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Introduction
Recently released cultivars of creeping bentgrass are available that possess more desirable agronomic characteristics. Traditional conversion involving the use of non-selective herbicides or soil fumigants is costly and time consuming. Interseeding has been proposed as an alternative conversion method. Interseeding is a method of conversion where seed is sown into an established grass sward. The goal is for the introduced cultivar or species to become the major component of the sward over time.

A variety of factors influence the success of interseeding. Gap size has been found to influence the early success of seedlings. However, significant disruption to the canopy would reduce the uniformity of the playing surface and may not be desirable to golfers.

Factors such as germination speed and seed size have also been found to influence the success of seed sown into areas with established plants. In addition, research shows that seeding at above average rates has been advantageous when trying to establish turf cover when traffic is present.

The objectives of this study were to evaluate the ability of converting an established golf course putting green via interseeding.

Materials and Methods
An interseeding study was conducted on an established practice putting green at Hyperion Field Club in Johnston, Iowa and on a research putting green in 2009 and 2010. Four replications arranged in a split block experimental design were used to evaluate three plant protectants (main plots) and two seeding regimes (subplot within main plot). Main plots treatments included a non-treated control, applications of Velocity herbicide, or Trimmit plant growth regulator. Sub plot treatments included seed sown into the canopy twice or nine times.

A Maredo seeder with vibratory spikes was used throughout the season to seed into the existing canopy at 1.5 lb/1000ft2 for seasonal totals of 4.5 or 13.5 lbs/1000ft2. Velocity was applied every 14 days at 2 oz/A starting 4 June 2010 and concluding 16 July 2010 for a total of four applications. A fifth and final application of Velocity was made 1 October 2010 at the same rate. Trimmit was applied every 14 days at 6 oz/A starting 4 June 2010 and concluding 10 September 2010 for a total of eight applications.

Regular maintenance practices were only slightly altered as the goal was to preserve conditions that would be conducive for the play of golf. Irrigation via hand-watering was conducted during the summer months in order to provide moisture to promote germination. Regular maintenance included mowing performed daily to a height of 0.125 in. and overhead irrigation was applied as necessary. Fertilizer (7N-7P-7K) was applied at a rate of 0.25 lb N/1000 ft2 each month of the growing season and diseases and insects were controlled as necessary.

Plant samples were collected during the fall of each year prior to snowfall and the following spring for evaluation of Penn A-4 populations. Cultivar identification was performed by using random amplified polymorphic DNA markers.

Results and Discussion
Five months after initial seeding, the 4.5 and 13.5 lb/1000 ft2 seeding regimes resulted in a 19 and 39 percent conversion to Penn A-4, respectively (Figure 1). However, twelve months after initial seeding, Penn A-4 populations were reduced to 1 and 8 percent for the 4.5 and 13.5 lb/1000 ft2 seeding rate, respectively (Figure 2).

Figure 1. Percentage conversion of an established putting green to Penn A-4, five months after initial seeding.  Values within seeding regimes followed by the same letter are not different at P = .05

Figure 2. Percentage conversion of an established putting green to Penn A-4, 12 months after initial seeding. Values within seeding regimes followed by the same letter are not different at P = .05

These data indicate a transient shift to Penn A-4 occurred but was not able to persist. Additionally, applications of Velocity or Trimmit did not hasten conversion to Penn A-4 (Figures 1 and 2). The lack of establishment is likely due to competition from the surrounding turf and mechanical and environmental stresses.

During the first year of the study, the percentage of annual bluegrass was reduced from approximately 60 to 20 percent in plots treated with Velocity. No loss of density occurred, but phytotoxicity was observed in plots treated with Velocity. Significant loss of density was observed during the second year of the study from Velocity applications.

Conversion was more persistent on the research putting green. The 13.5 lb/1000 ft2 seeding regime resulted in a 42% establishment of Penn A-4 the fall following interseeding (Figure 3). Evaluation of the plots the following spring revealed 45% Penn A-4 still present. Although interseeding was more successful in the research setting, the overall quality of the turf would not be acceptable for most putting greens.

Figure 3. Percentage conversion of a research putting green to Penn A-4. Values followed by the same letter are not different at P = .05

These results suggest that the level of maintenance and overall quality of the putting surface influence the success of conversion. Conversion through interseeding in this study was not successful when the plots were maintained under golf course conditions. Interseeding was only successful when conditions were allowed to deteriorate below acceptable levels. The overall conditioning of the putting surface in order to permit interseeding needs to be weighed against the cost of a traditional conversion when deciding on a renovation program.

On March 12, I posted some information on a late-fall (Oct. 25, 2011) seeding of bentgrass at Makray Memorial Golf Club near Chicago by Supt. Timothy Christians. Some of the newly seeded tees were covered during the winter and some were not. The subject of the post was the surprising success from such a late seeding.

The spring has continued to be unusually warm in the Chicago area. The following two pictures are from April 3. The first one is of the tee that had been covered and the second one is of the uncovered tee. They have both been mowed by April 3 and both will be open much earlier than usually would be expected.

Tee that had been covered during the winter.