Convert home lawns to new varieties

The rate at which breeders are commercializing new cultivars of virtually all species means a relatively new lawn can become outdated in a short time. Newer cultivars tend to have better disease resistance and stress tolerance and often are more tolerant of mowing abuses. As a result, lawn customers, both commercial and residential, often wish to improve their turf by converting to new and improved varieties.

Most communities have older neighborhoods in which most lawns contain a mix of species that have evolved over several years to create a unique combination of grasses. These represent, for the most part, the result of survival of the fittest. Many of these older lawns need a conversion to improved cultivars for a host of reasons: thatchiness, weed invasion, disease or insect problems. In the cool-season turfgrass areas, older lawns tend to evolve to Kentucky bluegrass with some fine fescues when nitrogen fertility has been adequate. When nitrogen fertility has been low or absent, these lawns tend to become predominantly fine fescues. In many cases, a significant thatch problem exists, particularly when lime applications have not occurred and the pH is below 6.3

In newer lawns-those established since the early 1980s-landscape contractors, unless otherwise specified, typically used mixes of cool-season grasses that included significant amounts of turf-type perennial ryegrasses (enough so that the established stand is dominated by this species). In the past 5 to 8 years, breeders have made significant improvements in perennial-ryegrass cultivars, especially with improved mowability and increased resistance to red thread, rust and snow mold. Therefore, in most communities in the cool, humid regions of the country, only lawns that have been established in the past 5 years or so consist of the best cultivars that the industry has to offer. For this reason, ample opportunity exists to improve older (and even not so old) lawns through various methods of conversion. After all, sound cultural practices can only bring turf quality up to the genetic potential inherent in the cultivars. You should consider increasing that genetic potential by incorporating cultivars with improved characteristics.

Choose the correct conversion strategy How do you go about the conversion process? To make the correct choice, you must carefully inspect the existing turf to assess whether it needs a complete conversion or whether merely shifting the turf population to predominantly improved cultivars will suffice. In older lawns, certain cultivars and species often develop particular niches where they thrive (for example, moist, shaded locations, heavy-wear areas or areas with unique soil conditions). In such circumstances it would be a mistake to alter the turf in any way. Time and nature have chosen the appropriate grasses for those areas. On other sites, however, it makes good sense to completely start over by removing the existing turf and establishing improved cultivars.

* Complete conversion. Clearly, it's easiest to accomplish a conversion when you need to remove the entire turf stand. To do so, spray the entire lawn with a non-selective herbicide such as Roundup Pro (from Monsanto) or Finale (from AgrEvo). It is important to wait a couple of weeks for any regrowth to occur before attempting to remove the dead turf. If regrowth does occur, re-spray and wait an additional week or so. When bidding such work, be sure to include the necessary time for spraying and the possible need to re-spray. Contractors often overlook the need to re-spray, and then the older, inferior cultivars persist via rhizomes and stolons and re-establish themselves, mingling with the newly seeded improved cultivars. Consequently, the intended complete conversion does not happen.

If a thatch problem exists, physically remove the dead turf and thatch, and prepare a new seedbed before seeding and growing-in the improved cultivars. In those circumstances where you achieve a total kill and no thatch problem exists, the dead "stubble" serves as an excellent mulch into which you can establish the new cultivars. To do this, core cultivate several times, verticut the site and then apply the seed by slit seeding and broadcast seeding, using half the total seeding rate for each method.

*Partial conversion. On lawns, where you have determined that a reasonable amount of desirable turf is present and you only want to establish a "presence" of new cultivars, using a plant growth regulator (PGR), such as Novartis' Primo or PBI/Gordon's Embark, can be effective. The PGR is a necessary part of the process because interseeding is only successful when you suppress the existing turf so that it will not compete with the overseeded cultivars for a time.

The order of conversion should be as follows: * Mow (scalp) with a rotary mower * Core cultivate (three or four times) and verticut to a depth of at least 0.5 inch into the soil * Collect clippings, thatch and debris * Apply starter fertilizer * Use a combination of slit and drop seeding of the chosen cultivars * Roll the site and then irrigate * Two days later, treat with the PGR at the label rate.

The combination of scalping and the PGR application suppresses the existing turf for at least 6 weeks, which provides ample opportunity for the overseeded cultivars to germinate and establish before the surviving turf begins to re-grow. By providing the new, introduced cultivars the chance to become established and competitive, they will eventually dominate the population and only the fittest of the original turf will remain part of the sward. Having these remaining "fit" grass plants is not a negative-it lends genetic diversity to the final turf population.

Regardless of the reasons for wanting to convert a lawn or introduce improved cultivars to a site, you can achieve considerable success with relative ease-the chemical and mechanical tools are available and reliable. An appropriate site evaluation is the key to the ultimate outcome.

Dr. Thomas L. Watschke is professor of turfgrass science at the Pennsylvania State University (University Park, PA).

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