Crabgrass (Digitaria species) is a common annual weed that completes its life cycle over a single growing season. This warm-season grass thrives during the heat of summer but cannot survive cold temperatures. The entire parent plant dies off completely in the fall. This annual cycle concludes with the arrival of the first significant cold event of the season.
Environmental Triggers for Crabgrass Die-Off
The death of the crabgrass plant is triggered by a drop in temperature that results in a hard frost. A hard frost is defined as a period when the air temperature falls to 32°F (0°C) or below for a duration of several hours. This prolonged exposure to freezing temperatures causes the water inside the plant’s cells to freeze and expand, rupturing the cell walls and killing the entire structure.
A light frost, where temperatures dip briefly near freezing, may only damage the outermost leaves or tips of the plant, but the core remains viable. Crabgrass lacks the physiological defenses to withstand a killing frost, unlike cool-season turf grasses that enter dormancy. The exact timing of this die-off is highly variable, occurring earlier in northern regions and later in southern climates due to the difference in when the first hard frost arrives.
Visual Signs of Plant Death and Decay
The immediate consequence of a killing frost is a rapid change in the plant’s appearance. The green foliage quickly loses its color and turns a noticeable straw-brown or reddish-brown. This sudden discoloration highlights the presence of the dead weed against the backdrop of a still-green or dormant lawn.
The dead crabgrass plant remains visible and structurally intact for a period following the frost before the process of decay begins. The dead, matted thatch will slowly break down over the winter months, gradually blending into the existing turf. Until it fully decomposes, the dead plant material can create bare patches or an uneven texture in the lawn.
The Survival Mechanism: Seeds Left Behind
The die-off of the parent plant marks the completion of its primary purpose: reproduction. Before the plant succumbs to the frost, it enters a reproductive phase, producing a massive number of seeds. A single mature crabgrass plant can produce between 100,000 and 150,000 seeds, which drop to the soil surface throughout the late summer and fall.
The seeds remain dormant in the soil throughout the cold winter months. This seed bank is resilient, with some seeds remaining viable for several years, waiting for the right conditions to germinate. The dead plant material can inadvertently protect the seeds by acting as a light mulch, shielding them from harsh winter elements until spring arrives.
Why Knowing the Die-Off Timing Matters for Next Year’s Control
Understanding the timing of the fall die-off is important for establishing a control strategy for the following year. Since crabgrass is an annual killed by frost, applying post-emergent herbicides in late fall is ineffective and wasteful. The plant is already dead, and the herbicide will not affect the seeds left behind in the soil.
Instead, the die-off signals the window for planning the application of pre-emergent herbicides for the spring. These must be applied before the overwintered seeds begin to germinate, which typically occurs when soil temperatures consistently reach 55°F for several days. Observing when the crabgrass dies in the fall helps predict the general timing of the next season’s germination, allowing for better-timed application of pre-emergent products in the early spring.