Lawn care calendar for Wisconsin: when to fertilize and seed

Spring conditions vary considerably from south to north; thaw timing sets the real start.

Which grasses grow where in Wisconsin

UW-Madison's calendar is for cool-season turf: Kentucky bluegrass, perennial ryegrass, tall fescue and fine fescues. Kentucky bluegrass, the dominant Wisconsin lawn grass, survives weeks of drought by browning and going dormant, though dormant lawns do not tolerate traffic.

Mow at about 3 inches all season, do not make the first spring cut very short, and return clippings, which recycle nitrogen and do not cause thatch. Never remove more than a third of the blade, and set the mower to its highest setting before summer.

Frost dates from NOAA

Dates are the NOAA 1991-2020 normals for the last spring and first fall 32 F freeze at 50% probability. Milwaukee Mitchell International averages April 19 and October 29. Madison averages May 1 and October 10, and Green Bay averages April 30 and October 9.

Our arithmetic: Milwaukee has about 193 days between those dates, Madison about 162 and Green Bay about 163. Milwaukee has roughly a month more than Madison or Green Bay.

Spring

UW-Madison says to avoid starting lawn work too early and to pay attention to soil thawing. Rake off debris, then look at the damage: spiraling trails mean voles, matted circular patches mean snow mold, and bleached irregular areas come from winter cold, wind and ice. Reseed as needed.

Wait until the second mowing to fertilize, with about 1 pound of controlled-release nitrogen per 1,000 square feet for a lawn in full sun and less in shade. Feeding at first thaw drives top growth at the expense of roots. If crabgrass was a problem last year, apply a pre-emergent when soil has been near 60 F for several days; forsythia in bloom, from late March to mid-April or later, is the reminder. If not, skip it, and do not use one where you will reseed the same spring. Seed in late April to early May if you must seed in spring, and core aerate with 15 to 20 cores per square foot if the soil is compacted.

Summer

Decide before dry weather whether to water all summer or let the lawn go dormant. If you water, about 1 inch a week keeps it green, applied early in the day and evenly, and split in two on heavy clay or slopes. Do not wait until the lawn is brown and then try to turn it green, which drains reserves.

Summer nitrogen goes only on irrigated lawns, at less than 1 pound per 1,000 square feet and in controlled-release form, and most lawns do not need it. Avoid vertical mowers, power rakes and seeding in summer, because cool-season grass recovers slowly and crabgrass moves into the open soil. Check for white grubs where brown meets green before treating anything.

Fall

September is the key month. Late August into early September is the ideal time to seed, sod and renovate, and UW-Madison suggests slit seeding for thin lawns. September 1 to 15 is the time to feed every lawn, about 1 pound of controlled-release nitrogen per 1,000 square feet, and mid-fall nitrogen is skipped to avoid growth surges that invite snow mold and voles. Fall is also good for core aeration and for spot-treating dandelions and other perennial broadleaf weeds while they are growing.

Keep mowing until the lawn goes dormant but do not let it go into winter tall, and clean leaves and debris off before snowfall. Dormant seeding in very late fall is not suggested in most cases.

Wisconsin's phosphorus law

Section 94.643 of the Wisconsin Statutes says no person may intentionally apply fertilizer labeled as containing phosphorus or available phosphate to turf. Exceptions are applying it to establish grass from seed or sod during the same growing season, and applying it where a laboratory soil test taken within the last 36 months shows a phosphorus deficiency. Nobody may apply fertilizer, manipulated manure or sewage sludge product to frozen ground, or intentionally to pavement.

A first violation carries a forfeiture of up to $50, and later ones $200 to $500. Retailers may not display phosphorus turf fertilizer. The lawn fertilizer calculator on this site works from the nitrogen figure on the bag, which is what UW-Madison's rates are written in.