Many golf course managers are attempting to switch their perennial ryegrass fairways to zoysiagrass, particularly seeded ‘Zenith’ zoysiagrass, in order to save money on irrigation and fertilization. Some may not be having the best of luck. It has been thought that the perennial ryegrass (PR) provides enough shade and root competition to disrupt germination and seedling growth of the zoysiagrass. Previous studies have proven that scalping the perennial ryegrass prior to seeding does improve germination rate; however, it does not remove all obstacles to success. Researchers at Kansas State University have shown that the mere presence of perennial ryegrass can inhibit germination and seedling growth of ‘Zenith’ zoysiagrass. They performed several studies, including chopping up perennial ryegrass leaves and roots and mixing them in with the top 1 inch of soil, and soaking perennial ryegrass parts in water, then using that water to irrigate zoysiagrass seedbeds. They also used glyphosate- vs. no-glyphosate-treated PR to mix in to the potting soil. In all studies, they found similar results: Perennial ryegrass inhibits the germination and growth of ‘Zenith’ zoysiagrass at some level. It did not matter if the soil mix contained only perennial ryegrass roots or shoots or simply the “tea” of perennial ryegrass. For example, use of the PR tea to irrigate samples resulted in at least 53 percent reduction in germination, and 57 percent reduction in zoysiagrass root mass. They did note that glyphosate-treated PR did not inhibit zoysiagrass growth any more than non-glyphosate-treated PR.

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