Crabgrass is a recognizable and frustrating weed problem for homeowners maintaining a healthy lawn. This aggressive, warm-season annual grass is not native to North America, but it has established itself across the continent. It successfully exploits thin, stressed turf and quickly colonizes bare soil patches, making it a ubiquitous nuisance in lawns and gardens. Understanding the plant’s nature and biology is the first step toward effective management.
Identifying Crabgrass and Its Origins
The crabgrass genus, Digitaria, includes several species, with large crabgrass (Digitaria sanguinalis) and smooth crabgrass (Digitaria ischaemum) being the most common types. These plants are distinguished by their pale green color and low, spreading growth habit, forming dense mats close to the ground. The common name comes from the way the stems radiate outward from a central point, resembling a crab’s legs.
Large crabgrass has long, sparse hairs on its leaf blades, while smooth crabgrass is smaller and has fewer hairs, often only near the base of the leaf. Both species produce distinctive seed heads consisting of two to six slender, finger-like spikes clustered at the top of the stem. Crabgrass originated in Eurasia and was introduced to the United States in the mid-1800s, initially as an animal forage crop, before escaping cultivation and becoming a common weed.
Is Crabgrass Truly an Invasive Species
The term “invasive species” has a specific ecological definition, referring to a non-native organism that causes significant environmental or economic harm to natural ecosystems. While crabgrass is non-native and aggressive in a horticultural context, it is not typically classified as a true ecological invasive species. It is more accurately defined as an agricultural or garden pest, or a noxious weed.
Crabgrass thrives in disturbed, cultivated environments like lawns and agricultural fields, competing fiercely with desired plants. Its harm is primarily economic and aesthetic, creating an eyesore in manicured turf and reducing crop yields. The plant rarely threatens the biodiversity or function of natural, undisturbed habitats. Its impact is concentrated on man-made landscapes rather than fragile natural ecosystems.
The Life Cycle That Makes Crabgrass So Aggressive
The success of crabgrass is rooted in its annual life cycle and reproductive biology. As a warm-season annual, the plant completes its life cycle within a single year, germinating in the spring and dying with the first hard frost in the fall. This process is driven by heat, making it a formidable competitor during summer when cool-season turfgrasses are under stress.
Germination depends on soil temperature, beginning when the soil reaches a sustained temperature of 55°F to 60°F for several consecutive days. This timing allows it to emerge when conditions are unfavorable for many turf varieties, giving it a competitive advantage. Crabgrass is a prolific seed producer; a single mature plant can generate up to 150,000 seeds. These seeds remain viable and dormant for years, creating a persistent seed bank. The plant’s prostrate growth habit is also aggressive, as its stems root at the nodes when they touch the soil, creating dense mats that choke out surrounding turfgrass.
Effective Strategies for Control and Prevention
Managing crabgrass involves a combination of cultural practices and targeted chemical treatments. Prevention is easier than control and starts with maintaining a dense, healthy lawn that leaves no bare soil for seeds to germinate. Mowing at a higher setting, typically two to three inches, shades the soil and keeps it cooler, inhibiting seed germination.
Deep, infrequent watering encourages the deep root growth of desirable turfgrass, while light, frequent watering favors the shallow root system of crabgrass. For chemical prevention, pre-emergent herbicides are applied in early spring, before the 55°F soil temperature threshold is met, creating a chemical barrier against germination. If the weed has already emerged, post-emergent herbicides are most effective when the plants are small, typically in the two- to five-leaf stage.