May, 2001

Overseeding Rates and How They
Affect Spring Transition -
The Long and Short of It.

David M. Kopec, Extension Turfgrass Specialist

Overseeding bermudagrass fairways is almost always a must for low desert elevation golf courses. Many courses and clubs may have 80 to 85% of the annual play during the times of the year when the turf is overseeded.

Over the last twenty years, there have been great improvements in the turf quality of perennial ryegrasses. The down side is that, in many cases, the overseed lasts longer than it should. It can last long into the summer, then die out suddenly, leaving precious little time for the surrounding Bermuda to grow back and become hardy enough for the next overseeding!

In the "old days", superintendents would overseed at high seeding rates in order to create a turf which maintained a high proportion of total plants which would have low tiller numbers per plant. This would produce a canopy of (juvenile) weaker plants in the long run, producing a "short" transition by early summer. Does the technique of high over-seeding rates still produce this effect on newer perennial ryegrasses which have greater heat tolerance, tolerate closer mowing, and produce a denser turf than varieties of just 15 years ago?

With the help of Cactus and Pine, the University of Arizona conducted a two-year study to address the effect of overseeding rates [450-500-55-600-650-700-750-800 lbs. PLS/Acre] on Spring transition.

The first week of October, "Quickstart" perennial ryegrass was overseeded on Tifway 419 bermudagrass after standard scalping operations and light vertical mowing. The turf was mowed 3 times a week at 11/16 inch, fertilized at 0.5 to 1.0 lb –N-/M per month, and irrigated to avoid stress. No other management practices were conducted at transition, which could cause unwanted interactions with the actual seed rate treatments.

Percent bermudagrass was measured on all plots starting in June and through July as the percent Tifway present on a percent plot basis. Heat injury using a laboratory procedure was conducted on ryegrass plants of all growth stages in early June for the seed rates of 500-600-700 and 800 lbs PLS/acre.

Four times each year, plugs were taken from each plot so that individual plants could be separated and counted into plants with different tiller stages (same as number of shoots per plant). This was done to 1) measure the total number of shoots per sample, and 2) determine the frequency of multiple tiller plant production. Seed rates which have high tiller-to-plant ratios have a higher amount of plants which have multiple shoots attached to the same crown (i.e. multiple tillered plants). Seed rates that have low tiller-to-plant ratios have more single tiller plants, and less multiple tiller plants.

1. Overseed rate affected heat injury in the lab test. As overseed rates increased, heat injury also increased. There was more injury at 800 lbs than 450 lbs, with 600 and 700 lbs being intermediate in heat injury. This was true over both years.

2. As heat injury increased, the actual transition decreased. The 500 lb seed rate always had more bermuda earlier in the season than the 800 lb rate. The 600 and 700 lb seed rates had transition similar to that of 500 lbs. This was true for both years. Based on the two-year average, the mean percent bermudagrass at rates tested for heat injury were as follows: 500 lbs 73%, 600 lbs 74%, 700 lbs 72%, and 800 lbs 60%. This corresponds to an 18% treatment effect between the 500 and 800 lb rates.

3. The tiller composition of the turf did not affect heat injury. Plants consisting of 1, 2, 3, 4, or 5 or more tillers per plant were no different from each other averaged across overseed rates. Also, there were no differences in heat injury for the different tiller classes, from one seed rate to another.

4. Therefore, tiller composition had no affect on heat tolerance or transition. But, seed rate alone did.

5. At seed rates of 750 and 800 lbs, rapid drop-offs occurred in total ryegrass density (total tiller counts) in late April-early May, into mid-June. The sudden loss of plants may result in poor transition due to perhaps a presence of a chemical inhibition (from necrotic ryegrass) or from physical resistance from a dense shoot complex.

6. The best transitions occurred at seed rates of 450-500 lbs, and at 600-650-700 lb/acre seed rates.

7. The 450 lb rate was slowest to achieve 98% or greater turf cover in the fall compared to others. By late December, it provided near full cover.

We now know the story - - and its very different from what we learned previously about ryegrass hardiness, overseed rates, and the affect of only seed rate on transition.

Be sure to observe the Pure Live Seed content (on the seed you purchase) when you calibrate at overseeding this fall. A hundred pounds over-or-under per acre, may affect your transition!

Cactus and Pine is grateful to its membership and tournament players in the Annual Karsten Challenge. Without this support, detailed scientific research like this would not be possible.

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Issued in furtherance of Cooperative Extension work, acts of May 8 and June 30, 1914, in cooperation with the U.S. Department of Agriculture, James A. Christenson, Director, Cooperative Extension, College of Agriculture & Life Sciences, The University of Arizona. The University of Arizona is an equal opportunity, affirmative action institution. The University does not discriminate on the basis of race, color, religion, sex, national origin, age, disability, veteran status, or sexual orientation in its programs and activities.

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