Aspens are high-elevation trees that need cool, moist conditions to thrive. Growing them at lower elevations in Colorado almost always puts them under stress, which in turn causes disease and insect problems to set in. Anything that you can do to provide conditions closer to those the trees experience in the mountains would be helpful. That includes mulching the soil around them with 3-4 inches of organic mulch and keeping them watered well. Especially in the first couple years after planting, the trees require extra moisture until their roots become established.