If it has not been done, April is a good month to core aerate the lawn. Aeration permits air and water with nutrients to penetrate the soil. It reduces compaction of the soil and controls thatch in the lawn. Inspect trees and shrubs for parasites, disease and damage. Should you notice any abnormalities on leaves, branches or bark, call your county office of Colorado State University Cooperative Extension.
Someone there can help identify the source of the problem and recommend possible solutions.
If you did not fertilize the lawn in the fall, April is a good time to do it. Grass is dormant during the winter and fertilizer provides a good breakfast as the grass wakens and begins to grow again. In Colorado, high levels of phosphorus and potash usually are not necessary. Concentrate on the nitrogen content of fertilizer because nitrogen is the nutrient grass needs most. More is not better in this case.