I recently completed a survey that asked questions such as, “What are the biggest problem weeds in turf?” and “What should the ideal herbicide look like in the future?” As I filled out my answers, I thought about the last few years in turfgrass weed management field research. For a variety of reasons, there hasn’t been the wave of new product or active ingredient introductions that we saw from 2000 to 2010. Actually, in the past two years, I’m aware of more products that have been discontinued than I am of new herbicides registered for turfgrass weed control. On a positive note, this has given us the opportunity to figure out how to better utilize all of the newer chemistries that are available.

Some products have been recently released with improved formulations. Amicarbazone (Xonerate; FMC) herbicide is an example of a product that has become much more effective for annual bluegrass control, mainly due to improvements in the formulation and refinements to application timing recommendations.

This month’s article discusses weed control strategies which vary based on the species of weed being controlled. This is a good chance to discuss how some of the newer herbicides and more recently found weed problems have combined to cause some confusion when determining product selection and application timing.

Identify and know when to treat

Essential for satisfactory control of weeds with herbicides is to properly identify the weed species that are present and to make sure your herbicide is labeled for control of them. There are many weeds that are similar in appearance and many herbicides that are effective only for the control of one of them. This is more of an issue with grassy weed control. But it can also be an issue with broadleaf weeds.

Equally as important is to identify the life cycle of the weed. Understanding this allows you to know when the weed will be a problem and the best timing for herbicide application in order to achieve control. For example, there’s no need to apply herbicide to a weed like henbit or common chickweed in late April. Both of those are winter annuals that will die naturally in May. The better strategy to control them is with a late fall postemergence herbicide application or the use of a preemergence herbicide (but again, applied in fall).

Tips for control of sedges

Remember that sedges are not grasses, but “grass-like.” This is important because the list of herbicides effective for sedge control is different in most cases than those that control grasses. Sedges have edges, in reference to the triangular-shaped stem. Also, the leaves radiate in three directions from the plant, not two as we see in the grasses. Yellow nutsedge has always been an issue in cool-season turf. More field managers in the Midwest are reporting encroachments of annual sedge, green kyllinga and other sedges that were once more of an issue on southern turf (Figure 1).

Imazosulfuron (Celero; Nufarm) is a new herbicide for the control of sedges. It has a broad spectrum of activity on sedges and is effective against yellow and purple nutsedge (Cyperus spp.), as well as annual sedges and kyllinga (Kyllinga spp.). It’s also labeled for control of certain broadleaf weeds, including chickweeds (Cerastium and Stellaria), Carolina geranium, henbit, parsley piert, purslane and California burclover. It’s labeled for use on cool-season turfgrasses including bentgrass, bluegrass, ryegrass and both tall and fine fescue. Because of its broader spectrum on sedge species, the product is being emphasized more for use on warm-season turfgrass.

If you’re dealing mainly with yellow nutsedge, then halosulfuron- and sulfentrazone-containing products continue to give very good postemergence control. In our herbicide trials at Ohio State University, we’ve found that control is more rapid with sulfentrazone (seven to 14 days versus 21 to 28 days for halosulfuron) – but control with halosulfuron may sometimes last longer. Both are excellent herbicides that should be used as part of a rotation program for sedge control. For preemergence control, we’ve seen activity with sulfentrazone, but the timing is important (as close to initial leaf appearance as possible). Mesotrione, when used during overseeding, can also give some preemergence control of yellow nutsedge.

Figure 2. Grassy weed identification is easier and much more precise if you can utilize the seed head.

Tips for control of grassy weeds

Some of our grassy herbicides have a relatively broad spectrum of activity. But there are many cases in which a particular herbicide will control well one grassy weed species but not give adequate control of another that may be similar in appearance. Yes, vegetative structures can be used for grass identification. But if you have the opportunity, you should always use the seed head in order to make the identification. For example, this past year I had a sports turf manager contact me about a herbicide recommendation for control of field paspalum. He was reasonably confident in his identification based on some vegetative characteristics, including a distinctive white midrib. He had already applied a product recommended for field paspalum control and, after getting less than stellar control, the remaining weeds flowered, only to learn that he was in fact dealing with a combination of green foxtail and barnyardgrass. Find a seed head when possible, or let a single plant grow up and go to seed so that you can use that structure to aid in making the proper identification (Figure 2).

It’s also essential to recognize weed life cycles and that certain types of weeds need to be controlled with different herbicides, or at different times of the year. A second sports turf manager also recently asked about tips for field paspalum control. He reported having previously tried dithiopyr and a tank mix of quinclorac and sulfentrazone. Field paspalum is a perennial, thus the application of a preemergence herbicide may prevent seed from germinating but won’t prevent any plant already present from reestablishing from vegetative structures in the spring. Similarly, neither quinclorac nor sulfentrazone have been found particularly effective against it. Currently, the best option for control of field paspalum or dallisgrass is with two applications (21 days apart) of topramazone (Pylex; BASF) or topramazone combined with either quinclorac or triclopyr. The other herbicides are mainly to mute the bleaching effect; topramazone is what is giving control. Some have also reported success with multiple applications of mesotrione. The point is, given the selectivity of some of our postemergence grass herbicides and the similarity in appearances of the grasses, which may be annual or perennial, proper identification is very important.

Oftentimes you won’t be able to use a preemergence herbicide on an athletic field because of the overseeding that’s required. If overseeding, siduron and mesotrione are great options (follow the label carefully). But if you aren’t overseeding, and for areas surrounding the fields, the preemergence herbicides haven’t changed much in the last several years. Products that contain dithiopyr, prodiamine, oxadiazon and pendimethalin tend to give the longest lasting control. Remember to rotate among herbicide groups in order to reduce chances of weeds acquiring resistance to a particular class of herbicides.

You should also document where you have infestations and only treat those areas. In most cases, blanket applications of preemergence herbicides aren’t necessary.

Broadleaf control

Figure 3. Winter annual broadleaf weed control is usually more effective with a late fall postemergence herbicide application (use the ester formulation or an herbicide shown to be effective in cooler weather). You can also apply an early fall preemergence herbicide as long as you don’t plant to overseed.

We’ve been seeing more issues with the winter annual weeds in recent years. This may be a product of the milder winters that we’ve been experiencing. Weeds like henbit and chickweed have always been an issue, but they seem to be becoming more prevalent. In addition, species like hairy bittercress are also being seen in areas further north (Figure 3).

Winter annuals germinate in the fall, persist over winter, flower in April and die usually by June. Control of winter annuals when they’re flowering can be very difficult. The best idea is to either apply a preemergence herbicide (but in the fall, which may interfere with your overseeding plans) or to use a postemergence herbicide in late fall (November). The late-fall application should be with a herbicide that works in cooler weather, such as florasulam (Defendor; Dow) or the ester formulation of a three- or four-way product. If overseeding did occur earlier in the fall, make sure the seedlings are sufficiently mature before making this application.

As with the grassy herbicides, there haven’t been a lot of new products or active ingredients introduced for the turfgrass market in the past few years. But our understanding of how to best use some of the more recent product introductions has continued to evolve.

For example, the herbicide florasulam was introduced in 2012, and one of its potentially very useful properties is that it is quite active in cooler weather, making it more effective for control of broadleaf weeds if used very early in the spring. Florasulam will also suppress dandelion flower development if applied early enough, which is unique among our herbicides and, in many cases, a desirable result.

I’ve conducted tests with this and other herbicides applied early in the year (March) – what can sometimes happen if you apply too early is that you get control of the perennial weeds that are present.

In addition, many of the cool-season annual broadleaf weeds haven’t yet germinated. In other words, you might get great control of white clover, only to have those areas fill in with black medic, which germinates after your herbicide application. Again, it’s important to document what types of weeds you’re trying to control. If your turfgrass has dandelions but doesn’t have an issue with black medic and other spring annuals, the strategy of applying florasulam early in spring can be effective.

Some studies have also indicated that, similar to the ester formulations of the phenoxy herbicides like 2,4-D, an application of florasulam in November can result in good weed control until the following summer.

Final notes

  • For the second time in about a year, a large amount of turfgrass acreage on a university campus was inadvertently damaged when an applicator mistook a glyphosate containing formulation for a selective herbicide. When glyphosate went off-patent, many new glyphosate-containing herbicides were introduced to the turf market. Unfortunately, this has caused some confusion for some sports turf managers trying to differentiate between glyphosate and nonglyphosate herbicides.
  • To me, it seems like as an industry we could standardize something with nonselective herbicide packaging, like the container opens the wrong way, has a safety cap or is black. In the absence of this, make sure you always know what you’re applying – one of the many reasons why it’s always a good idea to read the label before each application.
  • In the next couple of years, there should be some new herbicides on the market, as there are a few companies that are experimenting with new active ingredients. As I mentioned in a previous article, resistance of certain weed species is becoming an issue in cool-season turfgrass. One of these new active ingredients, in addition to being effective for weed control, is a member of a different class of chemistry than some of our current herbicides. This should provide managers with important tools to help reduce resistance issues.