The perennial herb Securigera varia, commonly known as crown vetch, was introduced to North America from Eurasia in the mid-1800s. Widespread planting began in the 1950s, primarily to stabilize soil on highway embankments and for general erosion control. Despite being valued for its rapid growth and ability to thrive in poor soils, crown vetch is now recognized across the United States as a highly destructive invasive species. Its vigorous, unrestrained growth creates significant ecological, physical, and agricultural damage.
Ecological Disruption and Biodiversity Loss
Crown vetch forms dense, sprawling mats that aggressively blanket the ground, suppressing the growth of nearly all other vegetation. Trailing stems can grow up to six feet long, creating thickets that shade out and smother delicate native plant life, including grasses and wildflower seedlings. This physical exclusion prevents the germination and establishment of diverse plant species, leading directly to a loss of biodiversity.
The proliferation of crown vetch leads to the formation of monocultures, transforming once-diverse natural communities into single-species areas. This reduction in plant variety negatively impacts the local food web, harming native insect populations and the animals that rely on them. As a legume, crown vetch alters soil chemistry by fixing nitrogen, giving it a competitive advantage in nutrient-poor soils. The resulting increase in soil nitrogen can favor the growth of other non-native species, hindering the recovery of native flora.
Structural and Physical Damage from Root Systems
The underground structure of crown vetch is a major source of physical damage to human-built infrastructure. Its root system is characterized by aggressive, multi-branched rhizomes that can grow horizontally up to ten feet in a single season. These rhizomes continuously seek out new ground and can exert considerable force.
When established near hardscapes, the expansion of the rhizomes can penetrate and compromise structural integrity. The force of growing roots is known to destabilize road shoulders and cause pavement lifting and cracking over time. The plant’s dense mass also clogs drainage ditches and culverts, impeding water flow and accelerating localized erosion. Its depth and complexity make mechanical removal extremely difficult and expensive.
Toxicity and Agricultural Interference
Crown vetch poses a threat to livestock, particularly non-ruminant animals such as horses, swine, and poultry. The plant contains nitroglycosides, including beta-nitropropionic acid (3-NPA), which acts as a potent neurotoxin. While ruminants like cattle, sheep, and goats can detoxify these compounds using gut microbes, non-ruminants cannot. Consumption in large quantities can lead to symptoms in horses that include incoordination, lethargy, poor weight gain, or even paralysis and death in severe cases.
Beyond the direct toxicity, crown vetch invades and competes with agricultural forage crops in hayfields and pastures. Its dense growth reduces the yield of desirable grasses and legumes, making cultivated land less productive or unusable for grazing and hay production. It also poses a problem in cereal crops and vineyards, where its prolific growth and persistent seeds can contaminate harvests.
Aggressive Growth Characteristics
Crown vetch’s destructive power lies in its combination of rapid growth and relentless persistence. It can colonize between 70 to 100 square feet within four years through its extensive rhizome network. This vegetative spread is compounded by prolific seed production, with seeds remaining viable in the soil for up to 15 years. This creates a persistent seed bank that guarantees re-infestation.
The plant’s ability to fix nitrogen in the soil gives it a significant growth advantage. This combination of deep, spreading rhizomes and long-lived seeds makes the plant exceptionally difficult to control. Both mechanical removal and chemical treatments require multiple, successive applications over several years to eliminate the entire plant and deplete the seed bank.