Planting grass seed in Colorado's arid climate requires special care to ensure germination and growth. The best times to seed grass in Denver -- and across Colorado -- are April and September, when cool season grasses germinate and the afternoon rain may provide enough water. Colorado's heavy, alkaline clay soil must be amended with organic matter to provide a seed bed where the grass seed can germinate and then develop a healthy root system.
Conduct a simple soil test to evaluate how much your soil must be amended. Colorado soils that have never been amended may have a pH as high as 8.5. Remove large rocks and other debris, and till the lawn area to loosen the top 12 inches of soil. Adjust the grade to drain away from your home and other buildings. Till organic material, such as compost, composted manure or sphagnum peat, into the top 6 to 8 inches of soil, using 4 cubic yards of organic material per 1,000 square feet of lawn area. If necessary, add fertilizer according to manufacturer's directions and based on the results of your soil test.
Types of Grass for Lawns
Cool season grasses are best suited to the Denver metropolitan area and parts of Colorado with even higher elevations. Seed cool season grasses in early spring or fall when they will not be exposed to hot weather and strong sun. If possible, plant grass seed just before a rain shower, but not if the forecast predicts a torrential downpour that may wash the seeds away. If rains do not provide enough irrigation, water to keep the seed bed moist but not so much that water pools on the surface.
Kentucky bluegrass (Poa pratensis) is the most common lawn grass in the Denver metro area; it's hardy in U.S. Department of Agriculture plant hardiness zones 3 to 8, with over 100 varieties available. Good quality seed mixes contain a blend of at least five cultivars, which create a high quality lawn that's more resistant to disease.
Blue grama grass (Bouteloua gracilis), Colorado's state grass, is drought tolerant and requires little maintenance, though it does not tolerate heavy foot traffic. Blue grama is a native warm season grass that will grow in Denver's warm summer climate. It's hardy in areas as cold as USDA zone 3, up to zone 9, and thrives at elevations up to 7,000 feet. Plant blue grama grass from late May to mid-July, when daytime temperatures are at least 65 degrees Fahrenheit, in areas that receive full sun.
Mountain and Desert Regions of Colorado
Fescues (Festuca spp.) and native grasses may require less maintenance and be better adapted to mountainous regions of Colorado, where temperatures can drop to minus 30 degrees Fahrenheit in the winter and much of the precipitation is in the form of winter snowfall. Tall fescue (Festuca arundinacea) is hardy in USDA zones 5 to 8 and fine fescue (Festuca rubra) in zones 1 to 7. Both are somewhat drought tolerant and grow well in shade. Tall fescue develops a deep root system in deep, rich soils, however Colorado's heavy clay soil will not allow this deep root system to develop unless it has been cultivated and amended to a depth of 12 inches. Fine fescue is a good choice for mountain regions because it may go dormant at temperatures above 90 and below 50 degrees F.
Smooth the soil surface with a rake. Broadcast seed in two directions, at right angles, using half the seed for each application. You can scatter the seed by hand, but it's easier to use a seed or fertilizer spreader, which you may be able to rent from a hardware store, seed supplier or local state extension office. Use 2 to 3 pounds of Kentucky bluegrass seed or 4 to 6 pounds of fescue seed per 1,000 square feet when seeding a new lawn.
Lightly rake the seed into the soil, using the back of a rake to gently cover it. Water gently and evenly, keeping the soil surface moist during germination. During hot, dry weather you may have to water a few times each day until seedlings emerge. Mulching with straw will help retain moisture and prevent birds from eating the seed. Walk on the surface gently to tamp the seed into the soil, but then avoid walking on the newly sprouting grass.
Grass seedlings may emerge within a few days, up to two weeks. If the grass has not sprouted within two weeks, reseed and water well.