Overseeding a permanent grass such as bermudagrass with perennial ryegrass complicates spring transition, because perennial ryegrass is usually growing optimally and its upright growth shades the base grass. Although interstand competition between species greatly impacts transition, ryegrass allelopathy has been proposed as another contributing factor. Allelopathy, the suppression of growth of one plant species by another through the release of chemicals into the environment, has been implicated as a mechanism of interference between many plant species. Allelopathic effects of perennial ryegrass on bermudagrass have been suggested but not examined.
Perennial ryegrass has been reported to have inhibitory effects on ‘Zenith’ zoysiagrass (Zoysia japonica Steud.) (Zuk and Fry, 2006), alfalfa (Medicago sativa L.) (Chung and Miller, 1995), nodding thistle (Carduus nutans L.) (Nicholson et al., 1990), white clover (Trifolium repens L.) (Mattner and Parbery, 2001), and lettuce (Lactuca sativa L.) (Buta et al., 1987). Koski and Newberry (2004) proposed that either intense competition or allelochemicals impeded kentucky bluegrass (Poa pratensis L.) establishment into perennial ryegrass stands at fairway height. Rao and Buta (1983) extracted 19 phenolic compounds from perennial ryegrass that negatively impacted lettuce seed germination. Crabgrass seedling inhibition, chlorosis, and premature death were observed when grown on ryegrass extract tainted agar (King, 1996).
Experiments that monitor indicator species germination and growth in the presence of leachates, extracts, or debris of potential allelopathic agents are acceptable ways of examining and understanding allelopathic potential (Inderjit and Keating, 1999; Inderjit and Weston, 2003). Unequivocally proving allelopathy would require identifying and extracting allelochemicals from perennial ryegrass, applying them to bermudagrass, and observing growth suppression. The chemical basis for inhibitory effects was beyond the scope of this experiment. The objective of this research was to determine if perennial ryegrass contaminated soil media or irrigation water containing perennial ryegrass extracts would inhibit common bermudagrass germination and growth in a controlled environment.
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