Colorado spans both grass families: cool-season lawns are more common toward the cooler, northern parts of the state and at elevation, warm-season lawns toward the warmer south.
Cool-season · fall window
September 5
August 19 (coolest parts) to September 22 (warmest)
For cool-season lawns (fescue, bluegrass, ryegrass), fall is the best time to plant grass seed in Colorado, with spring the second choice: fall seedlings face less weed competition and mature before summer heat. The window opens as 2-inch soil falls back through 70°F, and germination slows sharply once soil drops below 50°F, typically around October 23. Fall seeding and fall pre-emergent are an either/or on the same turf; see pre-emergent timing in Colorado.
Cool-season · spring window
April 11
March 29 (warmest parts) to June 4 (coolest)
Spring is the second choice: the window opens when soil first holds 50°F and narrows as soil passes 65°F, typically by June 4.
Warm-season · planting window
June 4
May 14 (warmest parts) to June 8 (coolest)
For warm-season lawns (bermuda, zoysia), the best time to plant grass seed in Colorado is late spring into early summer, when soil is warming and a full growing season lies ahead. The window opens when 2-inch soil reaches 65°F and keeps rising. Soil stays above 65°F until about September 21, but seed sown after the summer peak cannot establish before dormancy, so the practical window closes by midsummer; the month-by-month table below shows where that falls.
Typical dates are computed from historical 2-inch soil temperature averages at 8 monitoring stations in Colorado; your own window depends on your nearest station's readings.
