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Overseeding perennial ryegrass after preemergence herbicides

Philip Busey

"How long must elapse between the last preemergence herbicide application on fairways until it's safe to overseed with perennial ryegrass?"  This is an issue particularly with multiple and/or late summer preemergence applications.

Herbicide labels answer with 3, 4, up to 10 months, depending on the rate, location, and number of applications. One product label adds a vague, "for best results."

The Dimension Pro FAQs web site http://www.dimensionpro.com/super_faq.htm claims a shorter safety period, only 3 months.  Besides the variability of environment, which can be great, I suspect that the safety period has not been studied carefully and I have found no documented scientific information on the subject.

Preliminary data on perennial ryegrass germination in response to weeks after treatment with different herbicides light blue circles = Dimension 1EC 1.5 fl oz/1000 ft2 dark blue circles = Dimension 1EC 1.125 fl oz / 1000ft2 triangles = Pendulum WDG 2.5 lb/acre red squares=Barricade 65WG 1.125 lb/acre. Seedling numbers are based on visual estimates from weeks 3 and 4 after overseeding, transformed by actual seedling counts of selected plots determined at the same time.

How do you go about studying this?  Here's an experiment that is answering the question.  For eleven weeks I applied, every Monday, dithiopyr (Dimension®), pendimethalin (Pendulum WDG®), and prodiamine (Barricade 65 WG®) to replicated bermudagrass turf plots, both at UF-Fort Lauderdale and Oak Tree Country Club, thanks to Bill McKee.  On Monday 10 January 2000, I overseeded all experimental plots with Lesco Double Eagle Blend perennial ryegrass at 8.5 pounds per thousand square feet.  The point of this is that we should expect to see some ryegrass emergence in plots treated with preemergence herbicide 10 weeks before overseeding, no emergence in the plots treated on the day of overseeding, and in between we should be able to pinpoint a "for acceptable results" safety period.  By doing this so late in the winter, during drier and cooler weather, this may be a worst case scenario for all the herbicides, because the products have had less reason to break down than if they had been applied in the summer.

Preliminary data from these specific experiments under these particular experimental conditions suggests that Dimension® and Pendulum® allow very high (> 75%) relative germination when overseeded 4 to 7 weeks after treatment.  Considering the relatively high cost of seed, and the variability of results that might exist under different environmental condtions, 75% relative germination is probably not acceptable.  But overseeding even 10 weeks after Barricade® treatment results in unacceptable results.  Considering that preemergence herbicides show relatively little selectivity, the flip side of this is that we may not necessarily have extended weed control from the products which are safer for earlier perennial ryegrass overseeding.  Also, there is addtional danger in overseeding too soon in areas that received multiple preemergence applications during the season, or received higher rates of application than were used here.

Results appear to be consistent with the safety margins advised by manufacturers, with a little extra margin of safety.  The University of Florida plots will be seen during the 13 March 2000 UF Turf Field Day and 13th Annual Otto Schmeisser Research Green Exposition (the South Florida Turf Expo).