Rough Bluegrass (Poa Trivialis): A Menace to Lawns!

The Problem

This grassy weed is invading area lawns and is rapidly becoming a serious lawn problem. It tends to form dense patches in the turf during cool, wet weather. When the light intensity and temperatures rise during the summer months, these patches turn brown as they go dormant. However, the dormant areas will revive in the fall as temperatures moderate.

These off-color patches are frequently misdiagnosed as being the result of disease or insect activity. At this time, there is no selective control for this weed, although Lawn Pro is currently involved in efforts to develop effective controls.


Poa Trivialis (PT), or Rough Bluegrass as it is often called, is grown for seed production in the Pacific Northwest, as is 90% of the Bluegrasses, Tall Fescues, Ryegrasses, etc., whose seeds are sold for lawn planting. PT is commonly found in many shady grass mixtures, which are sold throughout the country in retail lawn and garden centers. PT is also used to overseed Bermuda golf greens in the South for Winter play. There is a lot of this grass seed being produced!

Prior to about five years ago, there was no weed problem in area lawns involving PT. Since that time, the infestation of this weed in area lawns has gradually increased until it is now a fairly common occurrence. Where did it come from? There are several possibilities.

Sources of Contamination

Most grasses produce seed and the smaller the seed, the easier it can be distributed by the wind. For example, crabgrass and foxtail are annual grasses that produce seed whose distribution is wind borne. This is why we must apply pre-emergent products to ensure that these grassy weeds do not infest our lawns. PT seeds are small and if any local patches are allowed to go to seed, windborne distribution could spread this weed far and wide.

Birds may also be a vector for infestation in two ways. They may ingest the seeds which then pass on through the digestive tract without damage and are deposited on turf areas in the droppings. Also, the birds may pick up runners or stolons of PT for nesting material. These runners could be accidentally dropped on a lawn as the bird flies back to its nesting site.

Another possible source of lawn infestation is from desirable grass seed contaminated with PT seed. As you may know, all packaged grass seed is labelled. Unfortunately, the label will not list PT as being present unless the label is on a bag of seed to which PT has been added as a definite component. This is why many shady grass mixtures will have the PT percentage listed on its label. If the PT is a contaminant and not a desirable component, we have a very different situation.

Grass Seed Labelling

According to state law, certain weeds are designated as Noxious Weed. The Kansas Department of Agriculture has a list of the Noxious Weeds whose presence must be listed on the label of any seed being sold within the state. Since PT has not been designated as a Noxious Weed in Kansas, its presence as a specific contaminant is not required on the seed label. However, there is another heading listed on the label: Other Crop. This category includes the total sum of all other grass seeds that are grown as crops and which are also present as a contaminant in the seed of choice. Thus, you may find a small percentage of Ryegrass seed in a bag of Tall Fescue seed or in a bag of Bluegrass seed. This particular contamination is common and does not present a problem because the grasses are all compatible. A more difficult situation is found when Orchardgrass seed (used to seed pastures) contaminates Tall Fescue seed. Orchardgrass is not compatible with Tall Fescue - its presence in a lawn sticks out like a sore thumb! Fortunately, Orchardgrass is eliminated by regular mowing within two to three years.

If PT is known to be present as a contaminant in a bag of Bluegrass seed, its weight percent will be a part of the total listed as Other Crop. However, there is no indication on the label as to the identification and percentages of the various constituents of Other Crop. You just cannot tell if PT is present!

Current Status

Two years ago, after we became aware of the PT problem, we began buying only seed that listed Other Crop at 0.0%. This is often hard to locate. In point of fact, this approach may not solve the contaminated seed problem. PT is a member of the Bluegrass family and PT seed very closely resembles good Bluegrass seed, which means that a seed analyst might underestimate the percentage of PT seed in a batch of good Bluegrass seed.

All of this basically means that no lawn service can guarantee that the seed planted is 100% free of contamination. We strive to use the best seeds available, but this is no guarantee that problems won't occur.

Even if 100% pure seed could be guaranteed, there are still the other potential sources of contamination discussed earlier, plus the possibility that the current or previous homeowner purchased and planted contaminated seed in the lawn.

We are continuing to investigate novel approaches to eradicate the PT already present in the turf. As you might appreciate, we are exploring uncharted territory with the outcome of any particular attempt being highly uncertain. We are continuing our collaboration with Dr. Bruce Branham of the University of Illinois, who is actively researching the challenge of selective PT eradication. Eventually, we hope to control this grassy pest.