From SportsTurf, Pamela Sherratt’s “Q and A” column:

Q: How do I introduce a more sustainable turfgrass, like tall fescue, into an existing perennial ryegrass/Kentucky bluegrass field?

A: If the existing perennial ryegrass and Kentucky bluegrass field is doing okay and not posing too many problems, maybe ask why you want to make the change, since the sward is adapted to the local environment and has an established root system. By some measure, we could consider that sward “sustainable.” The word sustainable is a bit ambiguous. For example, for some it means that the plants can sustain themselves without input (not true), or that we don’t use non-renewable resources to maintain plants (also not true), or that we maintain it in an environmentally friendly way, which we’ve been doing for decades and calling it IPM. A term that I think better describes what our goal is “resilient.” We have to be ready for whatever the future brings with climate change and we have to have fields that are resilient and can withstand floods, drought, diseases, insects, weed competition and excessive traffic.

If your goal is to introduce a new species like tall fescue because you want to reduce water and pesticide use then yes, there are options. Tall fescue has a high water use rate if there is water available, but during drought periods it has deep roots and is able to access water deep in the soil profile. For that reason it is considered to be a more resilient grass in drought conditions. Tall fescue typically requires less herbicide use, due to its density and possible allellopathic properties. Endophytes in the grass can also eliminate surface feeding insects like chinch bugs and billbugs. Add on shade tolerance and wear tolerance to tall fescue’s attributes and you can see why it is considered resilient. There may be times, however, when problems like brown patch disease and white grubs will need to be addressed.

Introducing a new species into an established population of a different species is commonly referred to an interseeding. Interseeding can be difficult because the existing sward has an established root system and has a competitive advantage when it comes to space, light, nutrients, air, and water. It is also adapted to the temperature, mowing height, soil conditions, and management regime. An established sward has plants of many types that are succeeding because they are in their niche. Your perennial ryegrass and Kentucky bluegrass field is successful because their needs are being met. In essence, simply slit seeding a new species into a full sward of existing turf is futile, and broadcasting seed over an existing dense sward is money wasted.

If you want to get tall fescue established into the existing sward, there are two renovation options: one is to kill the existing sward with a non-selective herbicide then wait the required re-entry period and seed with the desired turfgrass. This is the most efficient and successful of the two options. This option gives the introduced species the best chance to germinate and establish without competition. The practice is even more successful if weeds are controlled during establishment. Applying an herbicide like mesotrione, which will not affect the germinating seed but will control weeds, will help. If seeding is done during the months of May-September it is also a good idea to get apron-treated seed to prevent damping off/pythium. Fungicides like mefenoxam will give new seedlings protection from disease for about 10-15 days.

The second option is to weaken the existing sward and reduce its competitive advantage, so that the new species can germinate and hopefully establish. This is accomplished by removing as much of the existing turf by mechanical means as possible, like fraze mowing, scalping, or aggressive aerification and/or scarification. Applying a plant growth regulator (PGR) like trinexapac-ethyl at the same time could help to check the existing turf and stop it growing. Using the highest rate of PGR for the species will make sure it is regulated for the maximum amount of time, but also may cause some phytotoxicity.

The premise is that the existing turf is not dead, but it is checked enough that the new seed can get established. It sounds a lot simpler that it actually is. Research has shown that the most dominating factor in how well a turfgrass survives is root competition, so even if the existing turf is scalped, regulated by PGRs and slit-seeded, the existing sward’s roots system could help the original turf recover and ultimately out-compete the introduced seedlings. The resulting sward with option two will also not be texturally consistent and smooth, because tall fescue is a bunch grass that will look clumpy. The process may need to be repeated over many seasons, until the tall fescue is the dominating grass. If you don’t care what it looks like for that period of time then go ahead and go with option two. I know of at least one turf manager that is using option two to renovate a county fairground site, where it does not need to look perfect and everybody is on board with the goal of the project. Good luck!