Turf Magazine - October, 2012


Tackling Tough Broadleaf Weeds

Picking products with the right ingredients
By Aaron Patton, Ph.D.

To control tough broadleaf weeds, first start with proper cultural practices. Your goal is to increase turfgrass vigor and health, which will discourage weed competition. These practices include, but are not limited to, proper mowing, fertilization, irrigation, drainage and soil cultivation. Cultural controls can be effective on many weed species, although they reduce weed populations slowly. As such, herbicides are often used in addition to cultural practices to control weeds.

Ground ivy, sometimes called creeping Charlie, is a tough-to-control broadleaf weed usually found growing in the shade. Very few cultural practices influence ground ivy, although fertilizing turf with a nitrogen fertilizer will reduce it some. When cultural practices do not effectively control weeds, herbicides are needed.

Among your first considerations in developing a weed control program for tough weeds is to, first, know what turfgrass species you have. Not all herbicides will safely control weeds without injuring the turf. For example, while metsulfuron is a good herbicide to use on certain warm-season grasses like bermudagrass, it will kill most cool-season grasses.

After determining your turf species, identify problem weeds and note what time of the year they occur. Sometimes, turf managers struggle with weed control on certain properties because they have misidentified the weed. You might mistake henbit for ground ivy, or lespedeza for white clover. That's when it's a good idea to have a good weed identification book handy, or resource you can call up on your iPhone, iPad or Android.

Fix the cause of infestations

Next, determine why the weeds invaded the turf area and correct the conditions or cultural practices that caused the problem. Was it compaction, low fertility, a soil pH problem, over-irrigation or poor drainage, or some other poor cultural practice that caused this weed to become established? If you determine that a cultural factor needs correcting, make necessary changes to improve turf health to increase competition with weeds.

An integrated approach, which includes enhancing turfgrass competition and using chemical weed control methods, will be the most successful weed control program. Remember, although it can be helpful to view weeds as indicators of cultural problems, the presence of a weed doesn't always indicate a cultural problem.

When herbicides are needed:

  • Select a chemical that is effective on the weeds and labeled for use on the turfgrass species you are treating. The following sections provide more information on herbicide selection.
  • Follow all label directions - the label is the law.
  • Apply the herbicide at the correct time (time of year and weed life cycle) and rate.
  • Apply the herbicide uniformly over the turf area without skips or overlapping.
  • Repeat the herbicide application at the recommended interval when specified on the label.

Popular standby still strong

There are a number of herbicides on the market that contain phenoxy herbicides such as 2,4-D, mecoprop (MCPP), 2,4-DP (dichlorprop) and MCPA, as well as the benzoic acid dicamba for use in broadleaf weed control. These ingredients make up the majority of what are commonly referred to as "three-way" herbicides with 2,4-D, MCPP and dicamba serving as the core three ingredients in most (Table 2).

The reason that so many products on the market contain this combination of ingredients is because they're effective to use on both cool- and warm-season grasses; exhibit excellent turf safety (except during green-up of warm-season turf); they're low cost; and they provide consistent control of common broadleaf weeds.

Weeds like dandelion, curly dock, plantains, white clover, Carolina geranium, chickweeds and prostrate knotweed can be consistently controlled with these three-way herbicides when applied at the right time.

However, some other tough broadleaf weeds such as ground ivy, wild violet, India mock strawberry, thistles, yellow woodsorrel, corn speedwell and lespedeza are often not completely controlled with three-way herbicides. And, that's in spite of many of these tough-to-control broadleaf weeds being listed on the three-way herbicide label. The fact is that they may provide control in some situations, but often these tough broadleaves require the use of a different set of ingredients to improve control. The remainder of this article focuses on some other herbicides options to maximize your control of ground ivy, wild violet, yellow woodsorrel, lespedeza and thistles.

Turf managers call many species wild violet, including the common blue violet, wooly blue violet and confederate violet. Wild violets are a persistent perennial and difficult-to-control broadleaf weed.

Proven solutions

  • Ground Ivy or Creeping Charlie - Ground ivy (Glechoma hederacea), sometimes called creeping Charlie, is a tough-to-control broadleaf weed in the mint family usually found growing in the shade. Very few cultural practices influence ground ivy, although fertilizing turf with a nitrogen fertilizer will reduce it some. When cultural practices do not effectively control weeds, herbicides are needed.

    In cool-season turfgrass, herbicides that work best for control include fluroxypyr and triclopyr (Table 3). Although there are many products that contain triclopyr, triclopyr by itself at 1 quart per acre typically provides the best results in our research. Additionally, good control can be achieved by tank-mixing triclopyr at 1 pint per acre tank-mixed with another herbicide containing 2,4-D as the only ingredient or as one of the ingredients. In bermudagrass and zoysiagrass, metsulfuron (Manor, Mansion, MSM) or Celsius WG are other good options for control.

    Herbicides containing quinclorac (Table 3) have also been shown to be effective on ground ivy populations at most locations in most states, but has not provided adequate control in Purdue University testing in West Lafayette, Ind. Ground ivy populations are known to vary in their tolerance to herbicides, so if you are having trouble controlling it at your location, consider switching herbicides to one of the ingredients recommended in this article or tank-mixing herbicides to improve control.

  • Wild Violet - Collectively, turf managers call many species wild violet, including the common blue violet (Viola sororia), wooly blue violet (Viola papilionacea) and confederate violet (Viola sororia f. priceana). Wild violets are a persistent perennial and difficult-to-control broadleaf weed.

    In cool-season turfgrass, good control of wild violet is typically obtained with triclopyr (Turflon Ester Ultra or Triclopyr 4) or products that contain triclopyr. Many herbicides are available with triclopyr as the key ingredient (Table 3).

    In bermudagrass and zoysiagrass, metsulfuron and flazasulfuron are effective wild violet control herbicides. Repeat applications are often required regardless of the herbicide. Fall applications are best followed by spring applications.

Yellow woodsorrel has rhizomes, but usually spreads by seeds. As its seed capsules dry, they can break open and disperse seed several feet.
  • Yellow Woodsorrel - Yellow woodsorrel (Oxalis stricta) and creeping woodsorrel (Oxalis corniculata) are similar species that can be found in turf. Yellow woodsorrel is more common in turf; creeping woodsorrel is more common in landscape plantings. Yellow woodsorrel has rhizomes, but most commonly spreads by seeds. As its seed capsules dry, they can break open explosively and disperse seed several feet. Yellow woodsorrel is difficult to control with herbicides. Typically, the most effective postemergence products contain fluroxypyr and triclopyr (Table 3).
  • Lespedeza - Lespedeza (Lespedeza striata) is a summer annual weed that is often an indicator of insufficient nitrogen and occurs in areas that are not frequently fertilized or have been neglected, such as sports fields, sod farms and lawns. Therefore, increasing the nitrogen fertilization of these areas will make turf more competitive and reduce lespedeza populations. Herbicides that contain metsulfuron, fluroxypyr or triclopyr are very effective at controlling lespedeza. Using 2,4-D alone will not control lespedeza or white clover. Multiple products and product combinations that contain metsulfuron, fluroxypyr or triclopyr are available (Table 3). Metsulfuron is safe for use on Kentucky bluegrass (not mixtures of Kentucky bluegrass with perennial ryegrass or other cool-season turf species) when applied at 0.25-0.5 ounce per acre. When using three-way herbicides containing 2,4-D + mecoprop (MCPP) + dicamba, repeat applications are usually needed for complete control of lespedeza. Although clopyralid provides excellent white clover control it does not control lespedeza and is not labeled for residential turf.
  • Thistles - Canada thistle (Cirsium arvense), musk thistle (Carduus nutans) and bull thistle (Cirsium vulgare) are all common turf weeds in turf; Canada thistle is typically the most problematic. Bull and musk thistles are biennials, whereas Canada thistle is a rhizomatous perennial. Products that contain clopyralid or mixtures that contain 2,4-D + MCPP + dicamba typically provide the best thistle control. Regardless of herbicide selection, thistle may require two herbicide applications for complete control.

Canada thistle is a tough customer to control. Regardless of herbicide selection, it may take two applications for control.
Using proper cultural practices reduces weed competition in turfgrass. However, tough weeds can often still persist. Recognize also that herbicides provide may only partial control. Sometimes multiple applications of herbicides are needed to control tough weeds.

Picking the right product is important for controlling tough weeds. Do some research on which herbicides control your weed best while making sure that they are safe to use on your desirable turf species. If you are having trouble finding this information, consult your state turfgrass extension specialist for more research-based information.

Dr. Aaron Patton is assistant professor of agronomy and turfgrass extension specialist at Purdue University. More information about weed control for turfgrass professionals written by Dr. Patton is available in a Purdue University Extension publication AY-336, "Turfgrass Weed Control for Professionals", available from the Purdue Extension Education Store (www.the-education-store.com) for $12. Contact him at ajpatton@purdue.edu for more information.