Crabgrass is universally considered an invasive and undesirable weed. In the context of horticulture, a weed is simply any plant growing in a location where it is not desired. Crabgrass is a ubiquitous presence in lawns and gardens, universally considered an invasive and undesirable grass that actively competes with cultivated turf. This aggressive annual grass is a primary target of lawn care programs across the country due to its ability to quickly dominate a yard.
What Defines Crabgrass as a Weed
Crabgrass belongs to the genus Digitaria, with large crabgrass (Digitaria sanguinalis) and smooth crabgrass (Digitaria ischaemum) being the most troublesome species found in home lawns. Its prostrate, or low-growing, nature causes it to spread outward from a central point, forming dense, coarse patches that quickly crowd out finer, desirable turfgrasses. The plant’s light green color and wide leaf blades contrast starkly with the uniform appearance of a manicured lawn.
This grass is highly opportunistic and thrives in conditions that cause lawn grasses to struggle, such as compacted soil, poor fertility, and low mowing heights. It is a warm-season grass, meaning it flourishes during the intense heat of summer when many cool-season lawn grasses are under environmental stress. The stems of larger crabgrass species can root at the nodes as they spread across the ground, further anchoring the plant.
Understanding the Crabgrass Life Cycle
Crabgrass is a summer annual plant, which means it completes its entire life cycle—from seed to seed—within a single growing season. The plant will die completely with the first hard frost in the fall. However, before it dies, a single plant can produce tens of thousands of seeds, which remain dormant in the soil over winter.
The germination phase begins in the spring when soil temperatures consistently reach 55 to 60 degrees Fahrenheit for several consecutive days. This period marks the window of opportunity for preventing new plants from emerging, as the seeds will continue to germinate from spring through late summer. Once germinated, the plant quickly enters its growth phase, utilizing the summer heat to rapidly mature and spread with its characteristic low-lying habit.
The final phase is reproduction, which occurs in mid to late summer as the days begin to shorten. The plant puts its energy into creating seed heads that resemble thin, branching fingers. These can produce up to 150,000 seeds per plant, guaranteeing a new crop for the following year.
Effective Control and Prevention Methods
Controlling crabgrass involves a two-pronged approach that combines chemical treatments with sound cultural practices to maintain a dense, healthy lawn. The most effective strategy is prevention, which relies on the precise timing of a pre-emergent herbicide application. These products work by creating a chemical barrier in the top layer of the soil, intercepting the germinating seeds before the seedlings can emerge.
The application of a pre-emergent product must happen before the soil temperature reaches the 55-degree Fahrenheit germination threshold, typically in early spring. Active ingredients like prodiamine, dithiopyr, or pendimethalin are commonly used to establish this barrier, which slowly dissipates over several weeks. If the timing is missed, a post-emergent herbicide containing ingredients like quinclorac can be applied to kill actively growing plants. Post-emergent applications are most effective when the crabgrass is still young and has fewer than five tillers, or side shoots.
The best defense against crabgrass is a thick, vigorous lawn that naturally chokes out weed seedlings. This is achieved through proper cultural practices:
- Raising the mowing height to at least three inches, which shades the soil and keeps it cooler, thereby inhibiting the light-dependent germination of crabgrass seeds.
- Regular, deep watering.
- Appropriate fertilization to maintain dense turf that outcompetes the invasive grass for light, water, and nutrients.