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Our Newest #1 Turf Weed

Actually, it’s not new, because it’s been around for at least 15 years, but it’s growing in its ability to ruin the perfect sward. It’s called “European bluegrass”, or “roughstalk bluegrass”, which are polite names for Poa trivialis. Most of us just call it, “triv.” It is a bona fide cultivated bluegrass variety, often found in “shady mixtures” (not ours!), and it’s a frequent contaminant in Kentucky bluegrass seed, and even turf-type fescue seed. Because of it’s floppy habit and lime green color, it looks particularly offensive in a nice sward of dark green Kentucky bluegrass.While “triv” is horrible in bluegrass-based swards, it’s not quite as noticeable in turf-type fescue because the fescues are a medium shade of green, and thus, their colors match better than the dark green of bluegrass. The picture below shows a small patch of triv in Jeff’s front yard. The dark green, low growing species is Kentucky bluegrass (Poa pratensis).

Poa trivialis

In the “good ole days” yellow nutsedge enjoyed the distinction of being the “worst weed” in St. Louis. But nutsedge can be controlled very effectively with halosulfuron (ProSedger®). Unfortunately, there are no selective herbicides that remove triv from cool season turf species. Jeff has tried three different herbicides, with reputed efficacy against this sum beaching weed, but the triv just laughed them all off. Botanically, Kentucky bluegrass and triv are very closely related (kissin’ cousins), and thus, no herbicide can eliminate one without harming the other. Roundup® will kill both, though.

This nasty weed grows very rapidly in cool, cloudy weather. It will grow a week or two later into the late fall than Kentucky bluegrass, and it’ll commence growth earlier in the spring than either bluegrass or turf-type fescue. It will go dormant after just 3 to 4 weeks of stressful hot and humid weather, thus it ensures its survival as soon as cooler, wetter weather returns (September and October here in STL). It spreads very rapidly by rhizomes, and can make very large patches in short order. When that large patch goes dormant in July and August it’ll leave a huge bare spot in your sward. UGLY, UGLY, UGLY! This grass tolerates short mowing, too, and you may notice that it looks really crappy after you’ve mowed at higher mowing heights of the summer. It has a “floppy” habit…not capable of standing erect like our bluegrass and fescues, at our mowing heights (>2 inches).

There’s another bluegrass weed that you shouldn’t confuse with Poa trivialis, called “annual bluegrass (Poa annua), often referred to as “Poa”. Golfers in particular hate this weed because it’s a serious problem on putting greens. It’ll actually interfere with the path of the putt, thus decreasing the golfer’s accuracy on the green, increasing their handicap…and we all know that golfers tend to take their frustrations out on the greenskeeper! This weed isn’t the same as triv, but it’s just as much of a pain in the ass. Annual bluegrass tolerates compacted soils and spreads itself very effectively via seed production. It’ll grow in those areas that you frequently scalp with your mower or string trimmer. It can grow in compacted soil, such as along a driveway, where your wife repeatedly runs her front tire along the grass (despite your best effort to provide remedial instruction…aka, “training”). It’s not nearly as ugly in Kentucky bluegrass as triv, but when it dies in August it can leave brown patches. The products we use for preemergence crabgrass control can prevent the germination of annual bluegrass, but the homeowner can’t purchase the products that will provide control of existing clumps (“postemergence control”).

The shot below (again, Jeff’s yard) shows the visible difference between these two Poa weeds. Triv is distinct in its filthy lime green coloration, and it’s ability to spread rapidly. Annual bluegrass is distinct because it grows in small patches (at first) and produces its seed heads before any other turf species. It has a dark green color than triv, but a lighter green color than Kentucky blue. Triv is in the foreground of the picture, annual bluegrass in the background (with seed heads).

Poa trivialis (foreground) and Poa annua (background)

If you want to do something about triv, your ONLY OPTION IS THE NUCLEAR OPTION. You have to press that big red panic button early spring to late spring, before the heats pushes it into dormancy. You have to spray your entire yard with Roundup® herbicide. You can’t just spray the spots, because you don’t know how far the rhizomes have spread underground, PAST the patch. Wait about 4 weeks, and spray it again. You can reseed 1 week after your second application of Roundup, but be sure to use a certified seed blend that is free from Poa trivialis. Reseed at a light rate (dependent upon the species) and plan to reseed in September, too. You need to know that seeding in the spring and summer usually doesn’t work too well here in St. Loiuis…generally speaking, that turf won’t make it through August because the heat stress and diseases can destroy young turf. Around Labor Day plan to reseed you can reseed heavily with a quality seed blend.

Remember, don’t go nuclear on triv if it’s not cool and moist, because you’ll be very disappointed if you don’t kill 100% of it. Even 98% control is a failure, because little sprigs of triv will spread to become larger patches. If you don’t get it done by the end of May, chances are that you’ll miss some of it because some sprigs will be dormant with the summer heat, escaping your nuclear warfare.

A true story: Jeff went nuclear 6 years ago, in an attempt to rid his front and side swards of triv. It was a reasonably cool June, but he made the mistake of timing his first spray in mid-July, because the triv appeared to be growing vigorously. (BTW, his wife was very pissed, thinking him a moron for killing what appeared to be a decent lawn–at least to her untrained eye). Jeff sprayed a second time in August, and then reseeded in early September with some very expensive bluegrass. Things looked good in November, when the turf finally went dormant. However, just as the new bluegrass started to thicken the next spring, the triv roared though the new sward, and consumed his yard, stronger and thicker than before.

Moral of the story: Go nuclear in March or April! Be sure your wife is on board, first!

(Note: This post was originally published on July 11, 2012)

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