Charting Conversion

Kentucky bluegrass was once used extensively on golf course fairways in the transition zone. During the past 25 years, mowing heights decreased and Kentucky bluegrass was largely replaced with perennial ryegrass because of its superior tolerance to low-mowing heights and its establishment vigor. Unfortunately, disease problems on ryegrass have become commonplace. Recently, Kentucky bluegrass breeders have developed a number of new cultivars with improved low mowing tolerance. These new cultivars offer superintendents the potential for reduced fungicide use because Kentucky bluegrass is resistant to the diseases that have plagued perennial ryegrass fairways. Consequently, many superintendents may be interested in converting perennial ryegrass fairways to Kentucky bluegrass, or to a perennial ryegrass/Kentucky bluegrass mixture.

Unfortunately, establishing Kentucky bluegrass seedlings in a mature perennial ryegrass turf is a difficult task. Research has proven perennial ryegrass to be capable of out-competing many other grasses for both moisture and nutrients. Kentucky bluegrass, by contrast, is notoriously slow-growing during its establishment phase. For Kentucky bluegrass seedlings to have any hope of long-term survival, there must be good seed-to-soil contact in conjunction with suppression of the ryegrass growth during establishment.

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Fortunately, superintendents have a number of tools available to achieve these twin objectives. Core-aeration, verticutting and rolling all help to achieve good seed-to-soil contact while plant growth regulators (PGRs) and post-seeding low mowing can suppress the growth of the existing perennial ryegrass. In addition, you can increase seeding rates, or use multiple seedings, in an attempt to gain increased seedling survival.

But which combination of these techniques is most effective in enhancing survival of seedling Kentucky bluegrass in an existing perennial ryegrass fairway? We designed research studies to compare the effectiveness of core-aeration, PGRs, post-seeding low mowing, high seeding rates and multiple seedings, both singly and in various combinations, on establishment of Kentucky bluegrass interseeded into a mature perennial ryegrass fairway-height turf.


Our first study took place during the fall of 1999 in an existing perennial ryegrass turf, maintained at 9/16-inch, at the Rocky Ford Turfgrass Research Center in Manhattan, Kan. Before seeding, we tried scalping, core-aeration and the PGR trinexapac-ethyl, doing so singly and in all possible combinations. For comparison, we also seeded into undisturbed plots and plots that had been sprayed with glyphosate before seeding.

All plots were vertically mowed in two directions before seeding to facilitate seed-to-soil contact. ‘Apollo’ Kentucky bluegrass (KB) was seeded on Sept. 14, 1999, at 2 pounds PLS per 1,000 square feet using a drop spreader. Following establishment, the area was maintained as though it were a golf course fairway at a “higher end” country club.

At three and 13 months after seeding (MAS), we used a grid to determine the percentage of KB in the plots. The grid was made from a plastic pipe frame with nylon fishing line running length-wise and cross-wise on 4-inch centers. We simply counted the number of KB plants lying under the intersections of the grid, and divided by the total number of intersections to obtain the percent KB in each plot.

To make a long story short, the results were disappointing! At 13 MAS, no treatment had more than 3.6 percent KB. By comparison, the plots treated with glyphosate prior to seeding averaged 75 percent and 99 percent KB coverage at three and 13 MAS, respectively. Clearly, we needed a more aggressive approach.


In the second study, started in the fall of 2000, we began with a more aggressive seeding approach for all the plots. On the day of seeding, we mowed everything at ¼-inch, core-aerated and vertically mowed to enhance seed-to-soil contact. Then we tried several new treatments in small plots across the study area. These included “post-seeding low mowing” or PSLM (mowing at ¼-inch twice weekly for a period of four weeks following seeding), two seeding rates (2 and 4 pounds PLS per 1,000 square feet), multiple seedings (fall only and fall + spring) and a more aggressive PGR (mefluidide instead of trinexapac-ethyl). We again included a pre-seeding glyphosate treatment for comparison. Following establishment, the area was maintained as though it were a golf course fairway at a “higher end” country club, as in the first study.

The grid method was used to estimate the percentage of KB in the plots in March 2001 (six MAS) and July 2002 (21 MAS). This time, we found some things that worked (see Table 1 on page G6). Specifically, PSLM and higher seeding rates were most effective in enhancing KB establishment. Six MAS, the average percent KB across all PSLM plots was 8.1 percent, compared to 2 percent in non-PSLM plots. By 21 MAS, PSLM plots averaged 31.9 percent KB, compared to 18.1 percent in non-PSLM plots. By comparison, pre-seeding glyphosate-treated check plots averaged 84 percent KB six MAS, with complete coverage obtained nine MAS. However, using glyphosate would dictate that golf course fairways be closed during the establishment period, which means lost revenue for the club.

PSLM did reduce turfgrass visual quality during the 4-week period; however, the surface remained smooth and would still allow play. Fortunately, the plots recovered rapidly after PSLM ceased — only three weeks later there was no difference in visual quality between the PSLM and non-PSLM plots.

Along with PSLM, higher seeding rates were also effective. By 21 MAS, plots seeded in the fall with 2 pounds PLS per 1,000 square feet averaged 19.7 percent KB, whereas plots seeded in the fall and spring with 4 pounds PLS per 1,000 square feet (i.e., 8 pounds PLS total) averaged 29.8 percent. You should note that, all other factors being equal, the percentage of KB obtained with a single fall seeding at 4 pounds PLS per 1,000 square feet was never significantly different from that obtained by seeding both fall and spring at 4 pounds PLS per 1,000 square feet (see Table 1 on page G6).

Similar to trinexapac-ethyl in our first study, the PGR mefluidide proved to be of little value. By 21 MAS, the PGR-treated plots did not have significantly more KB than untreated plots when the other factors were similar (see Table 1 on page G6).


Why were PSLM and a higher seeding rate helpful in enhancing KB establishment? It really boils down to giving the KB a competitive edge. PSLM kept the perennial ryegrass stand in check until the KB seedlings developed some roots and formed a few leaves. Without PSLM, the slow-growing KB is shaded and crowded out by the more vigorous perennial ryegrass. It's also possible that PSLM allowed more light to reach the KB seeds, increasing germination.

Higher seeding rates probably helped by simply providing more potential KB plants. When interseeding into a mature turfgrass stand like we did here, seedling survival is lower than in non-competitive situations (such as our glyphosate-treated plots), so providing more seeds via a higher seeding rate makes sense.

Finally, I should point out that KB coverage increased under all treatments between six and 21 MAS (see Table 1 at left). Therefore, if time is not a concern, it appears that gradual conversion towards KB may occur even if you use lower seeding rates and don't implement PSLM. This is probably a result of the rhizomatous growth habit of KB. Usually, however, a faster conversion would be desirable to golf course superintendents. Overall, our research shows that you can achieve the fastest conversion — without closing fairways — by using a seeding rate of 4 pounds PLS per 1,000 square feet in September, combined with PSLM for four weeks following seeding. PSLM combined with a higher-than-normal seeding rate helps give those little Kentucky bluegrass plants a fighting chance.

Steve Keeley is an associate professor of turfgrass management in the division of horticulture at Kansas State University (Manhattan, Kan.).

Table 1. Post-seeding low-mowing (PSLM), seeding rate, seeding timing, and plant growth regulator (PGR) effects on establishment of Kentucky bluegrass interseeded into a mature perennial ryegrass fairway-height turf.
PSLM‡ Seeding rate (pounds PLS per 1,000 square feet) Seeding timing PGR§ 6 MAS¶ 21MAS
percent of plot area
no 2 fall only no 1.0 12.7
no 2 fall + spring no 17.0
no 4 fall only no 2.4 15.2
no 4 fall + spring no 21.8
no 2 fall only yes 1.6 13.0
no 2 fall + spring yes 18.5
no 4 fall only yes 3.0 21.8
no 4 fall + spring yes 25.2
yes 2 fall only no 3.5 28.5
yes 2 fall + spring no 29.4
yes 4 fall only no 8.5 29.1
yes 4 fall + spring no 32.7
yes 2 fall only yes 5.2 24.5
yes 2 fall + spring yes 31.8
yes 4 fall only yes 15.1 39.1
yes 4 fall + spring yes 39.7
lsd.05#: 6.3 14.2
†Check plots treated with glyphosate prior to seeding averaged 84 percent Kentucky bluegrass six months after seeding and 100 percent nine months after seeding.
‡Post-seeding low mowing consisted of mowing twice weekly at 0.25 inches for four weeks following seeding.
§The PGR mefluidide was applied at the label rate for perennial ryegrass one week before seeding.
¶Months after interseeding.
#This is a statistical tool to help determine significant differences. If the difference between two treatments is greater than the lsd.05, the results are considered significantly different.

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