Crabgrass is a common grassy weed often seen in lawns. This article explores whether crabgrass is considered invasive and examines effective strategies for its control.
What is Crabgrass
Crabgrass, a warm-season annual weed of the Digitaria genus, is found across the United States. Its low-growing, sprawling stems resemble crab legs, giving it its name. Two common types are smooth crabgrass (Digitaria ischaemum) and large or hairy crabgrass (Digitaria sanguinalis).
As an annual, crabgrass completes its entire life cycle within a single growing season. It germinates from seed in spring, thrives through summer, produces new seeds, and dies with the first fall frost. Its leaves are typically wide, flat, and a lighter green than most desirable turfgrasses, often forming dense clumps.
Why Crabgrass is Invasive
Crabgrass is considered invasive due to its rapid growth rate and prolific seed production. A single plant can produce between 150,000 and 700,000 seeds in one season, which remain viable in the soil for several years.
Crabgrass seeds begin to sprout when soil temperatures consistently reach 55 to 60 degrees Fahrenheit, typically in early spring. It is highly adaptable, thriving in poor soil conditions, compacted areas, and drought, outcompeting many desirable turfgrasses. Furthermore, its prostrate growth habit allows it to spread across the ground, rooting at nodes where stems touch the soil, forming dense mats.
The Impact of Crabgrass
Crabgrass competes with desirable grass for water, nutrients, and sunlight, leading to weakened and thinning turf. This competition can create unsightly patches, particularly as the crabgrass turns brown and dies off in the fall, leaving bare spots.
Crabgrass thrives in hot, dry conditions when many cool-season turfgrasses are under stress, further exacerbating its competitive advantage. Its ability to spread quickly means it can rapidly dominate weakened areas of a lawn. Beyond lawns, crabgrass can also invade garden beds and other cultivated areas, where its growth can hinder the establishment and health of other plants.
Controlling Crabgrass
Controlling crabgrass involves both preventative measures and treatment strategies for existing plants. A healthy, dense lawn is the most effective defense against crabgrass, as it leaves less room and light for the weed to germinate and grow. Proper lawn care practices are fundamental to prevention.
Applying a pre-emergent herbicide in early spring is a crucial preventative step. These products create a chemical barrier in the soil, preventing crabgrass seeds from germinating and emerging. Pre-emergents should be applied when soil temperatures consistently reach 55 degrees Fahrenheit, typically before crabgrass begins to sprout. Many pre-emergent herbicides can inhibit the germination of new grass seed, so reseeding should be planned accordingly.
Proper lawn maintenance further supports prevention. Mowing your lawn at the correct height, usually between 3 to 4 inches, shades the soil and helps prevent crabgrass seed germination. Deep and infrequent watering encourages the deep root growth of desirable turf, allowing it to better compete against weeds. Appropriate fertilization promotes a thick, lush lawn that naturally crowds out crabgrass.
For existing crabgrass plants, post-emergent herbicides can be used. These selective herbicides target actively growing crabgrass without harming most established lawn grasses. Common active ingredients include quinclorac and mesotrione. Manual removal is an option for small infestations, especially when the plants are young and before they produce seeds. Remove the entire root system; moistening the soil beforehand can make this easier.