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Annual Bluegrass (WA) — Poa annua — Family: Poaceae — Grass

© Copyright 2004-2021, Ronald Calhoun.
© Copyright 2004-2021, Ronald Calhoun.
© Copyright 2004-2021, Ronald Calhoun.

Annual bluegrass is unique among weeds. There is probably no other weed that is so widely adapted to variations in mowing height, site conditions and cultural practices.

Annual bluegrass is the most common and widely distributed grassy weed in the world. It is mentioned as a weed in nearly every plant commodity.

Turfgrass management professionals, including golf course superintendents, sports field managers, sod producers, and lawncare operators, have spent years trying to eradicate annual bluegrass from their turf swards. Annual bluegrass (Poa) is one of the most invasive weeds in turfgrass stands. It is also one of the most difficult to control.

Efforts to find chemical controls for Poa have been thwarted by its diverse genetic make-up. Poa is officially described as a cool-season winter annual. Winter annuals are plants that germinate in late summer to early-fall, overwinter, and produce seed in the spring. Typical winter annuals die soon after seed production as daytime air temperatures increase.

Poa annua, although commonly referred to as annual bluegrass, is actually a diverse group of different biotypes with varying characteristics. Annual bluegrasses in warmer climates like the southern U.S., do indeed perform as a typical winter annuals. These "annual" bluegrasses are classified as Poa annua var. annua L. Timm. In the northern part of the U.S. and much of Canada there are biotypes that produce seed in the spring and then continue to grow as perennials. This somewhat peskier bluegrass is termed Poa annua var. reptans (Hauskn) Timm.

The fun doesn't stop there. Somewhere between true bunch-type annual bluegrass and stoloniferous [perennial] annual bluegrass are hundreds if not thousands of different biotypes.

Clearly, identifying controls that have excellent activity on annua, reptans, and everything in-between has been difficult for good reason. These biotypes are not just segregated by climatic region or area of the country. It is possible, in-fact likely, to have several biotypes of Poa on the same property. The segregation is not only determined by climatic zone, but also by management and cultural conditions such as irrigation, mowing height, and compaction.

Poa populations are so diverse that they can easily adapt to everything from unirrigated roughs to closely maintained putting greens. This diversity makes Poa a bit of a moving target. Predictable Poa control would likely exist if 100 percent of the Poa population was truly annual.

Objections to annual bluegrass are most often related to seed production (which can happen in any month in moderate climates), surface interference, color and disease susceptibility.