Lawn care calendar for Colorado: when to fertilize and seed

Nitrogen rates are pounds per 1,000 square feet from the CSU schedule; water is the weekly total of rain plus irrigation. Skip March feeding if you fed the previous fall and the lawn greens well.

Which grasses grow where in Colorado

Colorado State University Extension builds its lawn advice around Kentucky bluegrass, ryegrass, turf-type tall fescue and fine fescue, plus buffalograss, blue grama and bermudagrass on its fertilizer schedule. Its sheet says the preferred mowing height for every Colorado species is 2.5 to 3 inches, with 2 inches as the floor, and that the height should stay the same all year.

Buffalograss is the low-water choice. CSU describes it as a warm-season grass that breaks dormancy in mid to late spring, long after bluegrass greens up, grows hardest from late May through early September and turns brown at the first hard frost. Seed can be planted from spring until late July; CSU warns that seed sown between August 1 and October 1 may sprout and then die over winter, while seed sown from October to December waits until spring warmth.

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 Denver International Airport the last spring frost averages May 5 and the first fall frost October 9. At Colorado Springs Municipal Airport the dates are May 3 and October 8. At Grand Junction (Walker Field) they are April 28 and October 16.

Counting the days between the two dates gives about 157 frost-free days at Denver, 158 at Colorado Springs and 171 at Grand Junction (our arithmetic). The western slope has the longest runway, but all three sites can freeze in early May, which is why CSU tells you to delay the spring feeding if green-up is already satisfactory.

Spring

The first nitrogen window runs from mid-March to April 1 at 1/2 pound per 1,000 square feet, rising to 1 pound on high-maintenance bluegrass and ryegrass. CSU adds that this feeding may not be needed if you fed late the previous year, and that a lawn that greens up well can wait until May or June. From May to mid-June the schedule is 1/2 to 1 pound, and 1 pound on high-maintenance bluegrass.

Mowing starts when the grass starts. Bluegrass and fescue can need mowing every three to four days in the spring flush and only every seven to ten days when heat, drought or cold slow it. CSU also says light power raking of wet, matted turf in spring can speed green-up.

Summer

Bluegrass, ryegrass and tall fescue get no nitrogen in July and early August. Buffalograss, blue grama and bermudagrass are the ones fed then, at 1/2 to 1 pound. CSU puts the weekly water need, rain plus irrigation, at about 1 inch in May and June, 1.5 inches in July and August and 1 inch in September, and says a high-quality bluegrass lawn can use up to 2.25 inches in hot, dry, windy weather.

Tall fescue often needs as much water as bluegrass to look good, while buffalograss and blue grama can stay green for weeks without any. The state sheet tells you to water when the lawn shows it: footprints that do not spring back within an hour, then a gray or bluish tint. The most efficient hours it names are late evening, between 10 p.m. and midnight, and 8 to 9 a.m., with a note to check local rules first.

Fall

Mid-August to mid-September is the main feeding window for cool-season turf, at 1 pound. The final feeding goes on from early October to early November while the grass is still green and at least two to three weeks before the ground freezes. On very sandy soils CSU says to stop after late September and to use slow-release nitrogen, because winter rain can carry nitrogen into groundwater.

Core cultivation is the practice CSU calls essential, especially on thatchy or high-traffic lawns. Its sheet describes pulling plugs 2 to 3 inches long, but gives no month; the one fixed number is a thatch layer over 1/2 inch, which is the point to start working on it. Keep mowing at the summer height into late fall, since CSU says there is no reason to cut shorter.

Drought rules and phosphorus

Water utilities can cap what the extension calendar recommends. On the Denver Water page I read on October 2, 2026, the utility prohibits lawn watering and all spray irrigation from October 1 under a Stage 1 drought declaration, until further action by its board. Trees, shrubs, perennials and vegetables can still be watered by hand or with drip, and watering permits for new seed or sod were not being issued after September 30. Other Colorado utilities set their own rules, so check yours before you follow a weekly water target.

No Colorado statute limiting phosphorus in home lawn fertilizer turned up in the sources I opened. CSU treats phosphorus as a soil test question: a lawn that stops responding to nitrogen may be short of phosphorus or iron, and a soil test settles it. It does warn that grass clippings and leaves washed into the street are a major source of phosphorus pollution in urban waters.

The bag math

The CSU schedule is written in pounds of nitrogen per 1,000 square feet, and the bag only states a percentage. The lawn fertilizer calculator on this site turns the nitrogen percentage on the label and your lawn size into pounds of product. Because CSU says clippings left on the lawn can cut nitrogen needs by a quarter to a third, the lower end of each range is a fair start on a mulched lawn.