Hairy Crabgrass (Digitaria sanguinalis) is an annual warm-season grass species belonging to the family Poaceae, widely recognized as one of the most common and persistent lawn and agricultural weeds across temperate and tropical regions worldwide.
Despite its weedy reputation, hairy crabgrass has a long history of use as a forage grass and was once intentionally cultivated as a grain crop in parts of Europe and Asia. Its common name derives from the conspicuous hairs covering its leaf sheaths and the low, spreading growth habit reminiscent of crab-like legs radiating outward.
• One of the most widespread weed species on Earth, found on every continent except Antarctica
• Germinates later in spring than most lawn grasses, emerging when soil temperatures reach approximately 15–18°C
• A single plant can produce up to 150,000 seeds per season, contributing to its notorious persistence
• Classified as a C4 photosynthetic plant, giving it a competitive advantage in hot, dry conditions
Taxonomie
• Believed to have originated in the Mediterranean region and temperate Europe
• Introduced to North America likely in the 18th century, possibly as a contaminant in seed stocks or through human migration
• Now found from southern Canada through the entire United States and into Central and South America
• Thrives in USDA hardiness zones 3–10, demonstrating remarkable climatic adaptability
Historical significance:
• Was cultivated as a grain crop in parts of Central Europe and the Balkans under the name "Polish millet" or "Polish finger millet"
• Seeds were ground into flour and used for porridge and bread in times of grain scarcity
• In some regions of Eastern Europe, it was grown as a forage crop well into the 20th century
Roots:
• Fibrous root system, often forming adventitious roots at lower nodes that contact soil
• Shallow but extensive, allowing rapid water uptake after rainfall
Stems (Culms):
• Slender, branching, and often decumbent at the base, curving upward at the tips
• Nodes are glabrous or slightly hairy; internodes smooth
• Multiple tillers radiate outward from the base, creating a characteristic rosette-like mat
Leaf Blades:
• Flat, linear-lanceolate, typically 5–15 cm long and 4–10 mm wide
• Both surfaces and margins covered with soft, fine hairs (pubescent)
• Ligule is membranous, approximately 1–2 mm long
• Leaf sheaths are prominently hairy — a key diagnostic feature distinguishing it from smooth crabgrass (Digitaria ischaemum)
Inflorescence:
• Composed of 2–9 digitately arranged racemes (finger-like branches), typically 5–15 cm long
• Racemes are slender and spreading, giving the inflorescence a distinctive "hand-like" appearance
• Spikelets are arranged in pairs along one side of the rachis, lanceolate, approximately 2–3 mm long
• Lower glume nearly absent; upper glume about half the spikelet length
• Lemmas are hairy, especially along the margins
Seeds (Caryopses):
• Small, oval, approximately 1.5–2 mm long
• Reddish-brown to purplish at maturity
• Extremely prolific — a single plant can produce 150,000+ seeds
• Seeds remain viable in soil for at least 3 years, forming a persistent seed bank
Habitat Preferences:
• Lawns, gardens, agricultural fields, roadsides, waste areas, and construction sites
• Prefers compacted, nutrient-rich soils but tolerates a wide range of soil types
• Most competitive in sandy or loamy soils with moderate to poor fertility
• Commonly found in areas with frequent soil disturbance
Climate & Temperature:
• Warm-season (C4) grass; optimal growth occurs at 24–35°C
• Germination triggered when soil temperatures at 5 cm depth reach 15–18°C consistently
• Killed by the first hard frost in autumn, leaving characteristic bare patches in lawns
• Most vigorous growth occurs during the hottest months of summer (July–August in the Northern Hemisphere)
Ecological Interactions:
• Provides food for granivorous birds such as sparrows, doves, and quail
• Seeds consumed by small mammals including mice and voles
• Serves as a host plant for certain butterfly and moth larvae
• Can form dense stands that outcompete desirable turfgrasses and native vegetation
• Allelopathic properties have been documented — root exudates may inhibit germination of competing species
Reproduction:
• Exclusively by seed; no vegetative reproduction
• Seeds germinate asynchronously over an extended period (late spring through summer)
• Rapid life cycle — can go from germination to seed production in as little as 6–8 weeks
• Seeds dispersed by wind, water, animal fur, mowing equipment, and contaminated soil
Light:
• Requires full sun; performs poorly under shade
• Needs a minimum of 6–8 hours of direct sunlight daily for vigorous growth
Soil:
• Adaptable to a wide range of soil types but prefers sandy loam
• Tolerates soil pH from 5.5 to 7.5
• Thrives in compacted soils where other grasses struggle
• Benefits from nitrogen-rich conditions
Watering:
• Drought-tolerant once established due to C4 photosynthesis
• Germination requires consistent soil moisture
• Supplemental irrigation during dry summer months promotes lush growth
Temperature:
• Plant after soil temperatures reach at least 15–18°C at a 5 cm depth
• In most temperate regions, this corresponds to late April through May
• Growth ceases when temperatures drop below 10°C
Propagation:
• By seed; broadcast at a rate of approximately 1–2 kg per hectare for forage use
• Seeds require light for germination — do not bury deeply (surface-sow or cover with <3 mm of soil)
• Germination typically occurs within 5–10 days under optimal conditions
Weed Management (for lawn care):
• Pre-emergent herbicides (e.g., pendimethalin, dithiopyr) applied in early spring before soil reaches 15°C
• Maintaining a dense, healthy turf through proper mowing height (7–8 cm) is the most effective cultural control
• Hand-pulling is effective for small infestations before seed set
• Avoid overwatering and excessive nitrogen fertilization, which favor crabgrass over cool-season turfgrasses
Anecdote
Hairy crabgrass holds a fascinating dual identity — reviled as one of the world's worst lawn weeds yet valued as a nutritious grain and forage crop: • In parts of Poland and Eastern Europe, hairy crabgrass (known as "Polish millet") was cultivated as a grain crop for centuries, with seeds ground into flour for bread and porridge • The seeds are nutritionally comparable to other millets, containing approximately 10–12% protein and rich in minerals • A single hairy crabgrass plant can produce up to 150,000 seeds in one growing season — one of the highest seed outputs of any common weed • Its C4 photosynthetic pathway makes it remarkably efficient at converting sunlight into biomass under hot, dry conditions, outperforming most cool-season lawn grasses during summer heat waves • The species name "sanguinalis" means "blood-red" in Latin, referring to the reddish-purple coloration of the mature inflorescences and seed heads • Hairy crabgrass seeds can remain dormant in the soil seed bank for at least 3 years, ensuring the species' persistence even after aggressive control efforts • In the United States, crabgrass is so ubiquitous that it has become a cultural symbol of summer — its appearance in lawns signals the arrival of hot weather and the transition from cool-season to warm-season grass dominance
En savoir plus
