Facts on Poa Annua Grass

Annual bluegrass can be an annoying weed in warm-season turf lawns.

Forming large mats of green grass tufts during the cool moist weather from fall to spring, annual bluegrass (Poa annua) looks lush. The huge production of seeds and its tendency to yellow and look awful in the heat or drought of summer finds it undesirable for lawns and usually a pesky weed that pops up in garden beds. In fact, 15 American states list Poa annua as an invasive weed.

Origins

Annual bluegrass, sometimes called walkgrass, is native to Europe. According to the University of Alaska at Anchorage, this short-lived plant was introduced into North Africa, the Americas and New Zealand and Australia where it is today a common weed. It thrives in lawns, gardens, crop fields, pastures, roadsides and other recently disturbed soil areas. It is considered a pioneer plant, among the first to grow after soil is cleared or unmaintained.

Description

Annual bluegrass is a fast-growing grass that completes its life cycle in one growing season. Texas A&M University mentions two forms exist: one that makes a tufted clump and another that creeps, sending out little stolons or rooting stems outward from the intial plant. It flowers and develops seed heads, dropping them on the soil or scattering them by wind on bare soil. Once the seed heads ripen, the plants turn an uneven yellow to white with green coloration before dying.

Growth Tendencies

According to Purdue University, annual bluegrass germinates when temperatures drop below 70 degrees F, and depending on climate, can occur in late fall until early summer. Texas A&M University further clarifies this as nighttime temperatures in the 60s with daytime highs never more than 85 degrees F. It prospers and out-grows and out-paces other lawn grasses from late fall to early spring. The heat of midsummer stunts the growth, but it usually recovers in autumn and continues with vigor. The University also states one plant can produce upwards of 360 viable seeds in its lifetime. The grass flowers and sets seeds repeatedly over the growing season and its seeds may rest dormant in the soil for years.

Uses

In regions with cool, moist summers or during the fall to spring season, annual bluegrass can serve an useful purpose to cover barren soil to reduce wind and water erosion. This benefit may quickly be offset by the production of seeds that will proliferate years later and sprout weeds if the area's use changes. Overall, annual bluegrass is regarded as an undesirable weed in lawns, particularly low-cut types at golf courses.

Control

Any non-selective herbicide can be sprayed onto annual bluegrass but any spray drift will also affect nearby desirable plants. The key is preventative tactics, especially the use of pre-emergent herbicides that inhibit the germination of bluegrass seed. The main concern is eradicating viable seeds from the landscape over time through multiple approaches such as hand weeding of plants before they set seed teamed with pre-emergent herbicides to diminish the number of plants appearing in subsequent years. Texas A&M University also recommends cutting off irrigation and letting plants grow tall, as the dry soils and abundant leaf growth can naturally lead to the grass' demise before having the energy to flower and set seed. This is particularly useful in annual bluegrass weed control in warm-season turf grasses that tolerate drought.

References

• University of Alaska, Anchorage: Annual Bluegrass
• Purdue University: Control of Poa annua and Poa trivialis in Lawns
• Texas A&M University: Annual Bluegrass
• Grounds Maintenance Magazine: Managing Poa annua Greens