefully, not necessary. Usually goosegrass and cupgrass are minor problems. There are major problems on disastrous greens (which are probably so, for other reasons). For bentgrass greens, Betasan, Betasan/Ronstar (called Regalstar) are labeled. For overseeded bermudagrass greens, you have more flexibility. Remember, always check for safety on the ryegrass. Poa trivialis is very sensitive to many pre-emergence herbicides.

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