Path to Conversion

Several products and methods have been developed to control Poa annua over the years. These include many different herbicide chemistries and the use of plant growth regulators to control seed head production or limit Poa annua growth. While some of these controls have been successful, they are effective only when specific conditions exist or on a particular Poa annua biotype. No consistent, universal control for this difficult weed has been developed to date. Many golf course superintendents either have decided to manage Poa annua as a turf species, or limit its population growth. As described by golf course superintendent W.A. Alexander in the 1922 Bulletin of the USGA Green Section, the best method of Poa control may still be to, “cut it out with a knife.”

The O.M. Scotts Company, along with the Monsanto Company, recently have developed lines of creeping bentgrass (Agrostis stolonifera L.) that are tolerant of the non-selective herbicide Roundup (glyphosate). And other seed companies are also developing their own varieties of genetically modified turfgrasses (see Grounds Maintenance's “Battle of the Bents,” August 2003). Roundup Ready creeping bentgrass (RRCB) was created by using molecular biology techniques to insert a Roundup Ready gene in the genetic code of the plant. Although currently not available, when released, Roundup Ready bentgrass may be a valuable tool to control Poa annua in creeping bentgrass greens, fairways and tees.

At Iowa State University, researchers have been working on the most effective means of converting conventional turfgrass areas to RRCB. Traditional conversion techniques require that you close your golf course for an extended period of time, resulting in frustrated golfers and lost revenue. It may be difficult to convince golf course owners, members, or the greens committee that closing the course to undergo a conversion process would be worth the end result. The objective of the research is to develop conversion methods that minimize surface disruption and shorten the period of time the area is out of play.


Traditional putting green and fairway renovation practices usually start with an application of Roundup, followed by seeding using a slit-seeder or seeding into aerification holes. Often, the time interval from the application of the herbicide to the time of seeding is 2 to 3 weeks. In some cases, you must repeat applications of Roundup to completely eliminate existing turf. The conversion to RRCB is unique because you can use Roundup to selectively remove existing turf while Roundup Ready seedlings are germinating. This makes it possible to shorten the conversion process. The objective of this study was to determine when an application of Roundup should be made to remove existing turf to ensure successful RRCB establishment.

The study began on September 3, 2002, when a creeping bentgrass green and fairway, and fairways established with Kentucky bluegrass and perennial ryegrass were core aerified. The study was conducted at Iowa State University's Veenker Memorial golf course with the help of CGCS John Newton. Aerification cores on the green were removed, the area was vertically mowed by using triplex-mounted verticut units, and topdressed with sand to fill aerification holes. Cores on fairways were broken up by using the triplex verticut units, and the area was dragged to fill in aerification holes with native soil. RRCB was seeded at a rate of 1.7 pounds per 1,000 square feet to the fairways and greens. These methods are similar to cultural practices that are regularly conducted on golf course greens and fairways.

Existing turf was killed with Roundup at various times before and after seeding. Roundup applications to individual plots were made 7 days before seeding, at the time of seeding, 7, 14 and 28 days after seeding, at the end of the 2002 growing season and in Spring 2003. The seed that used was 50 percent Roundup Ready with the remaining seed being susceptible to Roundup. This combination of conventional and RRCB is due to the process by which the seed is produced. The 1.7-pound rate is of the combined seed and the actual rate of RRCB was 0.85 pounds/1,000 square feet. Because of this mixture, plots that were treated before seedling germination were again treated with Roundup 28 days after seeding to remove any conventional bentgrass seedlings. To act as a control, the existing sod of one plot was removed after it was killed with Roundup and seed was applied to bare soil. This plot was treated with Roundup 28 days after seeding.

As the study progressed, the percentage cover of RRCB of each plot was recorded to determine the effect of Roundup application timing on the establishment to RRCB.


By late fall 2002, some of the plots were acceptable for play, although they were not in ideal conditions. The most successful Roundup treatment timings were 7 days before seeding, at the time of seeding, and 7 days after seeding. On May 7, 2003 (the spring after seeding), plots on the existing creeping bentgrass fairway, to which Roundup was applied 7 days after seeding, were 73 percent covered compared to the bare soil plot, which was 80 percent covered. Plots receiving a Roundup application 7 days after seeding on the Kentucky bluegrass and perennial ryegrass fairways were 90 and 80 percent covered by May 7, 2003, as compared to the bare soil plots that were 88 and 87 percent. Although some of the plots that were treated 7 days before seeding and at the time of seeding had greater RRCB cover, it was not significant enough to warrant the earlier Roundup application.

The conversion of the bentgrass green to RRCB was not as effective. On June 1, the year after seeding, none of the plots except the bare soil plot had RRCB cover greater than 75 percent. Furthermore, a green must have nearly 100 percent cover to make playing conditions acceptable.


The results of the 2002 study demonstrated that seeding into openings made by aerification and vertical mowing were not very effective for putting green conversion. In 2003, studies were initiated on a native soil creeping bentgrass green to develop methods that would result in the successful conversion of golf course greens RRCB. The objectives of these studies were to determine mechanical methods, seeding rates, and seeding dates that hasten the conversion of golf course greens to RRCB. The study was divided into two trials: one to study seeding rates and the other to investigate seeding dates.

Four different mechanical methods were used in the seeding date and seeding rate trials, a T.I.P. greens spiker (T.I.P. INC., Custer, Wis.), Terri Combi greens spiker (Wiedenmann North America LLC, Savannah, Ga.), Graden vertical mower (Graden Industries, Victoria, Australia), and aerification/vertical mowing.


On August 18, 2003, one day before seeding, all plots were treated with Roundup. The following day, all mechanical treatments were applied and seed was sown at rates of 0.5, 1.5, and 2.5 pounds actual RRCB per 1,000 square feet to sub-plots within the mechanical treatment. The seed used was 60 percent RRCB and 40 percent conventional bentgrass; therefore, seeding rates were adjusted to insure that RRCB seed was sown at the specified rates. Each plot was then topdressed with sand to smooth the surface and fill in any voids created by the mechanical treatments. The entire area was treated with Roundup 21 days after seeding, as the seed used was 60 percent RRCB and 40 percent conventional creeping bentgrass. This, again, is an unavoidable result of the techniques by which the seed is produced.


One day before the seeding dates of August 19, September 9 and September 30, Roundup was applied to each plot. Each mechanical treatment was applied to individual plots on the designated seeding dates. All plots were seeded at a rate of 1.5 pounds actual RRCB per 1000 square feet and were topdressed with sand to fill in voids and smooth the surface. Each plot was treated with Roundup 21 days after seeding to remove any bentgrass that was not Roundup tolerant.


Researchers determined the effectiveness of each method by recording percent cover and giving each plot a quality rating. Although the study is still being conducted, the researchers were able to successfully convert an existing bentgrass green to RRCB by June 1, 2004, when seeding occurred in mid August or early September of 2003.

Similar to results that you have probably seen when overseeding, seed tends to germinate only in holes created by the mechanical device. Therefore, it appears that the conversion process is quickest when the mechanical method used creates many, smaller soil openings for seed placement (e.g. Terri Combi spiker, TIP spiker) rather than fewer, larger soil openings (e.g. aerification/vertical mowing).

The results indicate there is no advantage to seeding at a rate of 2.5 pounds when compared to 1.5 pounds RRCB per 1,000 square feet; however, the rate of 0.5 pound was clearly inferior to the higher seeding rates.

As you might have expected, the quickest conversion to RRCB occurred at the earliest seeding date. An earlier seeding date does not guarantee a successful conversion to RRCB, but it does make establishment less dependent on ideal weather conditions later in the fall.


When released, RRCB will be a valuable asset for removal of Poa annua, as well as other weeds, from fairways, tees and putting greens. Establishing RRCB in late summer and using methods that provide uniform establishment over the entire surface appear to provide satisfactory results with a minimum loss of play. Work on the establishment of RRCB is continuing at Iowa State University and more information will be available in the future.

Luke Dant is a graduate research technician and Nick Christians, Ph.D., is professor of turfgrass science, both at Iowa State University (Ames, Iowa).

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