Crabgrass (Digitaria species) is a widespread weed. Its ability to reappear every spring, often in the same spots, leads many to mistakenly believe they are battling a persistent, long-lived plant. This recurring presence creates confusion about its true life cycle, which is a significant factor in why it is difficult to control.
The Defining Feature: Annual Life Cycle
The most common types of crabgrass, such as large crabgrass (Digitaria sanguinalis) and smooth crabgrass (Digitaria ischaemum), are classified as summer annuals. This means the individual plant lives for only one growing season. The process begins with seed germination in the late spring or early summer after the soil warms up. The plant enters a period of rapid vegetative growth throughout the hottest months, often outcompeting cool-season lawn grasses. As days shorten in late summer and early fall, the plant shifts its energy to the reproductive stage, producing numerous seed heads. The entire plant is killed by the first hard frost, confirming its classification as an annual weed.
Why It Appears to Be Perennial
Crabgrass gives the illusion of being a perennial because of its prolific seed production and the resulting long-term soil seed bank. A single, mature plant can produce anywhere from 150 to over 150,000 seeds in a single season. These seeds drop into the soil and lie dormant over the winter, ready to sprout the following spring. The seeds can remain viable in the soil for at least three years, constantly replenishing the seed bank that ensures a new crop of weeds emerges annually. The new plants that appear each spring are not the original plant regrowing, but new individuals germinating from the dormant seeds left behind. Germination is triggered when the soil temperature at a shallow depth consistently reaches between 55 and 60 degrees Fahrenheit for several consecutive days.
Control Strategies Based on Timing
Knowing the annual life cycle of crabgrass makes the timing of control efforts essential. The most effective strategy is to use a pre-emergent herbicide, which creates a chemical barrier in the top layer of the soil to kill the seeds as they begin to sprout. This barrier must be applied before the soil temperatures reach the 55-degree Fahrenheit threshold, which often corresponds to the time when plants like the forsythia bush are in full bloom.
If the pre-emergent application is missed, or if new plants emerge later in the season, post-emergent herbicides must be used. These products are applied directly to the foliage of the visible plant and are most effective when the crabgrass is still young, typically in the two-to-five leaf stage. Once the plant matures and begins to set seed in late summer, applying post-emergent controls is less impactful because the plant has already ensured the continuation of the infestation for the following year. Breaking the cycle of reseeding over several seasons is the ultimate goal in managing this annual weed.