Crabgrass is a warm-season annual grass and one of the most common and persistent weeds in turf. Its success is due to a highly efficient, dual spreading strategy. The plant uses both a massive seed-based reproductive cycle and aggressive lateral growth to rapidly colonize thin or bare areas in a lawn. This allows a single plant to dominate its immediate surroundings and guarantee widespread re-infestation the following spring.
The Seed Production Cycle
The primary mechanism for long-term spread is the prolific production of seeds. Since crabgrass is an annual plant, it dies completely with the first hard frost of autumn, meaning the subsequent year’s population must emerge from seeds produced during the previous season. A single, mature plant can produce an immense volume of seeds, ranging from 15,000 to 150,000 under optimal conditions. This massive output happens primarily in late summer and fall. These seeds drop directly onto the soil surface, forming a seed bank that can remain viable for several years and ensuring that weed pressure will continue for multiple seasons.
Methods of Seed Dispersal
Once shed from the parent plant, crabgrass seeds are moved to new locations through various physical vectors. Mechanical dispersal is effective, often occurring when seeds hitch a ride on gardening tools or lawn maintenance equipment, such as lawnmowers. Failing to thoroughly clean a mower after cutting an infested area can easily transfer seeds to a previously unaffected section of the lawn. Natural forces also play a significant role in distributing the seeds; light seeds can be carried by wind or washed by water runoff during rain or irrigation. Additionally, the movement of people, pets, and animals can inadvertently pick up and transport the small, sticky seeds on shoes, clothing, or fur.
Spread Through Lateral Growth and Fragmentation
Beyond its reliance on seeds, the existing crabgrass plant spreads aggressively during the growing season through a process called tillering. Tillering involves the development of lateral shoots that branch out from the base of the main plant near the soil surface. These stems sprawl horizontally, radiating outward from the central crown in a dense, mat-like pattern, which gives the plant its crab-like appearance and allows it to choke out surrounding turfgrass. The prostrate stems can develop roots at the nodes wherever they make contact with the soil, allowing the plant to anchor itself firmly and absorb nutrients from a wider area, rapidly increasing its footprint within a single season. Physical disturbance, such as hand-pulling, can sometimes leave behind a small root fragment or a rooted node capable of re-establishing the plant.
Conditions That Promote Successful Spreading
Successful germination and establishment of new crabgrass plants depend on a specific set of environmental conditions. The most important trigger is soil temperature, as seeds begin to sprout when the temperature of the upper inch of soil consistently reaches 55 to 58 degrees Fahrenheit for several consecutive days. Optimal germination occurs when soil temperatures are sustained between 60 and 70 degrees Fahrenheit. The presence of bare or thin patches of soil is another necessary condition for successful spreading. Crabgrass seeds require light to germinate, so a dense, healthy lawn canopy that shades the ground will naturally inhibit sprouting. Weakened or sparse turf provides the open soil and direct sunlight the seeds need to establish a foothold.