Poa annua, commonly known as annual bluegrass, is a widespread and persistent grassy weed that often plagues turf areas. As a cool-season annual, it quickly establishes itself in lawns and golf courses during cooler months. It becomes noticeable due to its lighter green color and prolific seed heads, which are often visible even in closely mowed turf. The majority of Poa annua plants die when exposed to sustained summer conditions. This die-off is a predictable part of its life cycle, leaving unsightly brown patches in the turf when the heat arrives.
The Impact of Summer Heat on Poa Annua
The reason Poa annua perishes in summer is its low tolerance for heat and drought stress. As a cool-season plant, its optimal growth occurs when air temperatures are between 60 and 75 degrees Fahrenheit. When consistent air temperatures climb above 85 degrees Fahrenheit, the plant’s physiological functions break down rapidly.
This susceptibility is compounded by the plant’s shallow, fibrous root system. Unlike desirable turf grasses, Poa annua cannot root deeply to access moisture from lower soil layers. When soil moisture is depleted by summer heat, the plant quickly succumbs to desiccation. Turf managers sometimes use controlled drying to deliberately kill the weed, which exploits this weakness.
It is important to note that Poa annua exists in different biotypes. While the true annual type (Poa annua var. annua) reliably dies, a less common perennial biotype (Poa annua var. reptans) may survive prolonged heat. This perennial form can persist through summer dormancy, especially in highly irrigated areas like golf greens where constant moisture mitigates the heat stress. For most residential lawns, however, the common annual bluegrass will die off, leaving bare spots.
Understanding the Annual Bluegrass Life Cycle
The life cycle of annual bluegrass dictates its appearance and disappearance from the lawn. It is classified as a winter annual, meaning its most active phase occurs outside of the summer months. Germination begins in the late summer or early fall when soil temperatures drop below the critical threshold of 70 degrees Fahrenheit.
Once the seed germinates, the small plant establishes itself and survives the winter as a low-growing plant. Throughout the cooler months, it grows slowly while preparing for its reproductive phase. Rapid growth and seed production occur in the early spring as temperatures begin to rise.
This spring phase is when the weed becomes highly visible, producing numerous whitish-green seed heads, even when the grass is mowed short. A single plant can produce hundreds of seeds, which can remain viable and dormant in the soil for several years. The plant completes this cycle before summer heat arrives, ensuring a new crop of seeds is ready to germinate later in the year.
Timing Control Measures for Prevention
Effective control of Poa annua relies on preventing the germination phase, as the plant is difficult to remove once established. Since the weed dies in summer and germinates in the fall, intervention must happen before the seeds sprout. This involves applying a pre-emergent herbicide in the late summer or early fall.
The timing of this application is determined by soil temperature, not the calendar date. Pre-emergent herbicides should be applied just before the soil temperature consistently drops below 70 degrees Fahrenheit. This application creates a chemical barrier in the soil that stops the seeds from successfully sprouting.
Cultural practices also support chemical control by discouraging germination. Maintaining a healthy, dense turf reduces the open space and sunlight needed for seeds to take hold. Avoiding excessive watering and reducing soil compaction create an environment less favorable for this invasive annual. Proper fertilization is also important to ensure the desired turf grass remains competitive.