Evaluation of the Allelopathic Potential of Selected Perennial Groundcovers: Foliar Volatiles of Catmint (Nepeta x faassenii) Inhibit Seedling Growth

Six perennial groundcovers including Alchemilla mollis, Nepeta x faassenii, Phlox subulata, Sedum acre, Solidago cutleri, and Thymus praecox were investigated for the allelopathic potential of their respective foliar tissues via evaluation of volatile constituents produced by foliage. These groundcovers were selected for further laboratory evaluation because of superior performance as weed-suppressive groundcovers in previous field experiments. Foliar volatile components of N. x faassenii exhibited the strongest inhibitory effects on seedling growth of curly cress (Lepidium sativum), but S. cutleri also showed allelopathic potential by reducing shoot growth of curly cress seedlings with extracted volatiles. Although A. mollis and P. subulata exhibited strong weed-suppressive traits in past field experiments, weed suppression is apparently associated with either competition for resources or other allelopathic mechanisms rather than an allelopathic effect caused by volatiles. Volatiles of N. x faassenii were further evaluated with gas chromatography coupled to mass spectrometry (GC-MS). A total of 21 chemical constituents were identified in the volatile cocktail; 17 components were identified from a direct crude leaf sample extraction, including sabinene, β-pinene, β-myrcene, 2-(2-ethoxyethoxy)-ethanol, 1,8-cineole, ocimene, neryl Acetate, 4aα,7α,7aα-nepetalactone, α-copaene, trans-caryophyllene, alloaromadendrene, 4aβ,7α,7aβ-nepetalactone, germacrene D, β-farnesene, χ-cadinene, germacrene B, and β-sesquiphellandrene. Five additional constituents were identified in a methanolic extract of dried of N. x faassenii foliage, but not the volatile cocktail collected from N. x faassenii foliage. These included methyl benzoate, 2,4-decadienal, neryl acetate, isodihydronepetalactone, and caryophyllene oxide. Three components, 2-(2-ethoxyethoxy)-ethanol, alloaromadendrene, and χ-cadinene, were not only detected in both the volatile mixture and the methanolic extract, but also in an aqueous foliar extract that exhibited potential allelopathic activity.