Q: With a mixed ryegrass/Kentucky bluegrass field, is it possible to shift back to a majority KBG surface through overseeding; or is that a lost effort and better to go all-in on ryegrass to have a more uniform surface?
– Dan, New Jersey
A: The short answer is yes, it is possible to shift back to a majority Kentucky bluegrass field if you are overseeding bare soil/thin areas with 100% bluegrass seed and you have prepared a good seed bed with a slit-seeder, weasel or similar. Overseeding a dense perennial ryegrass sward, and hoping that bluegrass will establish is futile.
Regularly overseeding thin and bare soil during optimum bluegrass germination and establishment times is key. The ideal time to seed is fall – from mid-August to late September – due to cooling temperatures, timely rains, no competition from crabgrass or goosegrass, and seven to eight months of root development and maturation before the summer stress period. Bluegrass does not do well mid-summer when average daily temperatures are above 75 degrees Fahrenheit, especially if it’s dry.
The challenge in spring will be weed control because bluegrass is just too slow to out-complete aggressive spring weeds such as annual bluegrass, prostrate knotweed, crabgrass, goosegrass and yellow nutsedge. Avoid renovations during peak crabgrass germination period and consider applying an herbicide such as mesotrione that will control weeds but not have an adverse effect on bluegrass germination. Dormant seeding in late winter is also an option to get a jumpstart on the season. Pre-germinating the seed and mixing it with divot mix is also a good tip. The divot mix can then be applied to high-wear areas and bare spots after each game.
The main reason bluegrass overseeding fails is lack of moisture. The seed needs to be kept moist until it germinates. This means that someone must commit to lightly watering (syringing) the seed several times per day for one to two weeks until the seedlings are visible. Even then, there must be a robust fertilizer, irrigation and mowing program to get a dense, healthy ground cover. In essence, it’s a high-maintenance grass and the reason that many sports field managers use perennial ryegrass.
In response to your question about going all-in with perennial ryegrass, many cool-season turf managers do overseed with just ryegrass, but you must be vigilant for disease. Three of the grass-killing diseases – brown patch, pythium and grey leaf spot – can kill ryegrass quickly. In some cases, the inoculation period is so quick that by the time you see the disease, it is too late to save the grass. Protecting the ryegrass with a preventative fungicide program can prevent major grass loss, but it can be expensive. There are more disease-tolerant cultivars available, but keep in mind that new, seedling ryegrass is more susceptible than established ryegrass, and that while some cultivars are more tolerant, no cultivar is immune. Thus, be vigilant for disease during hot and humid weather, and investigate whether a preventative fungicide program is warranted.
Having a mixed sward of the two species gives you the best of both worlds. The perennial ryegrass will give you quick ground cover, great color and striping ability, and a hard-wearing surface. The bluegrass will give you great color and striping ability, a hard-wearing surface that can recuperate via rhizomes, and pretty good disease tolerance (bar summer patch). Bluegrass can also withstand stress situations, such as drought, many times. If the crown stays hydrated, bluegrass can survive several weeks of drought stress and be able to recuperate; perennial ryegrass typically cannot. Having both species in the sward offers a diverse range of benefits. To further maximize sward diversity, several cultivars of each species can be added to the mix.
I rarely come across sports field managers who have their own seed mix custom made. Custom-made seed mixes and blends cost more and usually require larger amounts to be purchased. Most managers rely upon their seed supplier to make sure the mix is uniform in growth habit, color, cultural requirements and mowing height. This is especially important with bluegrass because there are ~200 cultivars, and they can vary greatly from each other – from light to dark green and slow-growing to aggressive. If your mixture contains Kentucky bluegrass cultivars that have a long winter dormancy and they are slow growing and compact, they will not marry well with perennial ryegrass – especially in spring when ryegrass greens-up quickly. Further complicating the issue of uniformity, certain cultivars of Kentucky bluegrass will react differently if you are on a plant growth regulator program. Some bluegrass cultivars will become very compact and dense when under regulation, while others will not. I always recommend putting out a test plot so you can evaluate how your field will respond to certain growth regulators and other products.
Pamela Sherratt is sports turf extension specialist at The Ohio State University.
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