The overall appearance of a lawn is directly related to the maintenance provided, so this week, the ISU Extension horticulture specialists answer questions related to summer lawn care. Gardeners with questions can contact the experts by emailing or calling the ISU Extension horticulture hotline at firstname.lastname@example.org or 515-294-3108.
Kentucky bluegrass and other cool-season grasses thrive in the cool weather of spring and fall. Hot, dry conditions in summer are stressful for cool-season grasses.
Kentucky bluegrass lawns should be mowed at a height of 3 to 3½ inches in June, July and August. Bluegrass lawns should be mowed at a height of 2½ to 3 inches in spring and fall. The additional leaf area during summer shades and cools the crowns of the turfgrass plants. Extremely high temperatures at crown level can kill the turfgrass. A taller mowing height also encourages the development of a deeper root system on the turfgrass.
When mowing the lawn, never remove more than one-third of the total leaf area at any one time. Accordingly, a lawn being mowed at a height of 3 inches should be cut when it reaches a height of 4½ inches. Removing more than one-third of the leaf area weakens the turfgrass and reduces its ability to withstand additional environmental stresses. Weakened turf also is more likely to be invaded by weeds.
Dormant lawns (those that have turned brown) should not be mowed. Pedestrian and mower traffic could damage the turf.
Gardeners have two basic options on lawn care when confronted with hot, dry weather. One option is to simply allow the turf to turn brown and go dormant. The alternative is to water the turfgrass to maintain a green, actively growing lawn.
Kentucky bluegrass lawns survive extended periods of drought by turning brown and going dormant. While the foliage is dead, the turfgrass crowns and roots remain alive. Most healthy lawns can survive in a dormant state for four to six weeks without rainfall or irrigation. Healthy lawns that have been allowed to go dormant will green up again when the turf receives sufficient water.
Most lawns in Iowa require approximately 1 to 1.5 inches of water per week. When watering the lawn, apply this amount in a single application or possibly two applications three or four days apart. Avoid frequent, light applications of water, which promote shallow rooting and lush growth. Lush, shallow-rooted turfgrass is less drought tolerant. It also is more susceptible to pest problems. To determine the amount of water applied by a sprinkler, place two or three rain gauges within the spray pattern.
Early morning (5 to 9 a.m.) is the best time to water a lawn. A morning application allows the water to soak deeply into the soil with little water lost to evaporation. When watering is completed, the turfgrass foliage dries quickly. Watering at mid-day is less efficient because of rapid evaporation, and strong winds may cause uneven water distribution. Strong, mid-day winds also may carry water onto driveways, sidewalks or streets, wasting considerable amounts of water. Watering lawns in late afternoon or evening may increase disease problems.
Do not fertilize Kentucky bluegrass and other cool-season grasses during the summer months (June, July and August). The best times to fertilize cool-season grasses in Iowa are spring, mid-September and late October/early November. When fertilizing the lawn, do not apply more than one pound of actual nitrogen per 1,000 square feet in one application.
White grub populations and damage to lawns vary greatly from year to year and place to place, even varying from spot to spot within the same lawn due to variations in beetle numbers, weather, turfgrass vigor, soil conditions and other factors.
There are basically three approaches to grub management in the home lawn. One approach is to apply a preventive insecticide to the lawn on an annual basis. The second approach is to wait and see and apply a curative insecticide only when damage symptoms or signs of a grub infestation appear. The final approach is to do nothing (in regard to insecticides) and repair damaged lawn areas when grub damage occurs.
Late June to early August is the best time to apply a preventive insecticide. Several insecticides are available to commercial applicators for prevention of white grubs. Preventive insecticides available to homeowners include imidacloprid (Merit®, Grub-Ex®) and halofenozide (Mach 2®, Grub-B-Gon®). When using insecticides, carefully read and follow label directions.