When people search for information on weeds, they are often confronted with the reality that some of the most common weeds are grasses. This overlap between desirable turf and unwanted invaders stems from the highly contextual nature of the term “weed.” The question of when a grass becomes a weed is not a matter of botanical classification but one of human perception and location. Understanding this distinction is important for successful management of a lawn or garden space.
Defining a Weed: The Principle of Location
The definition of a weed in horticulture is entirely subjective, referring to any plant growing where it is not wanted. This principle of location means that a plant’s designation as a weed is based on human intent, not its species. A desirable grass species can easily become an unwanted plant when it invades an area dedicated to another purpose. This definition is focused on a plant’s competitive behavior and its interference with human-managed environments.
When Turfgrasses Are Considered Weeds
Turfgrasses valued for lawn uniformity, such as Kentucky Bluegrass or Tall Fescue, are considered weeds when they appear out of place. This often occurs when one type of turfgrass invades a lawn composed of a different species. For example, aggressive, warm-season Bermuda grass (Cynodon dactylon) becomes a weed when it spreads into cool-season Zoysia grass, creating a noticeable difference in texture and color.
The invasion of non-lawn areas also causes desirable grasses to be classified as weeds. When the runners (rhizomes or stolons) of a lawn grass spread into a flower bed, vegetable garden, or even cracks in a sidewalk, they compete for resources and disrupt the intended design. These desirable species are then treated as weeds, often requiring non-selective herbicides or physical removal. The issue is their presence outside the boundary of the maintained turf.
Common Grass Species Classified as Weeds
Beyond the issue of location, certain grass species are almost universally designated as weeds due to their aggressive growth habits or incompatibility with managed lawns. These species are often annuals or perennials with a different texture, color, or growth rate than the desired turf. They compete intensely with the intended grass for water, nutrients, and light, ultimately compromising the lawn’s uniformity and health.
Crabgrass
Crabgrass (Digitaria spp.) is one of the most common offenders, a summer annual that germinates when soil temperatures reach 55°F to 60°F and produces thousands of seeds before dying off in the fall. It is identified by its coarse, light-green color and low-growing, sprawling, clump-forming habit.
Goosegrass
Goosegrass (Eleusine indica), another summer annual, germinates slightly later than crabgrass, thriving in compacted soils and areas subject to low mowing. It forms a tough, dense clump with a distinctive silvery-white color near its base, making it highly noticeable in a manicured lawn.
Annual Bluegrass
Annual Bluegrass (Poa annua) is a winter annual or short-lived perennial that thrives in moist, shaded conditions. It is recognized by its light-green color, fine texture, and prolific, white, pyramid-shaped seedheads that appear in the spring. While it often dies out in the heat of summer, its rapid seed production ensures it returns the following cool season. This rapid reseeding makes it a persistent problem for lawn managers.
Note on Sedges
Some common “grassy weeds,” such as Nutsedge (Cyperus spp.), are technically sedges, not grasses, but are grouped with grassy weeds due to their similar appearance and management challenges.