What Is Annual Bluegrass?

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Annual bluegrass (Poa annua) stands as a prevalent and persistent weed in lawns, presenting challenges for homeowners and turf managers alike. Recognized for its fine-textured, bright green blades, this cool-season grass species disrupts the uniformity of lawns with its bunch-type growth habit and distinctive V-shaped seed heads. 

The presence of annual bluegrass often signals the need for targeted management strategies, as its aggressive nature can compromise the aesthetic appeal and health of a well-maintained lawn. Come learn all you need to know about annual bluegrass so you can keep your turf defended against it!

Identifying Annual Bluegrass

Annual bluegrass looks identical to many types of turfgrass, especially before it matures and develops a seed head. It has narrow, light green blades that are slightly hairy or completely smooth, which roll up in the heat of the day. This rolled or "canoe" shape is one of the most reliable indicators of annual bluegrass, as it will likely differ from many types of desirable turfgrass. Spotting bunches or clumps in your lawn that appear lighter and more narrow than the surrounding grass is a good place to start when trying to identify annual bluegrass. 

It grows between 6 and 12 inches tall at a much quicker rate than most turfgrass. The seed heads of annual bluegrass are very fine and delicate, often appearing feathery or cottony in both color and texture. These seed heads tend to be white or faintly light green in color, and they will appear on the plant in mid-to-late spring. Below are just a few key points to keep in mind when trying to spot annual bluegrass in your lawn:

  • Brighter/lighter in color than surrounding grass.
  • Taller than surrounding grass.
  • Smooth leaves with canoe-shaped tips.
  • Leaves are spread somewhat far apart.
  • The seed head is very fine and cotton-like.
  • Similar in appearance to fields of oat or cotton.
  • Growth happens in a bunching pattern.
  • Shallow roots can be seen if you pull the weed.

Annual Bluegrass VS. Kentucky Bluegrass

Differentiating annual bluegrass from the surrounding turfgrass can be difficult, especially if the lawn is mostly made up of Kentucky bluegrass (pictured above). Annual bluegrass has much narrower leaves than Kentucky bluegrass. The leaf blades of annual bluegrass also have a rolled appearance, while the leaves of Kentucky bluegrass are flat. The roots of annual bluegrass are very shallow, while the roots of Kentucky bluegrass can grow up to 6 feet deep. Shallow roots are a notorious feature of most grassy lawn weeds, as they allow the weeds first access to nutrients and water in the soil. 

Kentucky Bluegrass:

  • Perennial
  • Produces a seed head
  • Deep green color
  • Cool-season, desirable grass  
  • Grows at a normal rate
  • Deeper, healthy roots

Annual Bluegrass:

  • Annual
  • Produces a seed head
  • Lighter green color 
  • Undesirable in all climates
  • Quick growth crowds lawns
  • Shallow, invasive roots

What Does Annual Bluegrass Do To Lawns?

Annual bluegrass can cause a variety of issues in lawns, ranging from a temporarily unsightly appearance to permanent damage. This grassy weed grows quickly and has shallow roots that make it difficult for your lawn to stay healthy or full. Annual bluegrass competes for resources and space with other grass in the area, and these shallow roots allow it to gain first access to nutrients and moisture in the soil.

As a cool-season grassy weed, annual bluegrass requires higher amounts of nutrients when its seeds try to germinate in lawns during hotter weather. Unfortunately, annual bluegrass is also able to thrive in cool-season lawns when most turfgrasses are going dormant. As a result of these factors, lawns are often left discolored, thin, and patchy due to a lack of adequate nutrition and/or space to grow.

Life Cycle Of Annual Bluegrass

Annual bluegrass will begin to germinate in late summer or early fall, when temperatures begin to cool down for the year. Once soil temperatures get down to 70 degrees or below, annual bluegrass seeds will begin to germinate. From germination, the seedlings will emerge in fall, go dormant during winter, and finish their life cycle the following spring, when temperatures get warmer. In late spring or early summer, annual bluegrass will flower and set seed, starting the cycle all over again for this annual grassy weed.

Annual bluegrass is commonly found in all types of turfgrass, including Kentucky bluegrass, tall fescue, and ryegrass, and it is common in lawns all across the country. This weed can be found in both sunny and shady areas of a lawn, but it prefers partially sunny areas with moist soil. Lawns with open, direct sunlight may have a better chance of withstanding annual bluegrass because it prefers indirect sunlight. This weed is commonly found in areas of a lawn that have been damaged or disturbed. Compacted and patchy lawns are also commonly home to annual bluegrass, as this weed thrives in areas that see heavy traffic.

How Does Annual Bluegrass Spread?

Annual weeds do have enough time to develop overly complex root systems by which they can effectively spread an invasion through rhizomes and stolons, so they have to rely on seed production and distribution. Seeds can be dispersed by things as simple as foot traffic, irrigation, wind, and more. A single annual bluegrass plant is able to produce over 100 seeds in as little as 8 weeks, and each plant can produce over 350 seeds in a single season. Once seeds are distributed, dormant seeds can stay viable in the soil for up to 5 years.

How To Prevent & Treat Annual Bluegrass

Treating annual bluegrass in your lawn starts with good lawn care practices. Prevention is a more effective form of weed control than corrective measures, so make sure you call a lawn care provider to apply pre-emergent herbicides in fall, before seedlings emerge. Applying products containing bensulide, dithiopyr, and prodiamine can be effective at controlling annual bluegrass, but there are other ways to prevent the weed. Keep the following tips in mind, and be sure to call SKB for all your commercial weed control needs in Georgia!

  • Mow High: Scalping your lawn allows annual bluegrass to gain a stronger hold and develop more seeds.
  • Water Deeply & Infrequently: Annual bluegrass loves moisture, so avoid overwatering.
  • Aerate In Spring & Fall: Annual bluegrass thrives in compacted soils, which can be alleviated by aeration.
  • Overseed: Applying seed to thin patches in your lawn can help your lawn stay lush, green, and ready to overpower annual bluegrass!