Kentucky bluegrass and other cool-season turfgrasses thrive in the cool temperatures and frequent rains of spring. However, the growing conditions for cool-season turfgrasses are usually much more difficult during the summer months. Hot, dry summer weather is stressful to cool-season turfgrasses. Fortunately, good cultural practices can help bluegrass lawns survive stressful summer weather.
Sound mowing practices are important during the summer months. Kentucky bluegrass lawns should be mowed at a height of 3 to 3.5 inches in summer. (During cool weather in spring and fall, bluegrass lawns should be mowed at a height of 2.5 to 3 inches.) 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.
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 4.5 inches. Removing more than one-third of the leaf area weakens the turfgrass and reduces its ability to withstand additional environmental stresses. Weakened turf is also more likely to be invaded by weeds.
If possible, mow in the cool of the morning or evening. Mowing at midday may place additional stress on the turf. Also, make sure the mower blade is sharp. Dull blades tear and bruise the leaf tips.
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 4 to 6 weeks without rainfall or irrigation. Healthy lawns that have been allowed to go dormant will green up again when the turf receives sufficient water.
While dormancy is a natural survival mechanism for cool-season turfgrasses, Kentucky bluegrass will not remain dormant indefinitely. Bluegrass lawns are at risk of dying if dormant for more than 4 to 6 weeks. To prevent serious damage, apply 1 to 1.5 inches of water in a single application to bluegrass lawns that have been dormant for 4 to 6 weeks. Water again 7 days later. The grass should begin to green up after the second application of water.
Gardeners who want a green lawn throughout the summer should begin to water the lawn when symptoms of moisture stress begin to develop, but before the grass becomes dormant. A good indication of water stress in turfgrass is leaf color. Bluegrass that has access to adequate supplies of moisture is normally dark green in color. The foliage turns bluish green when water-stressed. Water-stressed turfgrass is also less resilient. Footprints remain in the turf after walking across it.
Turfgrass requires approximately 1 to 1.5 inches of water per week. When watering the lawn, apply this amount in a single application or possibly 2 applications 3 or 4 days apart. Avoid frequent, light applications of water which promote shallow rooting and lush growth. Lush, shallow-rooted turfgrass is less drought tolerant. It is also more susceptible to pest problems.
Sprinklers are the best way to water lawns. The amount of water applied may be determined by placing 2 or 3 rain gauges within the spray pattern.
The best time to water a lawn is early morning (5:00 to 9:00 am). Morning applications allow the water to soak into the soil with little water lost to evaporation. When watering is completed, the turfgrass foliage dries quickly. Watering at midday is more wasteful because of the high rate of evaporation. Also, strong midday winds may carry the water onto driveways or streets and waste considerable amounts of water. Watering the lawn in the evening or at night may increase disease problems.
White grubs are the most serious and destructive lawn insect pest in Iowa. There are three basic 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.
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 home gardeners include imidacloprid (Merit®, Grub-Ex®) and halofenozide (Mach 2®, Grub-B-Gon®). When using insecticides, carefully read and follow label directions.
It is generally not advisable to fertilize Kentucky bluegrass and other cool-season grasses during the summer months. 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 1 pound of actual nitrogen per 1,000 square feet in one application.
It is generally not recommended to apply herbicides during the summer months. Crabgrass is best controlled with an application of a pre-emergence herbicide in spring. Dandelion, plantain, white clover, and other broadleaf weeds are best controlled with an application of a broadleaf herbicide in fall.