For most weeds that infest turf, we have an effective strategy for control. Annuals, like crabgrass, are controlled by preemergence herbicides applied before their seeds germinate. Perennials, like dandelion, are controlled by postemergence materials that kill the mature plant selectively in the turf. (See earlier posts on pre and postemergence weed control). However, there are certain weeds for which we do not have an effective strategy. The perennial weed grasses are among the most difficult to control. The reason for this is that we are trying to control perennial grasses in a perennial grass. These species are so close physiologically to our turf grasses that we generally lack a strategy or a selective chemical to kill them.

A good example of this in the midwestern US is quackgrass in Kentucky bluegrass turf. This species is best identified by its long, clasping auricles where the blade of the grass meets the sheath (Figure 1). Both are cool-season, perennial grasses and there is no good way to control this species. Quackgrass also has a more extensive rhizome system that Kentucky bluegrass, allowing it to outcompete the bluegrass. Bromegrass is another similar species for which there is no good selective control. This grass is identified by its closed sheath (Figure 2). Its texture is similar to Kentucky bluegrass, but it tends to grow faster in early spring and late fall and often stands out in the turf as a weed.

Sometimes our weeds are other turfgrasses that we do not want in a uniform turf stand. A good example is the common type of tall fescue in Kentucky bluegrass. The tall fescue is a bunch grass that grows faster than the bluegrass and can be a serious weed problem in certain situations (Figure 3). Creeping bentgrass, which is a very useful turf on golf greens, can also become a weed in bluegrass turf.

There are also cases where warm-season perennial grasses become a problem in cool-season turf. Likewise, cool-season perennial grasses may be a problem in warm-season turf. A good example of a warm-season perennial in Kentucky bluegrass is nimblewill (Figure 4) which is identified by the small hair (awn) at the tip of each seed. Annual bluegrass can also become a weed in warm-season grasses like bermudagrass in the south.

Another group of weeds that are neither grasses nor broadleaves are the sedges. Sedges look like grasses, but their leaves are three ranked, meaning that they are arranged on the plant in three directions, rather than two like grasses (Figure 5). There are several types of sedges that infest turf areas throughout the US and the type you have will depend on your location. They are most prevalent in wet years. In the Midwest yellow nutsedge is a common problem. Sedges can be controlled with selective herbicides, but not the common materials that are usually used in turf for weed control.

For many of these weed problems, there is no selective answer, meaning that you have to kill the entire turf area with glyphosate (Roundup) and start over. This of course is very difficult and expensive. (See Renovation in this series).

Many of these types of weed problems often occur when cheap seed is used to establish the area. These perennial grasses can be found as contaminants in cheap seed. Again, this is no problem if the seed is going to be used for roadsides or other low maintenance areas, but they are a big problem in a quality turf area. So, one of the most effective ways to avoid them is to use the best, weed-seed-free turf seed that is available.

There are a few herbicides available for some of these difficult to control weeds, but you may want to seek professional help from a lawn care specialist for their use. These materials will require considerable knowledge of both the weed and the herbicide to be used properly.

In the case of cool-season weeds in warm-season turf, there is a class of compounds known as the triazines. Materials like Simazine can be used to selectively kill cool-season weeds like annual bluegrass in bermudagrass lawns. Again, get professional help. If these materials are misused, they can sterilize the soil for years and prevent the use of cool-season grasses. The biggest problems occur when homeowners mistakenly use them on a cool-season lawn.

Mesotrione (tenacity) is a relatively new herbicide for use in Kentucky bluegrass. This material can be used to kill creeping bentgrass in Kentucky bluegrass turf, a common problem on northern golf courses. It will also kill nimblewill in Kentucky bluegrass. This unusual herbicide will turn the target weed white following application. It also can kill germinating seed and mature plants of crabgrass which can be a very useful. It will not kill Kentucky bluegrass seed at germination, but will kill crabgrass seed, making it an effective material for spring seeding of Kentucky bluegrass. Homeowners can buy tenacity, but it is tricky to use. Unless you have a thorough knowledge of turf management, this product is best left to professionals.

If sedges are your problem, there are selective herbicides that will kill established sedge in cool-season turf areas. Halosulfuron (Sedge Hammer) can be used for this purpose as well as a few other materials. They may need to be reapplied every 14 days for two to three applications because the sedges can reemerge from underground structures that are not killed by the first application. Again, you may want professional help when using these types of herbicides. There are a variety of sedge controls for use in warm-season turf such as bermudagrass. Materials like foramsulfuron (Revolver) and trifloxysulfuron sodium (Monument) will selectively kill sedges in bermudagrass. Read the label carefully before use. These products may not be recommended for all warm-season turf species.

Figure 1. Quackgrass showing long clasping auricles.

 

Figure 2. Bromegrass with its closed sheath.

 

Figure 3. Coarse textured tall fescue in bluegrass turf.

 

Figure 4. Nimblewill. Notice the awn (hair) at the tip of the seed.

 

Figure 5. Yellow nutsedge with its three ranked arrangement of leaves.

I have embedded a power point on weed identification in turf that may be useful. You will need power point to open it. I you do not have power point, you may be able to open it at https://www.extension.iastate.edu/turfgrass/search/content/what%20is%20that%20weed.

 

Nick Christians, University Professor of Horticulture, Emeritus, Iowa State University

books: Nick Christians, University Professor of Horticulture, Emeritus, Iowa State University

books: http://www.amazon.com/Nick-Edward-Christians/e/B001H6SB7A/ref=ntt_dp_epwbk_0

turf blog: http://www.iaturf.blogspot.com/

Fundamentals of Turfgrass Mgt. 5^th ed  http://www.wiley.com/WileyCDA/WileyTitle/productCd-1119204631.html

Web site:  https://www.hort.iastate.edu/directory/nick-christians/

turf blog: http://www.iaturf.blogspot.com/

Fundamentals of Turfgrass Mgt. 5^th ed  http://www.wiley.com/WileyCDA/WileyTitle/productCd-1119204631.html

Web site:  https://www.hort.iastate.edu/directory/nick-christians/

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