Annual bluegrass (Poa annua) is a common weed on golf courses, but is now also becoming a problem on higher mowed turf areas such as lawns and athletic fields. This grass is lighter colored than Kentucky bluegrass and perennial ryegrass and can be identified by its boat-shaped leaf tip and membranous ligule (see Turf Tip on its identification). Annual bluegrass is especially noticeable in May and June because of its prolific seedhead production (Fig. 1).
Poa annua is a winter annual that germinates in the late summer/early fall once soil temperatures fall below 70 °F. Seedlings mature in the fall, overwinter in a vegetative state, and produce seed in late spring and early summer. Annual bluegrass is a prolific seed producer. An individual plant is capable of producing more than 360 viable seeds. The seed may lie dormant in the soil for many years before germinating. Annual bluegrass flowers and produces seed over several months and at any mowing height. It grows well under short days and cool conditions, and it will out-compete most other turf species during late fall and early spring. This grass will typically thin and die out during the heat and drought of summer in Indiana (Fig. 2) although there are perennial types that can survive the summer stress. We are starting to see summer decline of annual bluegrass now as our daily highs start to warm. This may be more problematic in 2011 than normal as many turf areas thinned by last summer’s drought were invaded by annual bluegrass last fall and this spring.
Chemical control of annual bluegrass can be attempted with either preemergence herbicides and/or with a postemergence herbicide called ethofumesate (Prograss). Ethofumesate is applied mainly as a postemergence herbicide, but it exhibits some residual preemergence control. Ethofumesate can be applied to Kentucky bluegrass and perennial ryegrass lawns. Two or three applications of ethofumesate applied between September and November are recommended per year. The applications should be approximately four weeks apart. Results are rarely seen that fall; but are usually observed the following spring. Other research at Purdue has looked at fall applications of mesotrione (Tenacity) as a possible alternative for postemergence control of annual bluegrass. Our research at Purdue suggests that Tenacity can reduce annual bluegrass populations which is consistent with the label language that states “Avoid spraying these turf types (annual bluegrass included) unless control and/or injury can be tolerated.” However, the label stops short of recommending this product for postemergence annual bluegrass control. Read more about this research at Purdue (page 18). Both ethofumesate and mesotrione are general use pesticides meaning that homeowners can purchase these products; however, their limited availability (sold only through professional supply companies) and packaging size (more product than needed for a single lawn) typically means that these products are either not readily available to homeowners or are too expensive for most homeowners. Thus, homeowners are encouraged to hire a professional to make these applications.
Most preemergence herbicides (crabgrass preventers) and Tenacity are labeled for preventative (preemergence) control of annual bluegrass. Application timing is very important, so herbicides must be applied in early fall (late-August or early-September) prior to annual bluegrass germination. A second application will be needed at the normal crabgrass preemergence timing in March to control spring germinating annual bluegrass. This technique may take many years to reduce the annual bluegrass populations and it will not be effective on perennial types of annual bluegrass.
Fig. 1. Annual bluegrass that germinated in a poor quality lawn in the fall, overwintered, and then produced seedheads in the spring.
Fig. 2. Annual bluegrass turning yellow and then brown from summer heat stress.
Aaron Patton, Assistant Professor/Turfgrass Extension Specialist