Bob Mugaas, UMN Extension Educator
1. One of the very best times of the year to be fertilizing your lawn is from about Labor Day through the middle of September. Applications that put down about one pound of nitrogen per 1000 square feet will help provide the necessary nutrition through the late summer / fall period - a time of active grass shoot and root growth.
2. It is important to avoid serious water stress on lawns this time of year. As noted above, the late summer /early fall period is a time of active growth. Hence, not only is sufficient nutrition important but ample soil water is just as important to sustain and encourage growth. Early in September, the average rule of thumb of one inch of water per week including rainfall should be sufficient. As we get later into September and early October, that same one inch of water may be sufficient for two or even three weeks depending on weather conditions. That is, the warmer and drier the weather the more frequently plants will need watering. The cooler, and more moist the weather, the less frequently plants will need to be watered. Remember, soils should be just damp to the touch not soaking wet to provide a healthy place for roots to grow and beneficial soil microbes to flourish.
3. Early September is an excellent time to be doing some overseeding of damaged or thin areas of the lawn. Seed germination is usually much quicker due to the warmer soil temperatures. Hence, seedlings are able to get out of the ground and more quickly establish the area prior to the onset of colder conditions of late fall and early winter. If you're struggling with trying to get some grass growing under the shade of some maturing shade trees, try growing some of the fine fescues. They are well adapted to dry shade conditions and are tolerate much lower inputs of fertilizer and water while still remaining healthy. If seeding areas shaded by trees, be sure to keep lightly removing tree leaves as they fall. That will help ensure that seedlings receive sufficient sunlight throughout the fall resulting in better establishment. Keep newly seeded areas damp during the germination process and gradually back-off the water as they begin to get established - usually about three to four weeks.
4. One of the best times to be aerating the lawn is right around Labor Day. This minimizes the amount of germination from unwanted weed seeds making for less competition to the new grass seedlings. It also provides increased soil oxygen levels that encourage better root growth and a healthier soil microbial community. Lawn aerifiers that pull a core of soil and deposit that core on the lawn surface are the most effective units that are still relatively easy to use for homeowners. These soil cores can be left to decompose naturally over the next few weeks. There is usually not a need to remove these from a home lawn. If you are also planning some fertilizing, and/or want to do some overseeding, an excellent time to do that is right after you have aerified. Aerification can also be used to control the rate of thatch build-up as the decomposing soil cores help to reinoculate the underlying thatch with soil. In turn, that helps break down the thatch and keep the amount of thatch build-up to below damaging levels (i.e., less than ½ inch)
When you think of fall as an active period of growth for your lawn grasses, the extra effort to ensure good growing conditions during that time helps ensure a healthy lawn going into the winter and a lawn quicker to recover and resume growth in the spring. Besides, late summer and early fall are some of the nicest conditions of the year to be tinkering with your lawn. Enjoy!