A few years ago, two separate populations of Bidens alba strangely appeared in the Falls Lake Area. They persisted for two years and then completely disappeared. Bidens alba (Shepherd’s Needles, Butterfly Needles) is primarily a Florida plant, with only spotty distribution north of Florida, and has been reported from only one county in North Carolina (New Hanover). In the Falls Lake Area, one population of nearly one hundred plants was clearly associated with a large, newly planted screen of Glossy Privet (Ligustrum lucidum). The other colony came up in the disturbed soil of land being cleared for a new development.
B. alba can be up to 5 ft. tall with luxurious f0liage and bright white flowers.
The Bidens genus is known for its slender, needle-like seeds terminating in 2 barbs, which allow the seed to stick to animal fur and human clothing, helping to disburse the seeds. But Bidens alba made this large geographic jump by hitchhiking in soil at the base of landscape plants, and on equipment or supplies used for land clearing. Outside of Florida, B. alba is considered an annual, so it must have seeded successfully for the second year of growth and then failed to persist thereafter. It is one of the few members of its genus to have white rather than yellow flowers, which aids in identification.
Just a year previous to the appearance of B. alba, another visitor from out of state appeared at Falls Lake – Jacquemontia tamnifolia (Smallflower Morningglory). In keeping with its reputation as a landscape weed, it was spotted along with other weeds in a Falls Lake parking lot. An annual and a member of the morning glory family, J. tamnifolia did not return to that site again. It is primarily a coastal species and has been reported from only 2 North Carolina counties. Seeds can be dispersed by wind or birds, with the latter the most likely in cases of isolated occurrence. While the flowers are quite small and clustered, the leaves are large for the morningglory family.
Finding plants out of place, like those described above, may seem somewhat mysterious, but is an example of how dynamic plant distribution can be. William Cronon’s classic eco-history of New England – Changes in the Land – describes the forces that determine why a particular species might be located where it is. Ecological factors such as climate, soil, and slope provide the background to intermediate, sometimes catastrophic events such as fire, wind and disease (and development) that can shift the species composition of an area completely. The two plants described above were unable to persist in their new location, unlike many of the invasive species we deal with today, which were able to establish permanent residence.