Better manage Poa annua

August 15, 2019 -  By
Photo: Corteva Agriscience

PROLIFIC POA Poa annua is one of the most common grassy weeds in the country and begins germinating in the late summer or early fall. (Photo: Corteva Agriscience)

Gil Del Rosario, Corteva Agriscience

Gil Del Rosario, Corteva Agriscience

One of the most common grassy weeds in the country is about to start germinating: Poa annua, also known as annual bluegrass.

We asked two experts, Gil Del Rosario, market development specialist for Corteva Agriscience Turf & Ornamental, and Ian Rodriguez, Ph.D., technical services manager for Quali-Pro, to offer advice for identifying and treating Poa annua.

Planning starts now: Germination begins in late summer or early fall. Warm days and cool nights indicate when germination is near and create the ideal environment for the weed, according to Del Rosario. When soil temperatures drop to 70 degrees F, Poa annua begins to germinate. It grows throughout the winter; seeding starts in the spring.

Cool is king: Although it’s known as a winter annual weed, perennial varieties exist. With an aversion to heat, perennial varieties are more likely to be found in cool-season grass-growing zones, Del Rosario says. The weed prefers cool, shady and wet areas, but it can thrive in full sun if the turf stand is thin, he says.

Don’t pick up hitchhikers: Poa annua is so widespread because it’s a prolific seeder. It is found in lawns, landscape beds and planted containers. Seeds transfer by shoes and mowers, Del Rosario says. Early application of a preemergent herbicide is important since the weed is most susceptible as a seed. With obvious white tufted seedheads, the weed can be identified in early spring and summer, Rodriguez says.

Avoid a mix-up: Unlike creeping bentgrass, Poa annua has a prominent midrib on the leaves. With a folded appearance, the leaves have boat-shaped tips. Watch out for a bunch-type grass that is a lighter green than desirable turf types, Rodriguez says.

Keep turf happy: As with all weed control, preventive methods and maintaining a healthy, dense lawn are crucial. Poa annua takes advantage of lawns with poor fertility, compaction and a thin turf stand. With Poa’s shallow root system, overwatering can promote growth. Rodriguez says a low mowing height will favor the grassy weed.

Photo: Corteva Agriscience

Photo: Corteva Agriscience

Controlling Poa annua

Even with healthy, desirable turfgrass, it’s important to not waste any time contemplating a control strategy.

Ian Rodriguez, Quali-Pro

Ian Rodriguez, Quali-Pro

Del Rosario and Rodriguez both recommend a preemergent strategy instead. Preemergent applications should be made right before soil temperatures reach 70 degrees F. “Be sure to allow for adequate time to make the application and then irrigate before the seeds begin to germinate,” Del Rosario says.

It is also important to plan for a spring re-treatment. “The spring application needs to be timed before the fall application starts to lose its residual control,” he says. Preemergent programs are the best approach, Rodriguez says. “Postemergent options are more limited in cool-season turf,” he explains. “But there are a number of selective postemergent control options available to clean up breakthroughs in warm-season turf.”

For preemergents, products that contain the following active ingredients can be effective: prodiamine, dithiopyr, pendimethalin, indaziflam, ethofumesate or simazine.

Del Rosario recommends using postemergent treatments as a backup plan for persistent populations of Poa annua.

Some active ingredients to look for in postemergents include: pronamide, metsulfuron, glufosinate, glyphosate, rimsulfuron or foramsulfuron — which are effective on dormant warm-season turfgrasses.

Application timing and changing modes of action are strategies that can minimize resistance when treating the weed.

Poa annua has shown the ability to develop resistance to virtually any herbicide group,” Rodriguez says. “So, combining modes of action is becoming a more common strategy when using preemergents.”

Rotate products with different modes of action, Del Rosario says. “Look at the mode of action number on the label of herbicide, and when switching products, make sure the number is different,” he explains. Before using any product, be sure to read and follow label directions closely.

Danielle Pesta

About the Author:

Danielle Pesta is the associate editor of Landscape Management. She started writing for the green industry in 2014 and has won multiple awards from the Turf & Ornamental Communicators Association (TOCA). She can be reached at dpesta@northcoastmedia.net.

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