The summer’s heat and humidity have taken an unusually high toll on turf in lawns, athletic fields, and golf courses. Most of the decline can be attributed to poor root growth weakening cool-season grasses and starting an avalanche of secondary factors leading to further decline. Root growth of cool-season grasses reaches a peak in mid-spring and declines with summer, basically stopping once soil temperatures reach 70F. Roots are relatively short-lived and require regular growth to replace those that die. With increasing soil temperatures, root growth does not keep pace with root dieback, thus root systems naturally become shallower and lose density during the heat of summer. Above average temperatures and a longer duration of high temperatures exaggerate this effect. Other factors that further reduce rooting include soil compaction, heavy spring N, saturated soils from poor drainage, excess rainfall, over-irrigation, low mowing or frequent scalping. The root system may be further compromised by feeding grubs including Japanese beetle, masked chafer, bluegrass billbug and/or black turfgrass ataenius or by soil-borne pathogens. For example, summer patch symptoms are running rampant in bluegrass lawns. We’ve even had one documented case of summer patch in a 100% creeping bentgrass green, which is an example of a minor pathogen affecting a normally resistant species that is susceptible only because of a compromised root system. Plus low rainfall in many parts of IN, and compromised root systems cannot keep up with water requirements of the plant. No matter what is applied during the summer, you cannot increase root growth once soil temperatures exceed 75F. With the temperatures in September, our expected flush of root growth has not occurred yet. This is very apparent on football and soccer fields with excess devoting.
Above ground is no better. Maximum photosynthesis and shoot growth of cool-season grasses occur near 70F. In Lafayette, we’ve had 65 days of temperatures 85For greater, 22 of which were over 90F including a number in September. Our cool-season grasses cannot produce enough photosynthate to supply the plant, and the plants rely on stored energy to stay alive. With extended heat, energy reserves are depleted further weakening the plant. Additionally, high humidity and increased irrigation to compensate for shallow root systems favor damaging foliar pathogens like pythium, brown patch, dollar spot, and gray leaf spot.
In spite of these conditions, an experienced professional should be able to manage a solid stand of one species like a Kentucky bluegrass lawn or a creeping bentgrass fairway or green with increased irrigation, fungicides, and/or insecticides. However, if a stand includes some of our weaker species like annual bluegrass (Poa annua), rough bluegrass (Poa trivialis), or perennial ryegrass, even the most seasoned turf professional can be over-matched. These three species are now common in lawns, athletic fields, and golf courses whether intentionally planted (perennial ryegrass) or introduced as weeds (Poa annua and trivialis)Poa annua is normally an annual that thins and dies with the heat of summer. Poa trivialis is a weak perennial that is extremely susceptible to dollar spot and normally goes dormant (but appears dead) during a hot August. Perennial ryegrass is extremely susceptible to a myriad of diseases that will thin or kill. Though fungicides may be life-support, the combination of heat and diseases will thin or kill these species in a summer like this.
Looking Forward
Thin Kentucky bluegrass or creeping bentgrass turf with softball-size holes can heal with aggressive fertilization this fall. Perennial ryegrass or tall fescue, with golf ball to maybe baseball size holes can be healed with fertilization. If you haven’t done so yet, fertilize in now (early Oct) 1.0 lb N/1000 ft2 using a fertilizer containing no more than 25% of the N as slow release, such as sulfur- or polymer-coated urea, urea formaldehyde, methylenediurea, dimethylenetriurea or natural organic nitrogen. Fertilize a second time near the final mowing in early November at 1.0-1.25 lb N/1000 ft2 using urea. These two fertilizations will promote good recovery of the turf.
On areas that have more damage than what can be healed through fertilization, reseeding will be needed but it’s too late to seed (see accompanying Turf Tip). Thus sod will have to be used or wait to dormant seed in late November. Before you reestablish, take a step back and determine if this is only a temporary fix when other major improvements are really needed. Poor drainage, faulty irrigation, poorly-adapted grass species, perennial grassy weeds, compacted soil, buried construction materials, shade, and excessive traffic are just a few of the issues where reseeding is only a band-aid.
Post-seeding Care
On newly seeded areas, apply a starter fertilizer at 0.5-1.0 lb N and 1.0-1.5 lb P2O5/1000 ft2 at seeding and again 4 weeks after germination. Even if only part of the area was reseeded, it won’t hurt the rest of the area to receive this fertilization in lieu of the October fertilization discussed earlier. Depending on timing of seeding, the final application of the year should be 1.0-1.25 lbs N/1000 ft2 with urea as above. Be careful with herbicide applications after seeding because they may damage seedlings. Refer to the label for specifics, but generally waiting until late October to control broadleaf weeds will be safe on the seedlings while giving very effective control.
Aerification
Since poor turf can often be attributed to compaction, aerification is critical. Aerifying as often as practical, with the largest feasible tines, and punching 20-40 holes per square foot. Hollow tines are much preferred, but solid tines are acceptable. Wait until the turf has recovered before aerifying. The rule of thumb is that you can almost never over-aerify in terms of frequency and number of holes punched.
Grub Damage?
Grub control on undamaged turf stands is not justified because the forecasted cooler temperatures will cause the grubs to cease feeding shortly. However, on turf stands damaged from grubs or summer weather, it makes sense to apply Dylox prior to reseeding to insure no damage from grubs either feeding or disrupting seedlings as they move in the soil. Only apply to stands where you can verify significant grub populations though.