The commercial seed trade: An early disperser of weeds in the United States

Seeds, bulbs, and cuttings of exotic plant species, including plants cultivated exclusively as ornamentals, were being advertised for sale in printed circulars in the United States by 1800. By the end of the 19th century seed trade catalogs were prolific and often listed dozens of exotic species that were already naturalized in the U.S. or new introductions from foreign suppliers. Among the species routinely offered for sale were many now considered noxious (e.g.,Berberis vulgaris, Eichhornia crassipes, Isatis tinctoria) or at least undesirable (e.g.,Cannabis sativa, Eleusine indica, Lysimachia nummularia). This commerce was not only responsible for the introduction and spread of some species earlier than previously recognized (e.g.,Bryonia alba, Schinus terebinthifolius) but also caused the deliberate distribution of other species assumed to have been spread by accident alone (e.g.,Aegilops cylindrica, Bromus briziformis, Bromus mollis). Seedsmen’s introduction of substantial genetic variation through repeated introductions, inadvertent directional selection for local races in their gardens, their widespread use of seeds (compared to cuttings or non-fertile plant material), and the sowing of seeds at high density under cultivation all enhanced the opportunity for eventual naturalizations. The largely unrecognized proliferation of this industry in the 19th century and the ready access that consumers had to these species via mail order contributed to the rapid and extensive dissemination of at least 139 alien species throughout the United States.