This has been an unusual growing season, even by Minnesota standards.
First, we had an exceptionally long, cool spring, which was followed by tropical heat and humidity in July, and finally, a relatively average August. Most lawns seem to have come through in pretty good shape, but now's not the time to forget about your grass. Autumn is critical to maintaining a healthy, thick lawn.
The grasses we grow in Minnesota -- primarily Kentucky bluegrass, along with some perennial ryegrass and fine fescue -- all thrive in spring and fall's cooler, wetter weather. Because these grasses are growing more actively now, September is a perfect time to apply fertilizer. Ideally, you should make that application by mid-September.
The University of Minnesota no longer recommends fertilizing the lawn a second time, later in autumn. Research has shown the plants are unable to make efficient use of the added nutrients at that time.
September is also an excellent time to plant grass seed in thin or distressed parts of the lawn. In the Twin Cities area, you can plant through the end of September, though it's best to sow those seeds by mid-month, if possible.
Prepare the areas to be seeded by scruffing up the soil with a heavy rake. If the soil throughout your lawn seems hard and compacted, rent a core aerator before you seed. Run the aerator over the lawn several times, then spread the grass seed. Allow the plugs from the aeration to remain on the lawn. They'll gradually break down.
After you seed, apply fertilizer that's meant for newly seeded lawns, and keep the area moist, but not spongy-wet, until the seeds germinate. Once the new seeds are up, you can water less often, but more heavily. If we get rain regularly, you might not have to water so frequently, but don't be lulled by cool temperatures alone. If we have a dry autumn, you must water not only the seeded areas, but the rest of your lawn, as well.
Continue mowing the grass as long as it keeps growing. At first, maintain the same height you did in summer, leaving grass about 3 to 3 1/2 inches long. In October, you should begin gradually reducing your mower height so that by the last mowing, the lawn will be shorter by an inch. Don't overdo it. Just cut your grass to 2 to 2 1/2 inches.
Having shorter grass going into winter is important. Both tall grass blades, which fall over and become matted, and leaves from nearby trees and shrubs can create conditions for snow mold on your lawn.
Besides, you don't want to let those leaves go to waste. Rake them up and use them as mulch to cover perennials and bulb beds.
Fall is considered the best time to go after broad-leaf perennial weeds such as dandelions and creeping Charlie. Whether you dig them out with a dandelion digger or pull them out by hand, the perennial weeds you eliminate this fall won't be around to irritate you next spring.
If you have a lots of weeds or just prefer a less labor-intensive way of getting rid of them, now is the best time of year to use broad-leaf herbicides. Plus, the herbicides are less likely to damage non-targeted plants, because those plants are tougher than they were in spring when they were putting out new growth.
Don't use herbicides where you've planted new grass seed. And, as always, read the label carefully to make sure you have chosen the right product and you are using it correctly.
There's no need to deal with crabgrass or other annual weeds in autumn. They'll die this winter and you can easily rake them out next spring. If you typically have lots of annual weeds in your lawn, spreading a pre-emergent herbicide in May will cut back on their numbers.
Deb Brown is a garden writer and former extension horticulturist with the University of Minnesota. To ask her a gardening question, call 612-673-7793 and leave a message. She will answer questions in this column only.