Last week, I had a lot of calls and pictures sent to me from people concerned about a strange, pale green grass growing in their lawn. The grass became noticeable when it began to sprout seedheads. The offending grass is named Poa annua, otherwise known as annual bluegrass.
Poa annua is a strange weed for homeowners to wrap their brains around. It is a winter annual, meaning that it germinates in the late summer or early fall once soil temperatures fall below 70 degrees F. The seedlings grow in the fall, overwinter in a vegetative state, then flower and produce seed in late spring and early summer.
Annual bluegrass produces a lot of seed. An individual plant is capable of producing more than 360 viable seeds. Mowing will not solve the issue, because flowering and seed production can occur at any mowing height. And once seed is produced, it may lie dormant in the soil for many years before germinating.
Much of what we know about Poa annua comes from golf courses, where the turf managers have fought it for 50-plus years. It thrives is compacted areas. In my lawn, I see it along our gravel drive, and in trafficked areas where people, pets and the lawn tractor travel. As it got better established, it outcompetes regular bluegrass is semi-shaded or dry areas.
Poa annua is a very adaptable plant. We now know there are also perennial types of Poa annua that will live through the stress of the summer heat, primarily in northern Illinois and Indiana.
Of course, the question every homeowner asks is “what can I spray on it to kill it now?” I wish it were that simple. Once annual bluegrass seeds, the “annual” version dies back quickly, so spraying does little more than give you some feeling of revenge. A better tactic is to wait until fall and use seed germination preventers.
Timing is everything with preventers. Apply a preventer too late, and the seed has already sprouted. Herbicides must be applied in early fall (early-September) prior to Poa germination. A second application will be needed in November or March to control spring germinating Poa annua. This technique may take many years to reduce the Poa annua populations and it will not be effective on the perennial type of Poa annua.
The most effective combination of treatments is to let the lawn go dormant from drought, followed immediately by application of a preemergence herbicide. The drought will kill the existing annual bluegrass and the preemergence herbicide will prevent it from re-germinating, but it will not prevent the desired turf from greening up again. This is most effective in lawns with less than 10% annual bluegrass. This strategy drives lawn enthusiasts crazy, because the lawn is not green and lush during the heat of summer, something they take pride in.
If you go online, you will find some herbicides labeled for use on growing Poa annua. Those products are available for professionals, not homeowners. If you hire a professional for this job, you must understand timing is absolutely critical, or a great deal of money and time can be wasted.