The first problem is that both fields are contaminated with approximately 30% annual bluegrass and the second problem is that the Kentucky bluegrass I seed each year in worn areas germinates but just seems to get smothered by the annual bluegrass.

Poa annua: double whammy in Detroit

by Dr. Dave Minner, Iowa State

Q: I ‘ve got two problems with my field and need some advice, but don’t use my name because I’m embarrassed and don’t want my boss to think that it’s my fault. I mow my Kentucky bluegrass football field at 1.5 inches and my baseball field at 1 inch. We routinely topdress and aerify the football field; I guess we have about 65% sand by weight to the depth of the 3-inch aerifier tine.

The first problem is that both fields are contaminated with approximately 30% annual bluegrass and the second problem is that the Kentucky bluegrass I seed each year in worn areas germinates but just seems to get smothered by the annual bluegrass. On the baseball field some patches of Poa annua are 10 feet across.

How do I get rid of the annual bluegrass and get more of the good Kentucky bluegrass back in my fields without killing everything with Roundup and starting over?

Double Teamed in Detroit

First, don’t feel embarrassed. Golf courses have been plagued with this problem for years and now we are seeing increased Poa annua invasion on sports fields that are mowed close, irrigated, and worn thin just when the annual bluegrass is establishing in late summer and early fall. The good news is that mesotrione, trade name Tenacity, has been recently labeled for use on golf courses and sod farms and is currently being considered for commercial lawns and athletic fields.

Tenacity will certainly be useful for cool-season athletic fields because it has pre and post emergence activity on crabgrass and annual bluegrass. It can also be used at time of seeding; that is very convenient because of the constant overseeding and inter-seeding that is required to keep fields producing continuous turf cover. Here are some strategies to control Poa annua with Tenacity and Roundup, while using higher nitrogen rates to speed Kentucky bluegrass establishment:

High school and minor league baseball fields that are infested with annual bluegrass and complete their season in early September can be aggressively inter-seeded with Kentucky bluegrass and then treated with Tenacity at time of seeding. As an extra measure of control, large patches of annual bluegrass devoid of Kentucky bluegrass can be treated with Roundup one day before or after Kentucky bluegrass seeding.

Our research trials have demonstrated that Roundup or Tenacity, applied within a day of seeding Kentucky bluegrass, will not have any negative impact on Kentucky bluegrass seed emergence. A month after Kentucky bluegrass seedling emergence repeat applications of Tenacity can be applied every 10 days not to exceed an annual application rated of 16 fluid ounces per acre. Repeat applications of Tenacity are necessary for effective control of annual bluegrass.

After the first application Poa annua turns yellow, by the second it is white, and after the third application plants are shriveled and brown. Post emergence control of annual bluegrass does not work in the spring and summer because the annual bluegrass seems to outgrow the herbicide affect. Tenacity applied in the fall usually gives complete kill by spring because the injured annual bluegrass plants are not able to survive the winter.

The Major League Baseball season can extend into October so aggressive inter-seeding of Kentucky bluegrass and visible bleaching of the Poa annua from Tenacity treatment needs to be considered if applications are made during the playing season. Another consideration in baseball is that by the start of the spring baseball season in April the condition of the field will be the same as it was at the end of the growing season in November (i.e., dead patches of Tenacity killed annual bluegrass or dwarf Kentucky bluegrass plants still visible in drill rows). This leads to the second part of your question that involves how slowly Kentucky bluegrass establishes during the fall growing season.

In high traffic areas of the field where grass is going to wear out, seed early and seed heavy with Kentucky bluegrass even if there appears to be 100 percent turf cover when the football season starts in September; better yet seed in late August. What we have also learned is that you need to really feed the establishing Kentucky bluegrass plants to make them grow faster, tiller more, and ultimately cover more ground. The usual rate of 1 lb N/1000 sq. ft./month is just not enough to get the job done with such a short establishment season and in the presence of competing traffic. Instead our trials show (thanks to a tip from Tom Verips, CGCS who was using high nitrogen rates to quickly grow-in Kentucky bluegrass at the new Otter Creek Golf Course) that 4, 6, and even 8 lbs N/1000 sq. ft. from urea applied in split applications produced more turf cover as nitrogen level increased.

To force establishment after seed germination apply 0.5 to 1.0 lb N/1000 sq. ft. every 7 days until the end of October anywhere that you are pushing Kentucky bluegrass to fill in from fall seeding. So, if you’re double teamed by Kentucky bluegrass that is too slow and annual bluegrass that is too aggressive on your baseball and football fields, then add Tenacity and extra nitrogen to the team in the fall.