By Dr. Eric Reasor
From the February 2023 Issue
The basis of any turfgrass weed management program is maintaining an actively growing, healthy turfgrass stand. Using appropriate cultural practices such as mowing, fertilization, irrigation, and cultivation for the turfgrass species is the best way to help the lawn compete against weeds. However, several weeds can persist in even the most well-maintained turfgrass. While there is a growing trend to tolerate certain weeds that are pollinator-friendly (e.g., dandelions) or low-growing nitrogen producers (e.g., microclovers), many clients still desire a weed-free turfgrass stand. In these scenarios, herbicide use is often warranted.
Nine of the most common weeds affecting both warm- and cool-season turfgrass are: crabgrass; yellow and purple nutsedges; kyllinga; dandelion; white clover; oxalis (or yellow wood sorrel); plantain; prostrate knotweed; and chickweed. Since some of these weeds have similar growth and control characteristics, multiple species can be controlled with the same cultural and chemical measures. Therefore, it’s important to know which weeds are most problematic in your region and develop integrated weed management programs.
Here’s a description of each of the weeds mentioned, how to identify them, and an overview of the pre- and post-emergent herbicides that are the most effective in treating them.
In most regions, crabgrass is a Summer annual grassy weed. It germinates when soil temperatures warm to 55°F for several days. It is important to watch temperatures so herbicides can be applied before the crabgrass emerges since pre-emergent herbicides are the most effective control. Look for active ingredients such as dithiopyr, indaziflam, pendimethalin, and prodiamine. For maxi- mum efficacy, pre-emergent herbicides must not only be applied prior to crabgrass germination, but be followed by irrigation or rainfall for “activation.”
Post-emergent herbicides are a great tool to control crabgrass that breaks through the pre-emergent herbicides. Examples of these herbicides include fenoxaprop, fluazifop, pinoxaden, and quinclorac. Post-emergent herbicides are most effective at early crabgrass growth stages with the addition of a spray adjuvant (e.g., MSO, NIS).
Using the same herbicide repeatedly will eventually allow for crabgrass resistant to that specific mode of action to populate. Since there is only a small selection of herbicide modes of action for crabgrass, it’s vital to manage turfgrass properly through cultural practices, weed scouting, and using alternate herbicide modes of action. This will maximize herbicide efficacy and reduce the risk of herbicide resistance.
Yellow & Purple Nutsedges
Purple nutsedge is more common than yellow in southern regions. Both have triangular-shaped stems and a glossy leaf appearance. Distinguishing purple nutsedge from yellow nutsedge can be somewhat challenging without the presence of seed heads. Purple nutsedge leaves are abruptly pointed. Yellow nutsedge leaves slowly taper to a point. Both species have underground tubers and rhizomes that allow them to spread and return year-to-year. Nutsedges can also spread via seed. However, their viability is relatively low compared to other weed species.
Nutsedges prefer moist soil conditions, but can persist in many different environments. Cultural control measures may not provide acceptable control. Fortunately, there are several herbicide-active ingredients. Some active ingredients include sulfentrazone, halosulfuron, flazasulfuron, trifloxysulfuron, imazosulfuron, and pyrimisulfan. Be sure to read the herbicide label prior to any application for use on the specific turfgrass species.
Kyllinga species are weeds in the sedge (Cyperaceae) plant family, but they are not nutsedges. There are many different kyllinga species that can infest turfgrasses. The most common and difficult to control are green kyllinga and false-green kyllinga. These perennial weeds can form a dense canopy to outcompete turfgrasses year-after-year. Green and false-green kyllinga typically have a finer leaf texture and a lighter-green color than nutsedges. They are also likely to have small and burred spherical seed heads at the base of the leaves even when regularly mowed. Kyllinga do not have any underground tubers, but they do possess a dense rhizome system. Like nutsedges, kyllinga prefer wet soil conditions but also can persist in various environmental conditions.
Mowing height is one of the most important factors for managing kyllinga in lawns. Increasing the mowing height, especially with cool-season turfgrass, can give the turfgrass an edge in competition.
The same herbicide active ingredients used for nutsedge can be used for kyllinga. Depending on the turfgrass species, sulfentrazone, halosulfuron, flazasulfuron, trifloxysulfuron, imazosulfuron, and pyrimisulfan herbicides can provide kyllinga control.
Common dandelion is one of the most recognizable weeds in the world and one of the most prevalent in maintained turfgrasses. The iconic yellow flower on the tall, hollow stalk is easily identifiable. The yellow flowers then mature into the white “puff balls” or “blow balls” when the seeds develop and are ready for wind and traffic dispersal, often spreading throughout large areas.
Under mown conditions without the yellow or white flowers, dandelion leaves can be used to identify the species. Dandelion leaves are deeply lobed or toothed with an oblong or spatulate shape. The leaves form a rosette which can consist of many leaves. Dandelions also have a deep taproot and milky sap.
There are numerous herbicides labeled for dandelion control in turfgrasses. Most of these herbicide active ingredients are considered “broad spectrum” broadleaf herbicides; however, certain herbicides used for grassy weed control can provide dandelion control. Some of the most common active ingredients used for dandelion control are 2,4-D, carfentrazone, fluroxypyr, MCPP, MCPA, mesotrione, metsulfuron, penoxsulam, quinclorac, and triclopyr.
Herbicide selection depends on the turfgrass species, site, and cost. Herbicides are also less effective during peak dandelion bloom than they are pre- and post-bloom. Post-dandelion bloom may not be the best application timing either due to the seed production and dispersal of dandelion.
White clover is another highly recognizable weed in maintained turfgrasses. Found in warm- and cool-season turfgrasses, white clover is a perennial broadleaf and can tolerate a wide range of mowing heights and environments.
The weed can be easily identified by its leaves and flowers. White clover leaves are a trifoliate leaf arrangement, with three round/elliptical leaflets, and often have a white watermark that partially encircles the base of each leaflet. White clover produces white flowers on long stems that are mostly present in Spring and Fall. Most active white clover growth is in Spring and Fall and spreads from stems and stolons that root at the nodes and can form dense mats or clusters.
White clover is a legume, so it can fix its own nitrogen and can persist in N-deficient turfgrasses. Increasing N fertilization and reducing soil moisture are two excellent cultural practices for managing white clover. Since white clover is a perennial weed, pre-emergent herbicides
are not readily effective. However, numerous post-emergent herbicide options are available.
Herbicides that contain three or more phenoxy or phenoxy-type active ingredients have shown to provide great control. Some specific active ingredients for clover control are carfentrazone, dicamba, fluroxypyr, halauxifen-methyl, MCPP, MCPA, mesotrione, metsulfuron, penoxsulam, quinclorac, and triclopyr. Be sure to read all herbicide labels prior to use.
Herbicide resistance is not a widespread problem in white clover; however, it is always possible for any weed to develop resistance. It’s important to apply herbicides at labeled rates and alternate modes of action.
Oxalis is a group of species commonly referred to as woodsorrels. There are several woodsorrels problematic in turf, but the most prevalent is yellow woodsorrel. It’s a perennial, cool-season weed and grows in a more upright habit than white clover.
Yellow woodsorrel weed can look similar to clover with a trifoliate leaf arrangement. However, it has heart-shaped leaflets instead of round. It also produces small yellow flowers that distinguish it.
Herbicide control of yellow woodsorrel can be quite difficult. Herbicide mixtures that contain three or more phenoxy active ingredients might not provide adequate control. Additional herbicide active ingredients to include in applications to aid in herbicide control are as carfentrazone, halauxifen-methyl, mesotrione, metsulfuron, penoxsulam, and sulfentrazone.
Broadleaf and buckhorn plantain are perennial broadleaf weeds found in many varieties of turfgrass. Broadleaf plantain has round leaves, whereas buck- horn plantain has narrow, lance-shaped leaves. Both species have noticeable parallel leaf veins causing them to be easily misidentified with one another. The leaves grow in a rosette with a taproot, and seed heads grow on tall stalks.
Like dandelion, there are numerous herbicides labeled for plantain control. Some of the most common active ingredients used for control are 2,4-D, carfentrazone, dicamba, fluroxypyr, MCPP, MCPA, metsulfuron, penoxsulam, quinclorac, and triclopyr.
There have been documented cases of herbicide-resistant buck- horn plantain species due to overuse of group 4 herbicides (e.g., 2,4-D, dicamba, and triclopyr). This issue is not widespread across the country, though it’s essential to alternate or combine multiple herbicide modes of action to mitigate herbicide resistance.
Prostrate knotweed is a Summer annual broadleaf weed. It germinates in early Spring, typically prior to crabgrass germination. Knot- weed seedlings can resemble grass seedlings soon after emergence but will develop oval-shaped leaves and form dense mats. It can out- compete turfgrass in compacted soils and is considered an indicator of compaction. If knotweed is a persistent problem, consider decreasing compaction with aeration.
Most pre-emergent herbicides will provide acceptable control. However, application timing is important because knotweed germinates prior to crabgrass. The active ingredients in pre-emergent herbicides that provide acceptable control include, but are not limited to, prodiamine, dithiopyr, pendimethalin, indaziflam, and isoxaben.
Post-emergence knotweed control is typically much easier when herbicides are applied earlier in the season. The dense canopy and thick stems of mature plants can make control difficult in late Summer. Typical 3- and 4-way herbicide mixtures containing 2,4- D, dicamba, carfentrazone, fluroxypyr, or triclopyr can provide acceptable post-emergent control.
Chickweed is a group of Winter annual weeds found in both cool- and warm-season turf- grasses. Common and mouse-ear chickweeds are two that are typically problematic. They germinate in Fall with cooler soil temperatures. Common chickweed has small leaves that come to a point and small white flowers. Mouse-ear chickweed leaves are also small, but are very hairy and oval to oblong shaped. Both species can grow upright if not mown but will adapt to mowing by growing prostrate and forming a dense canopy.
Chickweeds will be problematic in Spring if not controlled in Fall or Winter. The plants will be more mature and reproducing via seed in Spring. Both species are shallow-rooted, so proper irrigation practices are important, especially in Fall and Winter when irrigation needs of turfgrass are much less.
Control of chickweed is simple with appropriate timing. If applied prior to germination in Fall, pre-emergents that provide acceptable control include, but aren’t limited to, prodiamine, dithiopyr, pendimethalin, indaziflam, and isoxaben. If using a post-emergent, most broad spectrum, broadleaf products provide acceptable control if applied prior to Spring seed production.
Reasor, Ph.D, is Southeast Research Scientist with PBI-Gordon Corporation where he is responsible for coordinating all research protocols, technical assistance to cooperators, and product support for channel partners and end-users in 10 states. Reasor holds a Ph.D. in Turfgrass Weed Science from The University of Tennessee, where he conducted research programs for “off-type” grasses in bermudagrass putting greens, participated in extension activities, and managed weed science research programs. He earned his Master of Science in Plant Sciences from The University of Tennessee, and a Bachelor of Science in Crop and Soil Environmental Sciences from Virginia Tech University.
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