The purpose of this study was to test competitive and allelopathic effects of invasive garlic mustard on American ginseng (Panax quinquefolius) seedlings under natural conditions. For comparative purposes, we also examined the potential effects of the native striped violet (Viola striata). In order to partition effects of resource competition and chemical suppression via allelopathy, field soils were amended with activated carbon or left unamended. Activated carbon positively affected ginseng growth as well as biomass of competitors. Ginseng mortality tended to increase with garlic mustard presence, though activated carbon alleviated this response. Garlic mustard had no significant effect on ginseng seedling growth, while striped violet suppressed shoot length in the absence of activated carbon. Our results showed a surprising effect of activated carbon on plant growth, a potential allelopathic effect of the native striped violet and suggest that newly invaded ginseng populations with low densities of garlic mustard may be able to withstand its effects. However, recruitment within invaded populations may decline.

Plant-centric sampling provides a novel approach to quantifying the potential impact of invasive species on native plant species. The aim of this study was to determine the level of exposure of individuals and populations of Panax quinquefolius to invasive plant species using this approach in thirty natural ginseng populations. A high level of invasion was found with 63-70% of ginseng populations containing at least one invasive species. Approximately one-third of all individuals were found in close proximity to invasive plants. The most prevalent invasive species were Rosa multiflora and Berberis thunbergii. The exposure to invasives of plants in different size classes varied among populations. Invasive species presence increased with greater ginseng population sizes and presence of harvest. The abundance of invasives plants within forest interiors near this valuable medicinal herb suggests that the economic and ecological costs of competitive interactions with native species could be high.

A Panax-centric view of invasive species and the competitive and allelopathic effects of garlic mustard (Alliaria petiolata) on American ginseng (Panax quinquefolius L.