This podcast and accompanying transcript were developed by Mike Goatley, Extension Turfgrass Specialist, and Shawn Askew, Extension Turfgrass Weed Specialist at Virginia Tech.
The hot, dry weather of mid-summer has resulted in a major surge in crabgrass and other warm-season grassy weeds in many Virginia lawns. Are you noticing these patches or clumps of typically light colored, wide-bladed grasses in your lawn now? These weeds are adapted to summer heat they are especially competitive under these conditions in cool-season lawns (fescues, bluegrasses, and ryegrasses). Many calls are received in late summer from folks asking what can be done to kill crabgrass now? For “do it yourselfers” in home lawn care, the answer is (unfortunately) not a lot. Let’s talk about what is likely in your lawn at this time, how to identify it, and finally, a little on control options (or lack thereof) for grassy weeds in late summer.
Weed Identification. On a walk through my neighborhood in Blacksburg, I identified the following grassy weeds as being most prominent in cool-season lawns in early August: smooth crabgrass (Digitaria ischaemum), large crabgrass (Digitaria sanguinalis), a few foxtails (Setaria spp.), goosegrass (Eleusine indica), thin paspalum (Paspalum setaceum), dallisgrass (Paspalum dilatatum), barnyard grass (Echinochloa crus-galli), and nimblewill (Muhlenbergia schreberi). And there are always a multitude of weeds that I can not immediately identify, leading me to do some research and making some inquiries with experts in plant identification (just as you can do as well). Knowing what weed you are managing can make all the difference in the world in deciding if, how, and when to treat. Dr. Shawn Askew’s “Turf Weeds” web site at www.turfweeds.net serves as a great place to start in identifying lawn weeds. This website is fully searchable by common and/or scientific name of the weeds, as well as broader categories such as weed-type (grass, grass-like or broadleaf) or life cycle (annual, biennial, or perennial). Each weed is accompanied by several color photos of prominent vegetative or reproductive (i.e. seedhead) characteristics that are unique to that plant. Couple the picture with the description of the weed that is provided, and you can probably become a pretty effective taxonomist in basic lawn weed identification with a little practice.
Another valuable identification resource is the Weed Identification Guide that is maintained by Extension Weed Specialist Dr. Scott Hagood (the easiest way to access it is to search for Weed Identification Guide on the VT website at www.vt.edu but for those of you using this podcast transcript, you can access it directly at http://www.ppws.vt.edu/scott/weed_id/rightsid.htm). While not specific to turf weeds, it has an excellent grass identification key that I find to be very helpful.
And if you are simply stumped on what any weed is, you can complete some paperwork and submit a sample of the weed to your local VCE office. Agents there will likely be able to tell you what the weed is, and if not, they will forward the sample on to experts at Virginia Tech for its identification.
Use the weed’s life cycle to chart control strategy. Another very important part of the weed description that was previously described and now needs to be reemphasized is the weed’s life cycle. Of the weeds previously detailed, thin paspalum, dallisgrass, and nimblewill are all perennial weeds that will continue to persist in your lawns season after season—the rest of the weeds I listed are summer annuals that will die at first frost this fall. Now, all summer weeds will typically be quite mature at this time of year (even the annuals) and as a general rule, the older the weed is, the more difficult it is to control. But knowing its life cycle allows you to make informed decisions regarding how to combat the weed. For instance, you know that a perennial weed is going to maintain its place in your lawn unless you aggressively attack it with cultural, mechanical, or chemical control methods. Perennial weeds can rarely be controlled with single applications of herbicides. On the other hand, annual weeds complete their life cycles within one growing season. They have now had several weeks to mature and most are producing lots of seedheads at this time (and subsequently completing their growing season). Annual grassy weeds like crabgrass, foxtail, and goosegrass will all die at first frost this fall. So, is it worth trying to kill annual grassy weeds so late in the summer? Maybe or maybe not. It’s not an easy answer.
Should you do it yourself or hire a professional? Methods of chemical weed control depend on the weed, the turfgrass, and the applicator. Professional lawn care companies have significantly more tools in their arsenal of herbicides to combat summer grassy weeds, so if you are looking for a reason to employ a licensed lawn care operator, late summer weed control can be very good justification. Licensed applicators fully understand the chemistry, the weeds that are being targeted, and how the turf is going to respond to the application. Paying for their expertise can often be a very affordable alternative to complete lawn renovation this fall. All this being said, you can certainly still be a very effective turfgrass manager with products off the store shelves, it is just that you have fewer options than professionals at this time of year.
What are mid-summer grassy weed control options? By far, the primary summer grassy weed problem in the mid-Atlantic continues to be crabgrass, and licensed applicators have access to chemical formulations containing active ingredients such as quinclorac (safe to use on almost all turfgrasses) and fenoxaprop (can be applied only to selected grasses) that are excellent crabgrass materials with good safety on specific turfgrasses. Quinclorac even offers great flexibility in terms of fall seedings/renovations of cool-season lawns. However, it is likely these chemicals can not control mature crabgrass in single applications now. And even if they do, you must consider that removal of the crabgrass in late summer leads to a lot of bare ground that will be subject to invasion by fall germinating weeds in late August/early September.
Other chemical options available to homeowners and professionals alike are the arsonates: DSMA or MSMA. These are the active ingredients in many broad spectrum postemergent herbicide formulations found on lawn and garden center shelves. These chemicals are labeled for almost all turfgrasses except centipedegrass (where the standard POST grass herbicide is sethoxydim) and St. Augustinegrass (unfortunately, this grass has virtually no POST options readily available to either homeowner or professional applicators alike). The arsonates require temperatures of 80 to 90 F to maximize activity and multiple applications on 7-10 day intervals are necessary to completely control the weedy grasses. Expect some turf discoloration with these chemicals and pay attention to label restrictions on reseeding if your plan is to re-grass this fall. Remember – a pesticide label is THE LAW when it comes to legal applications of the specific chemistry.
Finally, as Dr. Askew so often reminds me, don’t forget the value of hand-pulling or spot treating weeds if you only have a few patches scattered about your lawn. A little sweat equity now rather than large scale applications of herbicides can go a long way towards having the best lawn possible later this fall.
Summary. The herbicides I have listed are not an exhaustive list of all products available for grassy weed control. With some research you will likely be able to find other labeled control options. This six plus minutes of discussion on summer grassy weed control might have generated more questions than it answered, so be sure to use the expertise available at your local Virginia Cooperative Extension office.