The introduced exotic vines pale and black swallowwort rapidly have become invasive throughout regions of the northeastern United States and adjoining areas of Canada. Preliminary studies have reported that the species are allelopathic, possibly contributing to their competitive ability and invasiveness. Results from our laboratory assays indicated that swallowwort root exudates caused significant root length reductions (e.g., 40% for butterfly milkweed and 20% for large crabgrass) and reduced germination (e.g., 25% for lettuce) of indicator species. Additional bioassays with dried swallowwort tissues demonstrated that tissue leachates caused varied responses in indicators, with both significant stimulatory and inhibitory effects. In particular, significant congeneric interactions were noted between the two swallowwort species. Evidence from this study of swallowwort tissue phytotoxicity has important implications for developing effective management and habitat restoration strategies for the two invasive species.

Nomenclature: Annual bluegrass, Poa annua L.; barnyardgrass, Echinochloa crus-gallis L. P. Beauv.; black swallowwort, Cynanchum nigrum (L.) Pers., non Cav.; butterfly milkweed, Asclepias tuberosa L.; common milkweed, Asclepias syriaca L.; large crabgrass, Digitaria sanguinalis (L.) Scop.; orchardgrass, Dactylis glomerata L.; pale swallowwort, Cynanchum rossicum (Kleopow) Borhidi; lettuce, Lactuca sativa L.; tomato, Solanum lycopersicum L

Interpretive Summary: Our laboratory studies provided evidence for the negative impact of pale (PSW) and black swallowwort (BSW) tissue leachates and root exudates upon the growth and germination of indicator species, in particular common and butterfly milkweed. Additionally, we found important congeneric interactions between the two species, with results suggesting a possible competitive advantage for BSW. Our study findings indicate that allelopathic interference might help to facilitate the dominance of swallowworts in cases of interspecific competition. However, field studies are needed to demonstrate whether this interference occurs under natural conditions, and whether it ultimately has an ecological significance in the invasiveness of the swallowworts in natural settings.

With regard to management, these results suggest that there could be unexpected ramifications to what is currently one of the recommended control strategies (in addition to chemical controls) for the two species—mowing. Mowing can be effective at preventing populations from spreading if timed to coincide with seed set and repeated throughout the growing season. Our research suggests that the abundant swallowwort leaf and stem material remaining on the soil surface after mowing potentially could release biologically active compounds into the soil rhizosphere as it decomposes. These leachates have the potential to inhibit the germination and seedling growth of sensitive plant species in the seedbank, or those used to restore treated sites. Consequently, if mowing is used to control invasive swallowwort populations, mowed biomass should be collected and removed from the treated site if possible.