Poa annua, commonly known as annual bluegrass and also called annual meadow grass and simply Poa, is globally the most famous of the turfgrass species. It is one of the five most widely distributed plants in the world (Fenner, 1985), found on all seven continents, including Antarctica. Annual bluegrass follows where human disturbance has occurred, whether it’s explorers to Antarctica or a baseball player walking onto a field. The global adaptability of annual bluegrass is due to its multiple survival strategies. Numerous ecotypes of annual bluegrass exist that adapt to the local region down to a golf course fairway or green or to an athletic field. The diversity that exists is reflected in the common saying, “If every herbicide or product that was labeled to control Poa annua was as effective as stated, Poa annua would be an endangered species.”

A characteristic of annual bluegrass is its ability to produce copious amounts of seed. The life cycle of annual bluegrass is that of a winter annual. Simply stated, the sole purpose of a winter annual weed is to complete its life cycle prior to the arrival of summer. Annual bluegrass germinates in the fall and begins to grow as quickly as it can, putting most of its resources into growth that will lead to seed production. The plant sacrifices sustainable growth – relatively low tillering and shallow root system – in an effort to produce seed in the spring. And, it can produce seed!

Annual bluegrass can produce 75,000 to 225,000 flowering tillers per square meter per year, resulting in 150,000 to 650,000 seeds per square meter per year. The kicker is that the seed can remain viable for several years, resulting in a persistent and viable seed bank in athletic fields. This results in the appearance of small patches of light green annual bluegrass in athletic fields. Although annual bluegrass germinates in fall, the patches are most evident in early to midspring.

Table 1: Various annual bluegrass chemical control products

Many athletic field managers think these small patches are due to disturbances to the turf from divots, which is true. For many weeds, the soil needs to be disturbed and the weed seeds exposed to light in order to germinate. In the case of annual bluegrass, it has the ability to germinate in darkness (McElroy et al, 2004). Thus, just a small amount of space in the turf canopy, or maybe none at all, and annual bluegrass can germinate and gain a foothold.

So far we’ve focused on the annual type, but there are also perennial types of annual bluegrass. In low-maintenance athletic fields the annual bluegrass is usually the annual type. However, as the cultural intensity of a field is increased (lower heights of cut, fertilization, irrigation, pest control, etc.) the annual type takes on a perennial nature. The perennial annual bluegrass type is classified as Poa annua var. reptans. Perennials are characterized by a more stoloniferous growth habit, produce more tillers and shoots, have a deeper root system, and produce fewer seed heads when compared to annuals.

The evolution from an annual type to perennial type of Poa annua that is often seen on athletic fields is:

  • The turf is first invaded/colonized by annual type Poa annua, which might be quite obvious in the spring, but dies out after producing seed in the summer.
  • Each year the patches reappear and become more evident.
  • Eventually, the perennial type Poa annua predominates the field.

However, one question that has never really been answered is: How does Poa annua evolve from an annual type that initially invades an athletic field or a putting green to a Poa annua that exhibits perennial-type characteristics?

A characteristic of annual bluegrass is its ability to produce copious amounts of seed.
Photo by Pamela Sherratt.

Reasons given for the rise in perennial types include the environment and cultural practices. Some research points to the rapid rate and turnover of tillers of annual Poa annua that could actually account for the perennial nature of greens. However, a study was done by LaMantia and Huff (2011) at Penn State University that shed light on how perenniality might arise. To better understand the perennial nature of Poa annua on greens LaMantia and Huff characterized both the annual and perennial type of phenotypes in their Poa annua collection.

Focusing on inflorescence differences, they found when crossing putting green type (perennial) by annual type that the subsequent progeny did not follow expected Mendelian ratios that would be expected with single gene characteristics. Furthermore, when green type (perennial) by green type (perennial) Poa annua crosses were conducted subsequent generations past the first generation resulted in annual type Poa annua.

The are many biotypes of Poa annua. Some are annual (left) and some evolve into more perennial types (middle and right).
Photo by Karl Danneberger, Ph.D.

Thus, the green type or perennial Poa annua is unstable. This could mean that the perennial type of Poa annua found on greens will revert back to annual types. The authors hypothesized that close mowing induces an epigenetic effect on gene regulation. This means the functionality of the DNA may be affected, giving the perennial type of biotype, but the underlying DNA is not changed. As long as the mowing stress is present, the Poa annua on greens will tend to exhibit the perennial traits, but once mowing is stopped, eventually the Poa annua would revert to an annual.

Is that cool or what? Poa annua can be whatever it wants to be. It changes or adapts to a form that is best suited to the conditions it’s managed under.

Given the adaptability to a variety of environmental and management conditions, it’s no wonder annual bluegrass becomes the predominant turfgrass species. There’s an ecological explanation for why annual bluegrass can dominate a turfgrass stand, the competitive exclusion principle (Gause’s Principle). The principle says that two similar species cannot co-exist, and one species will eventually be driven to extinction. Annual bluegrass possesses a plethora of adaptive weapons, including the ability to adapt to low mowing heights and intensive cultural practices, as well as traffic and light situations, making it a formidable opponent for the more desirable athletic field turfgrasses like Kentucky bluegrass and perennial ryegrass.

So, how do we control or manage this opponent? No one thing or practice is going to control annual bluegrass. The first question to ask is “Why is it there in the first place?” The answer will lead to corrective strategies to minimize reinfestation. You may be able to kill annual bluegrass, but if you do not change the reasons why it is there in the first place it will come back.

Making conditions less favorable for annual bluegrass growth and more favorable for the desired turfgrass is not easy and requires careful management. Extreme measures taken to rid annual bluegrass from a stand, such as not applying certain nutrients or eliminating watering, rarely succeed, since the practices do little to promote the desired turfgrass species. Finally, specific chemicals (herbicides and/or plant growth regulators) will be needed to minimize the competitive nature of annual bluegrass.

Cultural management of Poa includes handpicking, avoiding soil disturbance (i.e., core cultivation) during peak germination in the fall, and regular overseeding with desirable grasses. Chemical control options are being studied by Dr. John Street at Ohio State University. His preliminary results are summarized in Table 1.

Dr. Karl Danneberger has been a turfgrass professor at Ohio State University since 1983. Pam Sherratt is a sports turf specialist at Ohio State University and served on the STMA board of directors from 2010-2011.