Lawn care calendar for Iowa: when to fertilize and seed

Fertilizing windows are the ISU Extension answers for Kentucky bluegrass; the number of feedings is your choice, from one in late fall to three for a vigorous, dark green lawn.

Which grasses grow where in Iowa

ISU Extension covers four cool-season species for Iowa lawns: Kentucky bluegrass, fine fescue, tall fescue and ryegrass. Fine fescue, tall fescue and ryegrass tolerate shade better than bluegrass, which is the least shade tolerant. The turfgrass choice guide tells you to think about sun now and in several years when trees mature, how the lawn is used and how much work you want to do.

Mowing frequency follows growth and height: a lawn kept at 2 inches needs mowing when it reaches 3 inches, and one kept at 3 inches when it reaches 4.5, so that no more than a third of the leaf comes off in one cut. In favorable weather the grass grows an inch in five to six days.

Frost dates from NOAA

Dates are the NOAA 1991-2020 normals for the last spring and first fall 32 F freeze at 50% probability. At Des Moines International Airport the last spring frost averages April 18 and the first fall frost October 20. At Cedar Rapids Municipal Airport the dates are April 27 and October 11, and at Sioux City Gateway Airport April 29 and October 5.

Counting the days between them gives about 185 frost-free days at Des Moines, 167 at Cedar Rapids and 159 at Sioux City (our arithmetic). ISU staggers crabgrass preventer by region: early to mid-April in southern Iowa, mid-April to May 1 in central Iowa and late April to early May in the north.

Spring

Crabgrass germinates when the soil reaches 55 F. ISU says to apply the preventer when forsythia blossoms drop or redbuds begin to color, which in a typical year is around the middle of April, and to aim for half an inch of rain or irrigation within three to five days afterward. If March and April run warmer than normal, apply early in the regional window; if they run cold, apply late.

April is also one of the two best months for core aeration. Remove cores about three-quarters of an inch wide and 3 inches long, with 20 to 40 holes per square foot, when the soil is moist. Clay soils and heavy traffic call for aeration twice a year, and well-drained lawns with little use for once a year. Spring seeding from early April to mid-May works if you can irrigate, but annual weeds move in unless you use a preventer such as mesotrione at seeding.

Summer

Do not fertilize Kentucky bluegrass or other cool-season grass in June, July or August. Seeding from late May to mid-August is extremely difficult and usually unsuccessful. Mow at 3 to 3.5 inches and remove no more than a third of the blade.

Late summer is when the work restarts. ISU says to seed from mid-August to mid-September because cool-season grass germinates fast in warm soil, fall days and nights favor growth and few weeds germinate. Without a soil test, it suggests tilling 5 pounds of a 20-5-10 type fertilizer per 1,000 square feet into new seedbed, and says another feeding may be needed 3 or 4 weeks after germination, when the grass is 1 to 1.5 inches tall.

Fall

Mid-September is an excellent time to feed, at no more than 1 pound of nitrogen per 1,000 square feet, with a second feeding in late October or early November. A moderate plan uses those two. A minimal plan uses the late fall feeding alone. A three-feeding plan adds the spring one. The mowing height drops from 3 to 3.5 inches to 2.5 to 3 inches in mid to late September, and mowing continues until the grass stops growing, which in Iowa is usually late October or early November.

Early September is the best time to aerate. Late September to early November is the best time to control perennial broadleaf weeds with a mix such as 2,4-D, MCPP, dicamba or triclopyr, because the plants are moving food to their roots. Dormant seeding is the fallback if the late summer window is missed: seed after growth stops but before the ground freezes, typically late November to early December.

Fertilizer rules

Iowa Code chapter 200 covers commercial fertilizer licensing, registration, labeling and inspection. I found no rule in it that limits phosphorus in home lawn fertilizer. The practical limit is the label: starter fertilizer such as 20-5-10 is for seeding, and the maintenance feedings in the ISU schedule are written in pounds of nitrogen only.

The lawn fertilizer calculator on this site converts the first number on a bag into pounds of product for your lawn size, so the 1 pound target is the same for any bag.