Key Information

Poa annua

Annual bluegrass (Poa annua) is thought to have originated from a hybrid of Poa infirma (weak bluegrass) and Poa supina (creeping meadow-grass) that occurred approximately 2.5 million years ago in the interglacial ice ages of Europe.

Annual bluegrass is widespread around the world. Its presence has been observed on all continents, including Antarctica; though, it is most prominent in temperate climates.

Annual bluegrass is typically classified as an annual comprised of numerous biotypes or “populations.” Many of these biotypes are also capable of perenniating, meaning that they may exist in a vegetative state – producing viable seed throughout the year.

Though perennials are much less common than the annual biotypes, they tend to occur in frequently mown or grazed scenarios in temperate climates with adequate year round moisture. In fact, some of the most lauded golf greens in the world are composed of annual bluegrass, including: Pebble Beach, Oakmont, and the more recently converted Chambers Bay.

Annual bluegrass has historically been an important weed of many, if not most, commodity and specialty crops. The extensive reliance upon herbicides as the primary means of control has led to almost overwhelming presence of herbicide resistance. There are very few commonly utilized herbicides that annual bluegrass has not developed resistance to – albeit often in isolated or unique populations.

The growing concern among many turfgrass managers and scientists is that we may no longer have effective chemical means of controlling annual bluegrass.

Principles of Herbicide Resistance

Herbicide resistance can have severe economic consequences for turfgrass managers and producers by affecting aesthetics, playability, surface stability, and yield. In order to control resistant weeds, turf managers may choose to increase their input costs by purchasing alternative herbicide chemistries and investing in additional labor to culturally and mechanically control resistant weed populations.

Repeated use of herbicides with the same site of action will select for resistant plants. These uncontrolled plants will reproduce, leading to an increased prevalence of resistance within a local population. That population may move off-site as seed on equipment or athletic attire.

Controlling resistant populations can be aided by understanding what herbicides they may be resistant or susceptible to. But ultimately, chemical weed control with multiple sites of action will be more effective than repeatedly relying on the same failed treatment.

It is also well known that larger plants are much less susceptible to postemergence herbicides. Treatments prior to tillering are most effective, and when possible, treatment when temperatures and light conditions are conducive to active growth and herbicide uptake may also aid in control.

Mechanisms of resistance are simply how plants are able to tolerate or recover from herbicide application. Common mechanisms include:
• Target site resistance occurs when the intended herbicide binding location undergoes a conformational change in structure. This change prevents or reduces the binding of a herbicide. Most populations have very low occurrence of this structural “defect”. In fact, these binding sites that are structurally different from that normally present in the natural population may have a negative consequence on the plant’s health, referred to as “fitness penalty” or “fitness cost.”
  It is the process of repeated selection using the same herbicide site of action that expands this trait across an entire population, leading to full blown resistance.
• Non-target site resistance may occur as a result of decreased herbicide absorption or translocation, as well as changes in plant metabolism.

Best Management Practices

Effective herbicide resistance management requires a balance of chemical and non-chemical tactics, including:
• Decreased reliance upon chemical weed control and increased diversity of traditional cultural practices, such as selection of appropriate turf species for the scenario or environment (e.g. aesthetic, use, or management capabilities). The emphasis should be on providing a dense and competitive turfgrass sward. A holistic approach of managing soil and plant health is required in order to do so.
• When herbicides are used, diversify sites of action between years and within seasons.
• Emphasis on both preemergence followed by early-postemergence means of control, which effectively reduces the amount of annual bluegrass exposed to postemergence strategies.
• Use labelled rates at appropriate timings and stages of growth.
• Use multiple herbicide sites of action (SOAs) with overlapping spectrums of weeds controlled.
• Scout after application and avoid allowing weeds to go to seed or proliferate vegetatively.