The question of whether clover is native to North America has a complex answer. While the common plants recognized as clover are non-native, the continent is home to its own indigenous species. The genus Trifolium, which includes all true clovers, has a global distribution with a notable center of diversity in western North America. However, the species dominating lawns, pastures, and roadsides today were intentionally brought over from Eurasia. This distinction is important because non-native species have profoundly affected the continent’s ecology and agricultural practices.
The Introduced Species That Dominate the Landscape
The clovers most frequently encountered across North America are non-native species introduced primarily for their agricultural value. Trifolium repens, commonly known as White Clover, is a prime example of a successful introduction from Europe and Western Asia. This low-growing, mat-forming perennial was widespread across the continent by the mid-1700s, included in seed mixes brought by European settlers for pasture and ground cover.
Its rapid spread earned it the historical nickname “White Man’s Foot Grass” among some Native American tribes, symbolizing the quick colonization of disturbed lands where European settlements were established. White Clover is a highly effective nitrogen fixer and spreads aggressively via creeping stems that root at the nodes. Its ability to thrive in disturbed environments, from manicured lawns to roadsides, cemented its place as one of the most common plants across the United States and Canada.
Another pervasive non-native species is Trifolium pratense, or Red Clover, which is native to Europe. Characterized by its reddish-purple flower heads, this upright plant became one of the most significant hay and pasture crops in North America. European settlers valued Red Clover for its high protein content, making it excellent forage for livestock. Widespread cultivation of both Red and White Clover led to their escape from agricultural settings and naturalization across the continent’s temperate regions.
The True Native Clovers of North America
While the most familiar clovers are introduced, North America has a variety of native Trifolium species that evolved independently. The greatest diversity of these indigenous clovers is found in the western part of North America, particularly in states like California and Oregon. These species are often less robust and occupy more restricted ecological niches than their non-native counterparts.
One historically significant native is the Running Buffalo Clover (Trifolium stoloniferum), once widespread across the midwestern and eastern United States. This perennial plant is distinguished by its long runners, or stolons, which creep along the ground and root to form new plants. Unlike most clovers, this species appears to have relied on the moderate, periodic disturbance created by grazing animals, such as migrating bison.
Another native species is Buffalo Clover (Trifolium reflexum), an annual or short-lived perennial native to the eastern and southern U.S. In the western states, species like Springbank Clover (Trifolium wormskioldii) and Tomcat Clover (Trifolium wildenovii) are native. Springbank Clover was once widespread in the wet areas of the Willamette Valley. These native species often have small, localized ranges and are threatened by habitat loss and competition from introduced species.
Ecological Impact of Non-Native Clover
The functional difference between introduced and native clovers is rooted in their primary ecological role as legumes. Clovers participate in a symbiotic relationship with Rhizobium bacteria, which form root nodules to convert atmospheric nitrogen into a usable form, a process known as nitrogen fixation. This nitrogen-fixing ability is precisely why introduced clovers were valuable in agriculture, helping to enrich the soil without synthetic fertilizers.
However, the widespread introduction of non-native clovers has significantly altered the soil chemistry in many natural ecosystems. Many native North American plant communities, such as prairies and woodlands, evolved in low-nitrogen environments, which limits the growth of aggressive, non-native plants. The excess nitrogen contributed by non-native clovers gives them a competitive advantage, allowing them to outcompete and displace native flora.
This alteration of the nitrogen cycle can also facilitate the establishment of other invasive species that thrive in nitrogen-rich soils. Although non-native clovers provide nectar for generalist pollinators, they do not offer the same specialized support as native plants. Native insects have co-evolved with native flora, and the proliferation of non-native species can reduce the available resources for specialized native pollinators.