Bob Mugaas, University of Minnesota Extension Educator
Each summer, temporary periods of hot, dry conditions commonly occur in this part of the country. Each summer these conditions prompt many questions about caring for and watering our lawns (as well as other landscape plants). Following are some lawn care tips to help cope with these dry conditions during Minnesota summers.
1. Where lawns are maintained in an actively growing condition, keep mower heights of cut between 2.5 to 3.0 inches to encourage deeper rooting. The cool season lawn grasses common to this area have naturally shallower, less robust root systems during the middle of the summer compared to the spring and fall periods of the year. Shorter mowing heights without an accompanying increase in water can make that situation even worse and unnecessarily stress the plants.
2. Avoid the use of postemergence broadleaf or crabgrass killers during hot dry conditions or any time the lawn is under drought stress, even during cooler periods. The term postemergence means the weedy plants are up and readily visible. Most of the products used during the growing season, with the exception of preemergence crabgrass weed killers, are applied as postemergence products to the actively growing weeds. The operative word here is 'actively' growing weeds. The same summer stresses that impact our grass plants can also slow the growth of many perennial broadleaf weeds. Thus, they may not be as sensitive to an herbicide application under these conditions compared to one applied in September to early October when they are again more actively growing.
3. During the summer months of July and August, actively growing lawns in a well watered soil utilize about 1 to 1.5 inches of water per week in the Twin Cities area. Historically, that has been the amount of water we have tried to replenish to our lawns on a weekly basis. However, the practice of "deficit irrigation" has become more and more common place over the last several years. In this practice, water is replenished at less than the amount of actual water lost during any particular growing period. Research has shown that under average summer conditions, lawn grasses will maintain acceptable quality even if less than the total water lost from the lawn isn't replenished. For example, applying an inch of water every two to three weeks instead of once per week could save a third to a half of the water used during the summer months while maintaining an acceptable lawn. Shorter intervals are needed during very hot, dry conditions while more moderate temperatures allow for a little longer interval between watering. While there may be some browning of the grass during these stress periods, most plants will remain alive and begin to grow again once more favorable conditions return.
4. Where soils are more clay like and/or compacted or soils are sandy, consider splitting the applications in half and apply more frequently. Since root systems will be somewhat shorter during this period, applying a little less water per time but a little more frequently will help ensure that water will not pond or runoff of the surface in the case of heavy clay and/or compacted soils. Likewise, in a sandy soil, it will help ensure that excess water does not drain past the grass plant's rootzone and hence be wasted.
5. It is generally best to allow the grass to grow more slowly during the summer stress periods rather than trying to force excessive grass blade growth by applying high rates of nitrogen fertilizer. The stimulus provided by the nitrogen fertilizer will result in increased water demand by the plant, more frequent mowing and usually a more rapid depletion of plant nutrition and energy reserves. All of those can make the plant more vulnerable to summer stresses, particularly heat and drought stress. For most average lawns, holding off applying fertilizer for a few more weeks, until around Labor Day, is more beneficial for the grass plant than applying during the summer stress periods.
6. There are instances where the heat and dryness of summer are just too much for our grass plants and some permanent injury can occur. This most commonly occurs where there is no possibility for supplemental water to be applied in amounts to at least keep the plants alive. On the brighter side, where loss of turfgrass has occurred, the period from mid-August through about the middle of September is an ideal time to do some overseeding and repairing of those areas damaged or destroyed by stressful summer conditions. Seeding at this time of year avoids significant competition from annual weeds and takes advantage of the natural cooling of temperatures. This also coincides with a time of year when rainfall is more frequent although supplemental irrigation can certainly be used to provide a uniform grass seed germination and early growth environment. This greatly improves the chances for successful establishment. In addition, this might be just the situation needed to allow for the introduction of more drought tolerant grasses such as the fine fescues into our lawns. While seeding is certainly one option for repair, resodding can be a quick and convenient way to repair an area. Making sure that the sod remains moist but not soggy for at least the first couple of weeks after installation will be necessary for successful establishment of the new sod. Never allow new sod or young grass seedlings to completely dry out as that will usually result in their death.