July 1998 - Volume V, Issue 7

Late Season Weed Control for Lawns

David M. Kopec, Extension Turfgrass Specialist

By August, everybody is tired of yard work. The lawn is no different from the garden or maintaining the rest of the property in this regard. You notice that the lawn has more weeds in it than you thought otherwise. The question is .........should I take care of the weeds now? The answer is sometimes yes, and sometimes no. Let's answer most of these questions right now.

Purple and/or yellow nutsedge. People call these weeds "nutgrass" but they are not actually a true grass (monocot). Nutsedge leaves grow faster than the turf and so within a day or so after mowing, the leaves stand up over the turf. To identify either nutsedge in a lawn, grab the plant at the base of the stem at the soil line. Rip it out. When you look down at the stem (from an overhead position) the leaves will be grouped in three positions (ranks of three). Each leaf will be 1200 away from each other. The leaves will thus appear in a triangle. Grass leaves (turf or grassy weeds) appear in ranks of two. When you look at the stem from overhead, the leaves of grasses are 1800 apart and all leaves reside on the same plane. This time of the year, nutsedge is sending food reserves down underground to make new nutlets. There are already lots of them down there, but you don't need to let the weed make more. You can hoe the plants out, or cut them at the stem. Or, you can spray them with MSMA (Bueno 6, or Daconate, or a similar product). This will burn the foliage of the weed and disrupt the plant. Bermudagrass will take this temporary discoloration when applied correctly. I have been told by die-hards that by applying 3-5 drops of either 2,4D, or Roundup (Doomsday) in the center of the leaf whorl will knock nutsedge for a loop for a while. Now is the time to knock it for that loop! Imazaquin (sold as the product IMAGE) and halsulfuron (sold as the product MANAGE) can also be used. Image should be allowed to dry on the leaf for 2-3 hours, and then watered in. IMAGE will kill all cool season grasses. Apply this on bermudagrass lawns only! IMAGE is less effective on yellow nutsedge. MANAGE is safe on all turfgrasses and is effective on both yellow and purple nutsedge. MANAGE needs to dry on the leaf for as long as possible (24 hours). MANAGE can be applied by certified landscape applicators only. Check the label for reseeding (mid-high elevation locations) or overseeding (low elevation) time intervals.

Another common weed is spurge. The low growing spurge plants can dominate any lawn in the summer time. If you have a history of spurge and did NOT apply a pre-emergence herbicide for it in the spring, chances are you can have a good crop of it by late summer. Unlike nutsedge, spurge is a summer annual weed which will start to die out in the late summer/ early fall. Spurge plants have milky sap --white sticky stuff inside the stems when you snap them. Wear a thin rubber glove. Some people are allergic to this milky latex-like sap. Should you control it? It depends how long the summer is. In Yuma, spurge will stay until early November. In Flagstaff it's caput by late August. In Kingman mid-September and in Tucson and Phoenix it's done its thing by mid-September. However, it has made lots of seed for next year already. The plants slow way down in growth and turn red or yellow, which signals the naturally occurring hasta-la-vista for this year. If you want to control it, apply a 2,4D compound (or similar phenoxy type), which will stunt it out of existence for the rest of this season. If you use MSMA, the spurge will turn bright red or purple. At higher elevations (perennial ryegrass, tall fescue, Kentucky bluegrass country) Turflon ester will knock it out real good. Do not put Turflon ester on bermudagrass. As always, be careful when applying broadleaf weed control agents in a mixed landscape. Avoid drift and use the amine salt formulation instead of the ester, if broadleaf plants are nearby (landscape or agricultural). If you treat it now or not, it will be back next year again, especially if you do not plan on using a pre-emergence application next spring. Pre-emergence is the way to go for spurge control.

Crabgrass, southwest cupgrass and foxtail can be prevalent grassy weeds and may contribute as a substantial part of an ill-maintained lawn at this point. All of these grasses are summer annuals. They germinate in the late spring and die in the fall. Crabgrass and foxtail are found at the mid and higher elevations in Arizona, while cupgrass predominates at the lower elevations. All these grasses will die in the late summer (night time temperatures first decrease and day length becomes shorter). Do I control them now ? If they exist in large patches, they have squeezed out the desirable turfgrass spp., or have colonized an area were turf has not grown. To kill them now in the above case may result in a sparse looking lawn, with dead grassy weeds leftover! If these weeds are interspersed between the desirable species, than you can treat with MSMA or DSMA. These grasses will turn purple, then yellow, and then fade away. Check the label for any reseeding or overseeding restrictions. It make no sense to treat these weeds post emergence as you get closer to fall (late summer in high elevation locations). Once again, these guys will be back! Pre-emergence in the spring followed by making a dense healthy lawn in the future is the best bet to long term control.

Products mentioned do not infer endorsement, nor does non - inclusion of similar or other suitable products infer exclusion. Products are included as examples only. Always read label instructions.

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