MTC TURF NEWS: Peter H. Dernoeden, Ph.D., Professor Emeritus, University of Maryland

Rough bluegrass (aka Poa trivialis, Poa triv., roughstalk bluegrass) is an extremely common perennial grass weed in lawns and golf courses in the mid-Atlantic. Rough bluegrass looks like Kentucky bluegrass in that leaves have a boat-shaped tip and there are two parallel lines down the middle of upper leaf surfaces. Unlike Kentucky bluegrass, RBG foliage has a yellow-green color. As noted below, however, from late autumn to late winter RBG leaves are darker green and blend into lawns composed of tall fescue and/or Kentucky bluegrass. It is mostly in spring and summer that RBG foliage appears more yellow-green. Hence, RBG often goes unnoticed for long periods, especially when plant numbers and sizes are small.

Rough bluegrass did not become a major problem in turf until the 1980’s. The reason is that RBG was being developed as the answer to growing turf in wet and shaded areas. It was introduced in shady seed mixes, which still exist today, and it can be a contaminant in non-certified seed. Rough bluegrass is now recognized to be an extremely noxious and invasive perennial weed in all cool-season turfs in our region.

Tufts of plants are easily detached or rolled back from lawn surfaces, exposing long (3” to 5” long), white tillers that trail close to the surface (i.e., procumbent). These white tillers behave like runners or stolons, but younger ones do not usually produce roots. Eventually, as one tiller trails over the next, the lower runners root and behave like stolons. The underside of RBG leaves usually is shiny, but this characteristic may or may not be evident. Further, Kentucky bluegrass produces an abundance of seedheads (usually in May); whereas RBG produces few seedheads, usually in late May and June and sometimes sporadically throughout summer. Conversely, the underside of Kentucky bluegrass leaves are not shiny; there are no long, white trailing tillers or stolons, and plants produce an abundance of blue-green seedheads-usually in May. The most reliable distinguishing characteristics of RBG are its yellow-green color in spring and summer and its white trailing tillers. Trailing stems enable one to easily detach tufts or roll back a mass of plants easily, thus exposing rooted stolons underneath the canopy. Rough bluegrass produces far fewer seedheads than Kentucky bluegrass, which appear in late May following the flush of annual bluegrass and Kentucky bluegrass seedheads. Unlike Kentucky Rough bluegrass plants cannot be darkened substantially by applying nitrogen fertilizer or iron. For most homeowners, RBG is highly objectionable, and when summer hits it can become a lawn nightmare. For LCO’s, it is an intractable problem. Rough bluegrass usually is most invasive in shaded and wet areas, but spreads rapidly into sunny areas, including non-irrigated lawns.

What makes RBG so noxious is that its trailing tillers and stolons out-compete the more desirable tall fescue and Kentucky bluegrass. I have seen lawns in mixed sun and shade environments to have up to 75% RBG cover. A major problem with RBG is that it rapidly turns brown during very dry periods, even during a spring drought. During summer, when there are heavy rain events, followed by sunny and hot weather, RBG develops a bronzed appearance and abruptly collapses and mats in sunny areas of the lawn.

Matting of shoots is a sure sign that too much water combined with heat was involved in the browning and/or apparent death of RBG. What is difficult to explain to home owners is how heavy rains play a major role in its demise in summer. After all, RBG is highly adapted to wet soils. Very wet soils (including sandy soils) persisting for several days in sunny and hot weather accumulate much more heat than well drained areas. Furthermore, warm water contains very little oxygen. Basically, a sunny, hot and wet environment “cooks” or “scalds” RBG causing plants to turn bronze or brown, collapse and matt. Conversely, cool to cold water holds more oxygen, does not accumulate a lot heat and thus does not cause problems in RBG during spring and autumn. While RBG foliage appears dead, it is NOT to be. All of the leaves and tillers may be brown and dead looking, but tiny buds on stolons and stem bases invariably survive wilt-induced dormancy and scald events. Where water puddles, however, the heat can kill all plants, however there invariably are plants close by that survive, recover and spread into voids. To be sure, simply collect several of the brown trailing tillers and stolons, and carefully peel off all of the dead tissue. Using a hand lens, look for either some green tissue underneath brown sheaths and/or for small, white buds (smaller than a pinhead) on stems. Recovery usually becomes evident by November, but sometimes a dense RBG cover does not appear until Spring

Management

As previously noted, the striking yellow-green presence of RBG in the spring and summer cannot be effectively masked with iron or nitrogen. Once RBG has been summer stressed (especially by drought or scald) and dead-looking, the best approach to improve the appearance of collapsed RBG lawn areas is to power-rake and otherwise physically remove the dead RBG tissue and re-seed or sod bare areas with tall fescue or other desirable species. Please appreciate that this will not eliminate the basic problem, but enough tall fescue may be able to compete and mask the problem.

Rough bluegrass cannot be safely and effectively removed with any herbicide currently on the market. There are a few herbicides that can phytotoxically suppress RBG, but turf injury is usually unacceptable, and RBG invariably survives. PoaCure (methiozolin) is an herbicide that has good activity. PoaCure, however, is only legal to use on golf courses and normally multiple applications are required for good results. For LCO’s –there are no good long-term options. You do, however have a fighting chance when only a few small plants are evident and before any seed has been produced by spot-treating RBG plants with Round-up (glyphosate), or remove them by digging, and re-seed or sod into dead spots.

Where RBG dominates, the recommendation would be to renovate with Round-up (glyphosate). The harsh reality is that one application of Round-up will not kill all stolons. Waiting 30-40 days between Round-up applications would help, but hardly would be practical. Even if you had spectacular results with a Round-up renovation, RBG invariably survives as seed or maybe an odd bud on a stolon. The only recourse is to renovate and then to vigilantly physically or chemically remove plants as they re-appear. Simply stated, RBG is an exasperating and intractable problem for homeowners and LCO’s. Until an effective and safe herbicide is commercialized for lawns, RBG will continue to be a major lawn nightmare.

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