Removing overseeded ryegrass from bermudagrass

Turfgrass managers in the southern United States often overseed bermudagrass (Cynodon spp.) with perennial ryegrass (Lolium perenne) to provide a dense, green turf during the winter months. While winter color is the only visible benefit, other potential benefits of overseeding include increased playing-surface uniformity, minimized damage from equipment and foot traffic, and reduced weed invasion during winter dormancy. Depending on the location, overseeded perennial ryegrass may last only 3 to 4 months. However, in cooler portions of the South, it may last up to 7 to 9 months.

Although the benefits of overseeding bermudagrass with perennial ryegrass are clear, the spring transition from cool-season back to warm-season grasses can be troublesome. Turfgrass managers need an effective and reliable way to transition perennial ryegrass out of overseeded bermudagrass. Recent research suggests that cultural methods that were thought to have worked in the past may not be so effective after all. Conversely, chemical methods seem to be the most reliable and effective way to promote a smooth transition in the spring and early summer.

Ideally, increasing temperatures in spring and early summer cause bermudagrass to come out of dormancy while, at the same time, the overseeded grass slowly dies out. However, weather conditions in spring and early summer vary, and natural transitions are not consistent from year to year. For example, cool, wet springs promote growth and development of perennial ryegrass while inhibiting transition to bermudagrass. This is a common problem in the Transition Zone of the United States where inconsistent spring and early summer weather makes it difficult to predict when the natural transition from perennial ryegrass to bermudagrass will take place.

In an ideal transition, the perennial ryegrass completely disappears just as the bermudagrass has fully greened up. However, when the ryegrass dies out faster than the bermudagrass fills in, the result is poor playability. Conversely, the newer heat-, disease-, and drought-tolerant varieties of perennial ryegrass used for overseeding may survive later into spring or summer than older cultivars. Thus, while these newer ryegrass varieties provide more attractive turf during winter months, they also create problems with spring transition.

Cultural methods for golf-course greens Management practices such as vertical mowing, scalping and aeration can aid in a smooth transition by stimulating bermudagrass growth as temperatures increase in the spring and early summer. Problems with spring root decline of bermudagrass dictate that you start these culturalpractices no later than several weeks before expected green-up. (Overseeding can inhibit bermudagrass greenup, which in turn may promote spring root decline.) The following procedures encourage bermudagrass at the expense of the perennial ryegrass. But be aware that these cultural practices may disrupt the playability of the turfgrass:

* Reduce mowing height, if possible, several weeks before expected spring green-up of bermudagrass. This reduces shading, warms the soil and sets back the overseeded perennial ryegrasses. * Spike golf greens every 2 weeks during cool weather. * Aerate, and pulverize the plugs, several weeks before expected spring green-up. This promotes bermudagrass growth by warming the soil. * Avoid spring fertilization until 2 to 3 weeks after spring green-up. * Maintain good soil moisture for the new set of roots being produced by bermudagrass. * Initiate light, frequent verticutting on greens during spring green-up of bermudagrass but while the ryegrass is still actively growing.

However, proving that this is not an exact science, research at Clemson University showed that traditional cultivation practices such as aerating, topdressing and vertical mowing are not effective in promoting bermudagrass emergence on overseeded greens. Typically, the vertical mowing causes a temporary decline in turfgrass quality and playability. Ultimately, when these cultural practices are credited for a successful spring transition, it actually is due to lucky timing and environmental conditions. Greens--regardless of treatment--transition well in a hot, dry spring and early summer. In contrast, a cool wet spring and early summer will promote perennial ryegrass and inhibit the natural transition to bermudagrass.

Chemical methods for golf-course greens Pronamide (Rohm and Haas' Kerb) is a relatively slow-acting herbicide that superintendents commonly use to transition ryegrass out of bermudagrass putting greens. Pronamide works well in cool conditions. Thus, if you make applications too early, and subsequent weather conditions do not favor bermudagrass growth, a poor transition will result. Research at the University of Georgia confirmed this, showing that pronamide, applied at 0.50 or 0.75 pound active ingredient (ai) per acre, can lead to browning of the perennial ryegrass and a poor transition. These treatments effectively removed the ryegrass, but turfgrass quality was not acceptable if bermudagrass was not actively growing. These research results stress the importance of application timing.

Pronamide applied at 0.25 pound ai per acre during early bermudagrass green-up also led to unacceptable turfgrass quality. However, turfgrass quality was acceptable when applications at this rate occurred when bermudagrass green-up had reached 50 percent. This resulted in a 4 -to 6-week transition. Because the higher light intensity and increased temperatures of late spring promote bermudagrass growth activity, this seems to be the ideal time to apply pronamide to smoothly transition out the overseeded ryegrass.

Glyphosate (Monsanto's Roundup Pro), if you apply it when bermudagrass is completely dormant, removes the perennial ryegrass but does not provide a gradual transition. The turf will be brown until the bermudagrass greens up. This is unacceptable for a golf course trying to maintain the best possible playing conditions in spring and early summer.

Cultural methods for fairways, tees and athletic fields At North Carolina State University, we've conducted research over the last 2 years that indicates that the use of cultural methods to remove perennial ryegrass at higher heights of cut are not effective. The cultural methods that the we included in our study were vertical mowing, scalping (decreasing the mowing height from 0.75 to 0.5 inch), aerating, vertical mowing and scalping (combined), and applying ammonium-nitrate. We applied the ammonium nitrate (at 2.0 pounds of nitrogen per 1,000 square feet) to wet turfgrass foliage to increase the burn potential of the fertilizer, and we performed all treatments, except the ammonium nitrate, every 2 weeks until the perennial ryegrass had naturally disappeared from the untreated (control) plots. Our results show that all of the cultural treatments transitioned the perennial ryegrass out at about the same time as the untreated plots. It was, again, environmental conditions--especially temperature and relative humidity--that dictated the speed of transition.

One evident difference among treatments was the density of bermudagrass plants when all the perennial ryegrass was gone. Plots we treated with ammonium nitrate, even though they did not produce faster transition, did have significantly higher bermudagrass shoot density (see graph, page G 24). Aeration produced significantly lower bermudagrass shoot density at the end of the season and turfgrass quality that was unacceptable.

Chemical methods for fairways, tees and athletic fields We also tested chemical methods (see Table 1, opposite page) to promote transition, and found that certain chemical treatments can be more effective than a natural transition. We applied treatments using two separate timings:

* Early application timing simulated deliberate chemical removal of the overseeding. These treatments included pronamide, glyphosate, glufosinate (AgrEvo's Finale), atrazine (Novartis' Aatrex) and metribuzin (Bayer's Sencor).

* Late application timing (mid-June) simulated a natural transition in mid-June (that is, a late green-up that might prompt a turfgrass manager that was hoping for a natural transition to use chemical methods). These treatments included pronamide, atrazine and imazaquin, which could injure the actively growing bermudagrass.

As previously mentioned, environmental conditions and overall turfgrass health are major factors in obtaining a smooth transition. Cool ambient temperatures at the time of application can decrease foliar uptake and translocation of systemic herbicides within plants. In addition, dry conditions can cause a plant to grow slowly. Thus, as with most chemical treatments, when these conditions exist, it is not the best time to apply these products.

* The early application treatment that worked best (see Table 2, page opposite page) was pronamide. Pronamide provided a good transition, and cool temperatures at the time of application did not negatively affect its activity. Nevertheless, you should carefully consider if, and when, to use pronamide. If temperatures are low and the bermudagrass has not broken dormancy, pronamide will readily remove the overseeding, leaving dormant bermudagrass that cannot fill in the spaces once occupied by the perennial ryegrass. For a smooth transition, you should make early applications of pronamide 6 to 8 weeks before full bermudagrass green-up.

Early applications of both glyphosate and glufosinate successfully removed perennial ryegrass but resulted in an unsuitable transition because the bermudagrass was still dormant. However, if you do decide to use these products, it's best to apply them before the bermudagrass breaks dormancy. An application when the bermudagrass already is breaking dormancy may not kill it--as long as the rates are low--but it will prolong green-up.

* Late application of atrazine (see Table 3, page G 22) provided a fast transition with some browning of the turf. If bermudagrass green-up is greater than 50 percent, this is an acceptable treatment. But if bermudagrass green-up is less than 50 percent, the rapid removal of the perennial ryegrass results in turfgrass quality that's unacceptable for golf-course fairways, tees or athletic fields that need consistent playing conditions. Note: Novartis considers the use of atrazine for removal of overseeded ryegrass from bermudagrass experimental and does not support or recommend such use.

A late application of pronamide yields a good transition while maintaining acceptable turfgrass quality during a 4- to 6-week transition. Even though pronamide works well under cool conditions, a late application still usually results in a shorter transition because environmental conditions favor bermudagrass over ryegrass as the spring and summer months progress.

If temperatures are warm at the time of application, imazaquin will provide a good transition with minimal discoloration. However, if cooler temperatures prevail at the time of application, a poor transition results. This is the major reason superintendents do not use imazaquin as an early treatment. If you do use it, don't apply it during the green-up phase because this herbicide can delay the green-up of bermudagrass.

Know your situation Each turfgrass manager has unique site conditions that determine the need for chemical transition products. Depending on the region, temperatures and relative humidity in a typical spring may be conducive for good natural transition. This is the case for some parts of Florida, Georgia and the Southwest. However, cooler temperatures in the northern regions of the transition zone are not conducive for a transition without additional inputs.

* For athletic fields that aren't played on in the spring, a non-selective herbicide such as glyphosate or glufosinate is the best treatment for removing the perennial ryegrass.

* Pronamide, properly timed, seems to be the best chemical method for transitioning perennial ryegrass out of overseeded greens. On fairways, tees and athletic fields, an early application of pronamide provides a good transition with acceptable turf quality. The transition takes 6 to 8 weeks, and you should not make applications before bermudagrass breaks dormancy.

* Late applications of atrazine, imazaquin or pronamide also provide a good transition, although some discoloration may occur with atrazine or imazaquin.

Dr. Fred Yelverton is associate professor and Brian Horgan is a graduate research assistant, both at North Carolina State University (Raleigh).

Several chemical tools are available for removing overseeded perennial ryegrass from bermudagrass. You must find the method most suitable for your situation: * For athletic fields that aren't played on in the spring, a non-selective herbicide such as glyphosate or glufosinate is the best treatment. * Pronamide seems to be the best chemical method for transitioning perennial ryegrass out of overseeded greens. An early application also works well on fairways, tees and athletic fields. The transition takes 6 to 8 weeks, and you should not make applications before bermuda-grass breaks dormancy. * Late applications of atrazine, imazaquin or pronamide also provide a good transition, although some discoloration may occur with atrazine or imazaquin.

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