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1995, No. 2
AUTUMN CONTROL OF BROADLEAF WEEDS IN TURF
Poa trivialis - A Pest on the Rise?
Recently, I have received several weed samples that contained rough (roughstalk) bluegrass (Poa trivialis).
Additionally, in conversations with turf managers from northern Illinois, as well as from other northern U.S. areas, I have been informed that this grass seems to be an increasingly common contaminant in seed mixes.
Rough bluegrass can be desirable when planted in wet, shaded settings, but it is normally considered to be undesirable when it invades other turfgrasses due to the yellow-green patches it forms.
Why is rough bluegrass becoming a more common intruder?
Currently, in Illinois, we are experiencing cool, wet weather that favors the development of Poa trivialis.
On a larger scale, the increase in rough bluegrass problems can perhaps be attributed to two reasons.
First, more Poa trivialis seed is being produced in the Pacific Northwest for use as a winter overseeding species in the southern U.S.
Because more seed is being produced, more seed is available to contaminate other seed lots and fields.
Second, there is difficulty in separating Poa trivialis seed from that of other species, especially Kentucky bluegrass (Poa pratensis).
Reportedly, seed laboratories vary in ability to distinguish between the two species.
The Plant
Rough bluegrass is a perennial, stoloniferous grass that forms dense, yellow-green patches, especially in moist, fertile, fine-textured soils.
The sheath is purple or green, rough, and split.
Its vernation is folded and the membranous ligule is large and either pointed and entire or somewhat toothed (see illustration).
There are no auricles. The blades have a rough margin, boat-shaped tip, and glossy underside.
Poa trivialis is intolerant of dry, warm weather, although it can survive these conditions by going dormant.
Reports also indicate it is less tolerant of postemergence broadleaf herbicides than Kentucky bluegrass.
Controlling Poa trivialis
There are no labeled selective controls for rough bluegrass; the use of nonselective controls, such as glyphosate, is recommended.
Be sure to read, understand, and follow all label directions before using any pesticides.
When trying to mechanically eliminate patches of Poa trivialis, be sure to remove all stolons that may be moving into desirable turf.
When purchasing seed, buy only high-quality seed with small quantities of crop seed and inert material, and deal only with reputable producers and dealers.
Unfortunately, even under the most demanding production conditions, trace quantities of Poa trivialis seed may be present in seed lots, so obtaining a lot that is totally free of this seed may be extremely difficult.
Finally, if you have questions dealing with seed testing (e. g., seed purity or germination rates), call the Illinois Department of Agriculture Seed Laboratory at 217-785-8487.
(TOM VOIGT)
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