Colorado offers a unique set of conditions for planting trees to encourage the most root growth and water absorption, but there are some common mistakes to be aware of.
Most of Colorado has thick clay soil which is both a problem and a solution to the dryness of our climate. The best way to make use of this type of soil when planting a tree is to dig a wide, shallow basin, two or three times the size of the root ball, where the edges will direct rainwater towards the roots. The earth will act like a big clay pot, collecting and funneling the water down to the roots. Fill this basin with water before you plant the tree to get a feel for how much water you will need and to see how quickly it sinks into the ground. You do not want the roots sitting in water for any period of time or they will rot. If the basin does not empty over the course of an hour, make it bigger and break the bottom up.
Tree roots generally spread outwards just under the soil with a few that sink straight down to find water. To encourage the roots to grow out, fill the basin with a mix of compost and soil, mounding it up because it will compress over time. The top of the compost should angle towards the edge of the basin so the water flows away from the very base of the tree and out towards where the roots will need it.
Mulch the entire top of the basin with three to five inches of wood chips to retain as much moisture as possible. The Colorado sun and wind dries trees out rapidly. Ideally, sheet mulch out to three or four feet from the trunk. Leave a few inches around the trunk of the tree bare because the trunk needs air circulation to be healthy.
Why wood chips and not straw or leaves? Tree roots have a symbiotic relationship with fungus and wood chips are the perfect environment for fungi. Straw and leaves harbor a more bacterial environment which is best for plants.
Water deeply once or twice per week around the top of the basin, going out to the edge.
The distance to plant trees apart depends on how big the tree will get. Dwarf trees and shrubs can be planted 5-10 feet apart, whereas semi-dwarf or medium-sized trees should be planted 12-20 feet apart. Full sized trees should be at least 20 feet apart to prevent them from shading each other. Some trees and shrubs work well as an understory, however, and can tolerate part shade. These can be planted underneath the larger trees.
Other things to consider are competition and companion planting for your trees. Field grass is one of the biggest competitors for nutrients and water, so it is important to mulch or sheet mulch deeply out to the tree’s drip line is to get rid of the grass as far out as possible. On the other hand, clovers and other herbs can be wonderful companion plants under and around a tree, providing the tree with nitrogen and other nutrients, living mulch and homes for beneficial insects and pollinators.