Landscape Architect Articles

March 06 LSMP - Turf Talk - Weed Seed Strategies03-02-06 | News

Weed Seed Strategies

by Stephen Kelly, regional editor

The preemergent needs to be down before the soil temperatures rise and the seed germination begins, which can make the timing tricky. Frank Lopes, Seed Research of Oregon, recommends Dithiopyr (Dimension) to take some of the guesswork out of timing pre-emergent applications, as it gives good post-emergent control on crabgrass up to the three-leaf stage, thus extending your timing window. Photo courtesy of Dow AgroSciences LLC.

Before we discuss preemergent usage, i.e., herbicides that kill germinating weed seeds while having little effect on the turf, integrated pest management (IPM) teaches that a healthy lawn indicates good cultural and fertility practices and is the best defense against weed problems. Turf mowed at the proper height, fertilized properly (at the correct rates and times) and efficiently irrigated results in minimal weed problems. Such weed conditions in turf can be spot removed by excising the roots with a spade or trowel. Table 1 reveals under what turf conditions certain weeds propagate.

What the Experts Use

It?EUR??,,????'???s always nice to learn from on-the-job experts. I?EUR??,,????'???ve had the occasion recently to interview two grounds managers: Jeff Kaufmann, a horticulturist with the National Gallery of Art Sculpture Garden in Washington, D.C., and Chris Burrows of Pinehurst Resort in North Carolina. Burrows told me that controlling the growth of weeds on such a large property (he?EUR??,,????'???s responsible for 70 acres of hotel landscapes) is essential. He reports brown spot and crabgrass are somewhat problematic.

Burrows employs a preemergent (Ronstar) with a medium grade fertilizer (5-10-17) and Pendamethali to control the crabgrass for his common bermudagrass, ?EUR??,,????'??Otherwise we?EUR??,,????'???ll get wrapped up in weeds,?EUR??,,????'?? he says. His crews apply the pre-emergents on the turf in March.

Kaufmann explained the turf at the Sculpture Garden is tall fescue with a small amount of rye blend. He reports a limited use of pre-emergents, sometimes Roundup, something as basic as Weed-B-Gon, or a 2,4-D product.

?EUR??,,????'??We try to use as few products as possible, because we have to keep the grounds people-friendly,?EUR??,,????'?? notes Kaufmann. ?EUR??,,????'??We want to stay away from products with re-entry times whenever we can.?EUR??,,????'??

When he does use herbicides he relies on extension service recommendations, what other horticulturists in the area are using and follows IPM practices. IPM recognizes that selecting turfgrasses well adapted to the environment, which the Sculpture Garden?EUR??,,????'???s tall fescue is, is key, plus maintaining a good thatch layer and soil with good organic matter.

The North Carolina Cooperative Extension Service advises that a thick layer of compost or mulch can adequately kill weeds, as long as new weed seed is not introduced in the compost.

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OK, So You Need a Preemergent

The Virginia Cooperative Extension writes that some weeds (e.g., crabgrass, as Chris Burrows has already attested) occur so routinely that a preventive approach is necessary.

Certainly, controlling weeds before they appear with a preemergent herbicide is easier and more economical approach to weed control than the postemergent (after-the-fact) approach.

Spray or Dry?

Frank Lopes, a division manager of Seed Research of Oregon, writes in ?EUR??,,????'??Pre-Emerge… Why? How? When? Where? & What??EUR??,,????'?? that spreading a dry granular preemergent product is the easiest and fastest way to accomplish this task.

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Timing

The preemergent needs to be down before the soil temperatures rise and the seed germination begins, which makes timing tricky.

Lopes recommends Dithiopyr (Dimension) to take some of the guesswork out of timing pre-emergent applications. He writes that the product also gives a good post-emergent control on crabgrass up to the three-leaf stage (at labeled rates, a.i.), which extends the timing window.

Lopes also recommends rotating preemergent herbicides every four or five years to reduce the chance of developing a preemergent resistant species of weed, noting that each of the grassy weed preemergents control a slightly different set of broadleaf weeds.

A number of preemergent herbicides are available for annual grass weed control.

All of the herbicides in Table 2 were tested at Ohio State University for several years and can be safely used on Kentucky bluegrass, perennial ryegrass, tall fescue and most fine fescue cultivars.