Crabgrass is a common, yet frustrating summer annual weed that completes its entire life cycle within a single growing season. Understanding the specific timing of its reproduction and germination is foundational for effective lawn management. Since this weed reproduces solely by seed, the goal of any control strategy is to interrupt this cycle by preventing the current generation from seeding and dormant seeds from sprouting.
When Crabgrass Plants Mature and Produce Seed Heads
Crabgrass plants spend the peak summer months focused on vigorous vegetative growth, expanding into low-lying, spreading clumps. This growth phase continues until environmental conditions signal a shift toward reproduction. As the days begin to shorten in mid-to-late summer, typically starting in July and August, the plant’s resources are redirected to seed production.
This reproductive stage is marked by the appearance of seed heads, which are visually distinct from the rest of the plant. These finger-like spikes, or racemes, emerge from the tips of the stems, carrying the next generation of seeds. Production of these seed heads continues until the first hard frost of autumn.
A single, well-established crabgrass plant is capable of producing an astonishing volume of seeds, often exceeding 150,000 throughout its reproductive period. This prolific output explains how a small patch of the weed can rapidly become a widespread issue in subsequent years. Even plants mowed at low heights can successfully generate and disperse viable seeds. Once seed production is complete and temperatures drop, the entire plant dies, turning a reddish-brown color with the arrival of frost.
How Seeds Are Dispersed and Remain Viable
After the seeds mature in late summer and fall, they drop directly onto the soil surface. They are then spread by various mechanisms across the lawn and surrounding areas. Seeds lack specialized structures for long-distance travel, but their high density in the soil makes them easily moved by external forces like water runoff, wind, and the movement of garden tools or lawn equipment.
Human activity, such as walking across an infested area, can also inadvertently transport seeds on shoes to new parts of the yard. This continuous dispersal ensures that the problem area expands each year if the plants are allowed to mature. Once on the ground, the seeds enter a period of dormancy, allowing them to survive the cold winter months.
These dormant seeds contribute to the “seed bank”—a reservoir of viable seeds stored within the soil. Crabgrass seeds are resilient and can remain viable and capable of germination for at least three years, and potentially longer, even when buried. This longevity means that controlling the weed requires preventing the current year’s seeding and addressing the large supply of dormant seeds waiting in the soil from previous seasons.
Soil Temperature and Next Season’s Germination
The most important factor determining when the new generation of crabgrass sprouts is the sustained temperature of the soil, not the air temperature. Germination is triggered when the soil temperature at a shallow depth (one to two inches) consistently reaches 55°F to 60°F for three to five consecutive days. This precise temperature requirement ensures the weed emerges reliably each spring, often starting in early to mid-spring depending on the local climate.
The initial burst of germination occurs when the soil first hits the 55°F threshold, but the process continues throughout the spring and into the summer. Areas that warm up more quickly, such as those with sandy soil, full sun exposure, or borders next to pavement, will see the first seedlings emerge. The most significant amount of germination occurs as soil temperatures climb higher, sometimes reaching 70°F.
Turf professionals and gardeners use natural indicators to help time the application of pre-emergent herbicides, which must be applied before the soil hits the germination threshold. The blooming of the forsythia shrub is a common visual cue, as its bright yellow flowers often appear when the soil is nearing 55°F. Applying a pre-emergent herbicide at this time creates a chemical barrier in the top layer of soil, killing the seedlings shortly after they sprout.
A late application, after the soil has reached the required temperature, will not prevent early-germinating seeds from establishing themselves. The volume of seeds produced by the previous year’s plants directly influences the density of new seedlings that emerge, making successful control reliant on precise timing based on soil temperature readings.