Knowledge is power

Before turfgrass managers act to knock out a weed on the course, they need to know what they’re knocking out.

Weed identification is the foundation of what superintendents need to do, says Ronald Calhoun, Ph.D., a specialist in Michigan State University’s department of crop and soil sciences. Calhoun spoke at the Golf Course Superintendents Association’s education conference at the recent Golf Industry Show in Orlando, Fla.

Calhoun, who talked about managing weeds in cool-season turfgrass systems, says herbicides are just one approach to dealing with weeds. The best approach is finding out what type of weed is growing and what likely caused the weed growth. Weeds can be an indication of something else happening, so treating the other issue can help with the management of the weeds.

“Don’t think of the weeds in isolation of everything else that’s going on,” Calhoun said. “If you do that, any agricultural practice you’re going to provide is just going to be temporary relief.”

For example, if there happens to be a high incidence of knotweed in a heavily traveled area, maybe the weed thrives in compacted situations. By assessing the entire situation and knowing the characteristics of the weed that’s present, it can be determined what factors are helping the weeds to thrive, and a fitting treatment method can be employed.

Cultural tools

Controlling weeds is easier said than done, but a few preventive steps can make a difference. If weeds emerge, be patient and let mowing, watering and the other cultural practices have a chance to work before turning to herbicides, Calhoun said. For starters, learn about how weeds reproduced. They tend to produce a large amount of seed.

“Eighty percent of crabgrass seeds make a run at germinating the following year,” he said.

Mowing can help, or hurt, in the fight against weeds. On one hand, many weed species can’t tolerate mowing, and it prevents seed production. On the other hand, mowing can lead to more weeds if the proper mowing height isn’t used. For example, 1.5 inches is becoming a common height for roughs, and crabgrass is growing on more courses, Calhoun said. Traffic caused by the mowers also can lead to more weeds.

Fertilization also can help or hurt the weeds’ cause. Fertilizer doesn’t know the difference between turf and weeds. Calhoun recommends fertilizing cool-season grass in the spring and fall, when it has benefits to the turf but less availability to the weeds.

Like the other practices, irrigation also can be used in the fight against weeds. Water uniformly to avoid areas that are too wet or too dry. These areas weaken the turf, making it more susceptible to weeds.

Other cultural practices, including drainage improvement and aerification, help manage weeds by depriving weeds of the environment that helps them to thrive.

Know your enemy

When cultural practices aren’t enough to manage weeds, herbicides can be effective if the right mode of action is employed. To pick the right herbicide, know the weed. This is of the most helpful tools for fighting them. There are about 60 weeds superintendents with cool-season turfgrass are concerned about. Calhoun breaks them into five groups:

• Establishment weeds – most are annuals. Includes chickweed, yellow rocket, shepherds purse, lambsquarters, spurge;
• Annual grassy weeds – consists of summer annuals. Includes crabgrass, goosegrass and foxtail;
• Easy-to-control broadleaf;
• Difficult-to-control broadleaf; and
• Special situation: for whatever reason, they need special treatment that only applies to that weed. Most are perennial grasses. This can include tall fescue, quackgrass, rough bluegrass, bentgrass, nimblewill, annual bluegrass and nutsedge.

Each classification group, with the exception of the last one, has a general group of herbicides that best works against the weaknesses of the weeds in that group. Preemergence and postemergence herbicides have different qualities that respond differently to weeds.

The best time to apply a preemergent in the spring can depend on the region. Applying herbicides on a certain date each year is the least effective way to apply, Calhoun said. More dependable methods include keeping track of the soil temperature or paying attention to when the lilac or forsythia plants bloom, he said.

For more help about effective weed control, Calhoun recommends visiting www.msuturfweeds.net, a weed identification Web site he manages (see the sidebar for more information). He stresses that the most effective way to control weeds is to control what causes them.

“Don’t use herbicides as a sledgehammer and try to rule the day,” he said.

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