Crabgrass (Digitaria spp.) and quackgrass (Elymus repens) are two of the most common grassy weeds that invade lawns and gardens, but they possess fundamentally different biological characteristics. Correct identification dictates the timing and type of control measures that will be effective. Though both are coarse, unwanted grasses, their distinct growth habits, life cycles, and root structures require separate strategies for successful removal.
Identifying Features of the Blades and Stems
The most immediate distinction between the two weeds is observed above ground in their appearance and growth pattern. Crabgrass is a warm-season grass that typically grows outward in a low, clump-forming, or prostrate manner, often compared to the legs of a crab. The blades are generally a lighter, yellowish-green color compared to desirable turfgrass, and they are noticeably broader and coarser in texture. As the season progresses, crabgrass produces distinctive seed heads that branch out like fingers from the end of the stem.
Quackgrass, conversely, is a cool-season grass that exhibits a much more upright growth habit and tends to grow taller than the surrounding lawn. Its foliage is usually a darker, sometimes bluish-green color, which makes it stand out conspicuously against a lighter green turf. A key identifying feature of quackgrass is the presence of clasping auricles, which are small, claw-like appendages that wrap around the stem where the leaf blade meets it. Quackgrass actively grows upward.
How Root Systems and Life Cycles Differ
The most significant biological difference lies beneath the soil, specifically in the root systems and life cycles of the two weeds. Crabgrass is an annual weed, meaning it completes its entire life cycle within a single growing season. It germinates from seed in the spring and dies completely after the first hard frost. It possesses a shallow, fibrous root system that is relatively easy to pull out of the ground. Each plant produces a prolific number of seeds, sometimes up to 150,000, which remain dormant in the soil until the following spring.
Quackgrass, however, is a perennial plant, meaning its root system survives the winter and allows the plant to return year after year. This weed spreads aggressively through long, white, underground stems known as rhizomes. These rhizomes are specialized underground runners that can grow horizontally, sending up new shoots every few inches and forming dense, interconnected networks. The rhizomes have sharp, pointed tips that allow them to penetrate compacted soil, making quackgrass exceptionally difficult to eradicate, as a small piece of rhizome left behind can regenerate an entire new plant.
Control Methods Dictated by the Weed Type
The annual versus perennial distinction fundamentally dictates the control methods necessary for each weed. Because crabgrass is a summer annual that relies entirely on seed germination each year, its most effective control involves pre-emergent herbicides. These products must be applied in the spring before the seeds germinate, typically when soil temperatures reach 55°F. Once crabgrass has emerged, post-emergent herbicides like quinclorac can be used, but they are most effective when the plant is young, having fewer than four tillers.
Quackgrass, due to its perennial nature and extensive rhizome system, is not effectively controlled by crabgrass preventers. Pre-emergent herbicides target seeds, but they cannot stop the established underground rhizomes from producing new shoots. Since there are very few selective herbicides that kill quackgrass without harming desirable turfgrass, control often requires using non-selective herbicides, which kill any green plant tissue they contact. Alternatively, manual removal must be meticulously done, ensuring every piece of the deep, tough rhizome is extracted to prevent regrowth.