Gardening with Allen: Prune shade trees for strong trunk

I need help pruning three shade trees that I planted last year that are now about 12 to 15 feet tall. They had bare trunks up to 4 feet when I planted them. Is it time to prune up to 6 feet now? A few small sprouts have developed below 4 feet. Should I remove those? The top of one tree was broken and now has three sprouts.

Most shade trees do best if they have a single trunk. So your first job is to pick the straightest sprout and remove the other two.

University research has shown that young trees develop a stronger lower trunk if lower branches are allowed to develop below 6 feet but are pruned so they only grow about 12 to 18 inches in length. These lower branchlets feed the lower trunk where they are attached and cause it to grow to a larger diameter more quickly. These short lower branches can be removed after there is at least 10 to 12 feet of growth above them.

When the time comes to remove them, small branches should be pruned flush with the trunk. When branches larger than 2 inches in diameter are removed a one half inch collar should be left where it is attached to the trunk. The healing tissue to repair the wound is located in this collar tissue.

Most shade trees need some thinning of inner branches to remove those that are crossing or rubbing against each other. In choosing which branch to cut, remove those growing inward or upward. The horizontal, outward growing branches are the strongest. No more than one-fourth to one-third of growth should be removed with any single pruning.

Sometimes upright branches, referred to as water sprouts, grow after heavy pruning. These can be easily removed while tissue is still soft by snapping. Snapping is accomplished by a quick downward jerk. Snapping is preferred to pruning because it removes latent buds that can develop into another sprout.

Young trees will develop faster if a 6-foot diameter circle of grass and weed free soil is maintained around them. Grass and weeds are very competitive for water and nutrients. Metal or plastic edging can be used to prevent grass from growing into the circle. Almost all weed seeds require light to germinate. A 2-inch mulch of compost or bark dust will prevent most light from reaching weed seeds in the soil. Weed barrier fabric will eliminate 100 percent of weed growth.