Crabgrass, a common summer annual weed, is a persistent problem for many homeowners seeking a pristine lawn. Its rapid spread often leads to the mistaken belief that it uses underground or above-ground stems, known as runners, to aggressively colonize turf. Understanding the actual mechanics of how this weed grows and propagates is the first step toward effective and long-term control. Managing this pervasive plant hinges on recognizing its life cycle and unique spreading mechanism.
Understanding Crabgrass Growth: Tillers vs. Runners
The short answer is that crabgrass generally does not have true runners (stolons or rhizomes), which characterize perennial grasses like Bermuda grass. True runners are specialized stems that establish new, independent plants at their nodes, allowing a single plant to spread across a large area. Crabgrass, specifically Large crabgrass (Digitaria sanguinalis) and Smooth crabgrass (Digitaria ischaemum), spreads primarily through seed and a process called tillering.
Tillering involves the production of multiple side shoots, or tillers, from the plant’s central base, resulting in a dense, mat-like clump. A single crabgrass plant can produce hundreds of tillers, creating a rapidly expanding circle of growth often confused with runners. This growth habit is prostrate, meaning the stems hug the ground, allowing the plant to evade mower blades and protect its growing points.
Although the plant does not use true runners, it can develop roots at the lower nodes of its stems, especially in the large crabgrass variety, contributing to its stability and ability to form small colonies. This characteristic, combined with its C4 warm-season metabolism, allows it to thrive in the heat of summer when desirable cool-season grasses are stressed. The dense, basal branching and low-growing stance of the tillers create a smothering effect on the surrounding turf, leading to the perception of an invasive weed.
The Annual Life Cycle of Crabgrass
Crabgrass is a summer annual, meaning its entire life cycle—from germination to seed production and death—occurs within a single growing season. The plant’s life begins in the spring when soil temperatures consistently reach 55 to 60 degrees Fahrenheit for four or five consecutive days. This germination window marks the beginning of the annual infestation, often occurring before desirable lawn grasses have fully emerged from dormancy.
Once germinated, the plant enters a period of rapid vegetative growth throughout the summer, characterized by prolific tillering that creates its dense, clumping appearance. A single, mature crabgrass plant is capable of producing up to 150,000 seeds, which are shed in the late summer and fall. The plant’s growth slows as days shorten, and it is killed entirely by the first hard frost, leaving behind a brown patch in the lawn.
The seeds dropped in the fall do not germinate immediately but enter the soil’s seed bank, where they can remain viable for several years. This massive seed production and long-term viability ensure that the weed problem is recurring, even if the current year’s plants are successfully eliminated. This annual cycle of seed production, dormancy, and spring germination makes timing crucial for successful prevention.
Timing and Methods for Effective Control
Since crabgrass spreads exclusively by seed, the most effective control strategy is prevention before germination occurs. Pre-emergent herbicides are designed to form a chemical barrier in the top layer of the soil, killing the seedling as it attempts to sprout and root. This product must be applied just before the soil temperature reaches the 55-degree Fahrenheit threshold, which often correlates with the blooming of local plants like forsythia.
If the application window for pre-emergent products is missed, post-emergent herbicides become necessary. These products are sprayed directly onto the growing plant and are most effective when the crabgrass is young, ideally before it develops more than a few tillers. Once the plant has matured and produced multiple tillers, it becomes significantly more difficult to control, often requiring multiple applications and higher rates.
Maintaining a dense, healthy lawn also plays a significant role in prevention, as thick turf shades the soil surface. Keeping the soil cooler and darker inhibits the germination of crabgrass seeds, which require light and warmth to sprout. Mowing at a higher setting, generally three inches or more, helps achieve this shading effect, creating an environment less favorable for the weed.