Kentucky Bluegrass (Poa pratensis)

DESCRIPTION: Most of the cool season grasses that begin growing early are not native to Wisconsin prairies. Bluegrass can be distinguished vegetatively from other early grasses by its narrow blade, which is V-shaped in cross section, and by the leaf tip, which is shaped like the bow of a boat. Kentucky bluegrass is distinguished from Canada bluegrass by the shape of the stem. In Kentucky bluegrass the stem is round; Canada bluegrass has a flat stem.

Many of the other cool-season European grasses (brome, timothy, orchard grass, quack grass, etc.) have similar growth habits and can be controlled using the techniques discussed below.

Picture of Kentucky Bluegrass

Kentucky Bluegrass
(c) John M. Randall/The Nature Conservancy

DISTRIBUTION AND HABITAT: Kentucky bluegrass was previously thought to be indigenous to the mountains of the Northeast along the Canadian border. However, it was introduced as a cultivar from Europe, and has been bred into multiple cultivars since its introduction. Because of its extensive use for lawns and in pastures, it is common in most grasslands, even those managed for native species. Canada bluegrass is also naturalized from Europe. Their effects on the natural systems are equivalent and therefore should be treated as one problem.

Kentucky bluegrass is a common lawn and pasture grass. It is found in meadows, fields, and semi-open woods throughout the United States. Canada bluegrass is often mistaken for Kentucky bluegrass, but is distinguished by forming extensive sods in dry, sterile soils (especially acidic soils) that cannot sustain the more common Kentucky bluegrass. Kentucky bluegrass is usually found on more mesic and fertile soils, although it will grow on dry neutral or alkaline soils.

LIFE HISTORY AND EFFECTS OF INVASION: Because bluegrass grows early in the season (when most other species are still dormant), it can spread very quickly. However, its shallow root system makes it susceptible to high soil temperatures and low soil moisture.

In spring, leaf growth accelerates after temperatures in the upper 1/2 inch of topsoil exceed 50o F. Production increases during April and peaks in May when soil temperatures average 60o - 64o F. Production declines when soil temperatures exceed 80o F and does not significantly resume (even with abundant soil moisture) until soil temperatures drop in late summer. Bluegrass has successfully invaded both remnant and restored prairies, savannas, and barrens. Establishment can be attributed to intentional introduction, past mowing, grazing, or cessation of fire. Kentucky bluegrass has the ability to replace prairie grasses that are shaded for half of the day. If left unattended, bluegrass can out-compete native prairie grasses and forbs, and will dominate shaded areas resulting from woody species invasions.

Picture of Kentucky Bluegrass

Kentucky Bluegrass
(c) John M. Randall/The Nature Conservancy

CONTROLLING BLUEGRASSES
Mechanical Control:
A controlled fire can dramatically reduce bluegrass in a native or planted prairie, savanna, or barrens. Fire will also set back the woody species whose shade encourages the proliferation of cool-season grasses. In southern Wisconsin, a late April or early May burn will destroy three to eight inches of new growth. Timing of burns may change on a year-to-year basis depending on weather conditions. Observing bluegrass growth is essential for effective control by burning. Fire is most effective when bluegrass is three to eight inches high. Burning at this time kills new growth and removes accumulated leaf litter. Burning off the moisture-retaining blanket of leaf litter increases stress on the shallow-rooted bluegrass by exposing the darkened surface to the sun. This helps reduce the competitive ability of bluegrass by encouraging summer dormancy and decreasing the chance of flowering and seed production. The effect is most pronounced on dry prairies and barrens. Burning can reduce bluegrass by more than 90%, but it is rarely 100% effective. Burning at the right time also improves the competitive advantage of native, warm-season grasses and forbs. Native species emerge later and benefit from the elimination of duff and a darkened soil surface.

When converting areas dominated by cool-season grasses into prairie, it is helpful to reduce the grass cover and seed bank before planting native seeds. This can be accomplished by any combination of tilling, smothering the grass, or applying herbicide. Till several times a year for at least one season to expose the seed bank and prevent further growth of the grass sod. Herbicide use followed by a season of tilling is also effective. On small sites, grasses can be killed by covering with black plastic or layers of newspapers during the growing season.

Chemical Control: Herbicide use is not recommended to control bluegrass on grasslands or savannas where there are native prairie plants. However, herbicide may be required on severely degraded areas or where prairie restoration is beginning. In such cases, the herbicide glyphosate has proven effective when used according to label applications.

More Information on the Web (link exits DNR)
Kentucky Bluegrass
- Wisconsin State Herbarium

Last Revised: September 3, 2004