Primo Influences on Overseeded Perennial Ryegrass and Poa trivialis

Dr. Grady L. Miller

Each season golf course superintendents in the south overseed warm-season turf with cool-season grasses to improve appearance and playability of their golf course. Plant growth regulators are becoming more popular for decreasing mechanical mowing and as an aid in suppressing growth to enhance establishment of cool-season species. Little is know about the effect of PGRs on cool-season grass seed germination. The objective of this study was to evaluate the effects of Primo on two cool season grasses used for overseeding.

For this study, Tifdwarf plugs were removed from the field and grown in fifty small pots in the Turfgrass Envirotron. After Tifdwarf was sufficiently established, the greenhouse temperature was lowered to typical fall temperatures before overseeding with cool-season grasses. Primo applications were applied either 7 days, 2 days, 1 day, 6 hours or 2 hours before overseeding. Primo was applied at 1 oz a.i. per 1000 square feet, which is double the recommended rate. Primo treatments were staggered so that after spraying treatments were completed all pots could be overseeded on the same day with either perennial ryegrass or Poa trivialis. Weekly observations were made from six weeks to evaluate % germination and injury to Tifdwarf.

Approximately two weeks after Primo application, Tifdwarf was showing signs of discoloring and some stunted growth. In between the third and fourth week, stunted growth due to Primo applications was very apparent. By the sixth week, very little of the Tifdwarf was noticeable due to cover of the cool-season grasses. Tifdwarf was suppressed without injury, although a lower rate of Primo would be recommended to avoid excessive discoloration.

Germination of cool-season grasses was successful. Germination data plotted on a graph depicted a sigmoidal (s-shaped) response curve for growth and led us to believe that Primo had no influence on germination of the two species. One observation made during the experiment was that little or no germination occurred in bare areas of the bermudagrass. Seeds of both types were visible and remained dormant in these spots. A conclusion drawn form this study was that maybe Primo remained active on the soil surface whereas, that which contacted bermudagrass foliage was absorbed and out of contact with the cool-season grass seeds. Recent studies reported by Ohio State1 indicated that significant germination inhibition of perennial ryegrass and Poa trivialis seeds when it came in direct contact with the seeds. Germination inhibition was directly related to the length of time Primo was in contact with the seed and higher chemical concentrations resulted in greater inhibition. Germination may not have been affected in our study because there was sufficient time for the Primo to dry or because sufficient Primo had been absorbed by the bermudagrass reducing the concentration that came in contact with seed.

In summary, this experiment led us to believe that an application of Primo before overseeding would not inhibit germination regardless of timing of application before overseeding so long as sufficient time passed to allow the Primo to dry on the leaf surface. Although bare areas had significantly lower seed germination, further experiments would be necessary to prove that Primo in the soil was the primary factor reducing germination.

1 Bell, G.E., T.K. Danneberger, and M.B. McDonald. 1997. Chemical inhibition of cool season turfgrass germination. International Turfgrass Society Proceedings. Vol 8(1):411-417.

Cool Season Grass response to Primo Application

Bermudagrass injury at 3 weeks after overseeding following PRIMO application