Crabgrass photo from senske.com
According to Ronald Calhoun, Ph.D., the foundation of any successful weed-management program includes the following: command of weed ecology, weed identification skills, and an understanding of herbicides and their active ingredients.
Calhoun is with Residex LLC, a Michigan-based supplier of turf and pest management products with 25 distribution centers in the eastern U.S. He spoke at the 2014 Lawn Care Summit in Nashville.
"Look at the lawn and ask yourself, 'Are the weeds telling me a story?'" Calhoun rhetorically asked the LCS crowd. The types and locations of weeds will tell you a lot about the overall health of that lawn—and what you need to do as a lawn care pro to whip it into better shape.
Weed ID. Lawn care pros should know the names, key identifying features and lifecycles of common weeds. Then, it's helpful to first classify the weeds you encounter into management groups, i.e. establishment weeds, annual grassy weeds, easy-to-control broadleaf weeds, hard-to-control broadleaf weeds, and special-situation weeds (typically 20% of the weeds you'll encounter, Calhoun says).
To more specifically identify the different weeds you encounter, start by examining broadleaf morphology. "Look at what's important to identify a given weed," Calhoun says. With respect to leaf arrangement, are leaves directly opposite one another or do they alternate? Or perhaps they are trifoliate (like clover) or whorled (like carpetweed). Whatever the case, note it.
You can also examine leaf attachment. Does the stem attach to the base of the leaf (sessile) or does the weed have more of a stalk (petiolate)? Finally, you can look at other things such as overall leaf shape (linear, ovate, round or heart/spade), leaf margin (the boundary area along the edge of the leaf), and the growth pattern (upright vs. spreading vs. clumps).
Gathering this information will help you hone in on the specific weed you're dealing with. There are numerous online resources and even mobile apps now available to help you search for weeds by the different criteria described above.
Product Selection. Once you identify the weed, you have to figure out how to manage it. Come to understand the different types of pre- and post-emergent products on the market, along with the non-selective products. "Don't just rely on trade names either, such as Specticle (Bayer) or Momentum (LESCO)," Calhoun adds. Become familiar with the active ingredients in today's products. It's pretty simple: In order to implement a good weed-management plan, you need to understand which products to use where, when and how much.
Developing Your Program
Taking into consideration everything discussed thus far, Calhoun says you can develop a four-component weed-management program: identify problems, develop strategies for each weed category, implement your plan, evaluate the outcomes.
Your strategy for each weed-type category should include consideration of timing and weed-lifecycle issues, pre- and post-emergent treatments, along with the cultural practices of fertilization, cultivation, proper mowing and proper irrigation. Finally, you have to choose the right products to accomplish all of this.
Establishment weeds. Most are annuals, many of which are winter annuals. Regular mowing often handles these weeds. Examples include chickweed, yellow rocket, shepherds purse, spurge, pigweed and purslane. "If the product you are using allows, you can be very successful in controlling these weeds at spring seedings," Calhoun points out. "But the best solution is patience. Give the cultural practices a chance to work. If you don't have time to wait, weed suppression at establishment is vital."
Calhoun outlined some options for those instances:
- Pre-emergent: mesotrione, quinclorac or siduron at day of seeding
- Early post-emergent: bromoxynil, carfentrazone, dithiopyr, mesotrione or quinclorac at 30-45 days after seeding
Annual grassy weeds. Summer annuals prefer hot weather, thinning areas and south-facing slopes. We're talking about things like crabgrass, foxtails and goosegrass. They germinate in spring, flower in summer, and then die with frost. "They become more of a problem with spring seedings or following fall grub damage or winterkill," Calhoun says.
There are both pros and cons to both pre- and post-emergent treatments.
- Pre-emergent advantages: most effective and least expensive, one application per season (no scouting for weeds necessary), fewer interruptions to turf uniformity, desirable environmental profile
- Pre-emergent disadvantages: potential for root inhibition/injury, must treat entire area, limits potential for overseeding
- Post-emergent advantages: only use in heavy seasons, spot treat, ability to overseed, ability for rescue treatment
- Post-emergent disadvantages: must wait until germination, interruption of surface uniformity, more expense than pre-emergent, diminished control vs. pre-emergent
What about a pre- AND post-emergent approach? The goal is to catch weeds during their germination window. Options include dithiopyr, which provides early post control plus an 8- to 10-week residual, and mesotrione, which provides early post control plus a 4- to 6-week residual. A repeat application then increases post-emergent activity. Another option, Calhoun shares, is to use fenoxyprop or quinclorac plus a pre-emergent.
- Pre/Post-emergent advantages: only use in heavy seasons, spot treat, allows for spring re-establishment, extends the pre-emergent residual in exposed areas
- Pre/Post-emergent disadvantages: must wait until germination, frequent scouting required, more expensive then pre-emergent, more potential for misses
Proper cultural practices can play a pivotal role in preventing annual grassy weeds. Proper mowing, fertilization, cultivation and irrigation can help prevent turf from drying out and/or thinning, which opens the door to these types of weeds.
Easy-to-control broadleaf weeds. Calhoun says you can count on pre-emergent grass herbicides to control these. Stand-alone phenoxy products are still the least expensive, and there are some hard-to-beat three-way herbicides when factoring price and spectrum of control. Timing is usually Oct-Nov and May-June. "Avoid tender turf and mid-summer," Calhoun points out. "Late fall can also be a good time, as plants begin to store food in their roots. You won't visibly see curdling, though."
Hard-to-control broadleaf weeds. These include summer annuals including spurge, oxalis, knotweed and purslane. Pre-emergent options include isoxaben or a proper pre-emergent grass product. If you're going the route of a post-emergent, "Get them while they're young," Calhoun advises. Carfentrazone or pyraflufen-ethyl are effective.
There are also some perennials in this category, such as white clover and black medic. A post-emergent treatment can include clopyralid, fluroxypyr or quinclorac.
Here are some other tips from Calhoun:
- Repeat applications of three-way herbicides
- Ester formulations of three-way herbicides are effective
- Primary application timing is in the fall – better translocation to roots, ability to use esters, winter annuals are still young
"You should also identify herbicides with specific activity on your target weeds," Calhoun says. "Learn the strengths of the different active ingredients." For example, triclopyr (T4) gets what 2,4-D misses; quinclorac is good for bindweed, clover, dandelion and speedwell.
You also have some late spring/summer options. For example, you could ‘spike’ with carfentrazone or quinclorac, though this can get expensive.
Special-situation weeds. These are mostly perennial grasses that don't fall into the other classifications. We're talking about tall fescue, quackgrass, rough bluegrass, bentgrass, nimblewill, annual bluegrass and nutsedge. "Unfortunately, there are very few selective herbicide options," Calhoun says. "And it usually involves two to four applications in the same season."
Options in the way of selective herbicides include:
- Yellow nutsedge: halosulfuron, sulfosulfuron, sulfentrazone, MSMA
- Bentgrass, nimblewill: mesotrione
- Quackgrass, tall fescue, ryegrass: sulfosulfuron
- Annual bluegrass: bispyribac, sulfosulfuron, ethofumesate, paclobutrazol, flurprimidol, mesotrione, linuron, primisulfuron
- Bentgrass out of fine fescue: fluazifop
- Grasses out of ground covers: fluazifop, sethoxydim, clethodim
You also have some non-selective options, Calhoun points out. You can spot or area treat with glyphosate, fluazifop, clethodim or sethoxydim. However, Calhoun says glufosinate is not recommended.