Interview with Alex Shigo

Alex Shigo is a world-renowned authority in arboriculture, the science of trees. His delving curiosity and sharp scientific analyses carried him beneath the bark into a realm previously little understood. He worked 26 years for the US Forest Service investigating decay in trees, and through the process of dissecting over 15,000 trees with a chainsaw, he has uncovered much valuable information about tree structures and tree systems that have revolutionized the practice of tree care. As he puts it in the preface to his book Tree Pruning, “A major problem throughout the history of tree pruning has been the scant attention given to pruning as it affects the health of trees, while great attention has been given to pruning as it affects the desires of man.” Even though he is busy writing, giving workshops and talks and doing consulting, he agreed to answer some pressing questions for The Colorado Gardener.

  1. Colorado Gardener (CG) People ask all the time, When is the best time to do pruning? Is there a clear answer to that question? Are there times when it is better not to do pruning?

Alex Shigo: If pruning cuts are made at the branch collar, then trees can be pruned at any time. In general trees are most vulnerable when they are putting on or dropping their leaves, and these are times when trees will be damaged most by improper pruning. Back before people knew about leaving the branch collar, the timing of pruning was more important.

  • CG: What is “topping” and what is “tipping” and why are they bad for trees?

Alex Shigo: “Topping” is the drastic removal of the tops of vertical stems where cuts are made between the nodes. “Tipping” is the making of internodal cuts on lateral branches. Nodes are the positions on a stem where a leaf, bud or branch arises. To cut between these nodes leaves a stub which is food for organisms that start rot and cankers. The swollen branch collar is not a stub. Also use common sense, don’t cut off a 10” branch at a node with a 1” leader branch; the branch left at the nodal cut should be at least 1/3 the diameter of the branch cut off. However it is also important to understand that the age and vitality of a tree determines the best approach to pruning and training. A young tree is much easier to train than an old tree, and a young tree can tolerate the removal of more branches. Old trees should be pruned only to remove dead, dying, diseased or hazardous branches; this will conserve their energy.

  • CG: Recently some university and other plant professionals in Colorado have been recommending the practice of planting trees high, that is, digging the hole shallower than the root ball so the top of the root ball is 2”-4” higher than the surrounding ground. What is your view of this practice? Is this approach valuable in heavy clay soil, in high water tables or not at all?

Alex Shigo: To me that approach is overcompensating for the past mistakes of planting too deep. Why not plant a tree at the correct depth? Where the trunk flares should be the ground level. For special reasons a tree can be planted on mounded soil where the earth has been opened below, but don’t just dump a pile of dirt and plant a tree on top of that.

  • CG: Utility companies used to prune openings through trees for the wires to pass. Now they remove all branches above the wires. The effect of this approach is often to deform the tree and to encourage the tree to grow lopsided and more vulnerable to snow and wind. Is there a better approach, which would serve the needs of the utility companies and not deform the trees?

Alex Shigo: I respect the utility companies; they are trying and have made much improvement. I would suggest that they think of what they are doing as pruning trees from utility lines, not as “line clearing.” In general 90% of the time, removing 3 branches will provide 90% of the needed clearance. This is preferable to cutting a branch in half only to have it sprout at the cut and grow back into the wires. Even more preferable is to stop planting conifers and other trees with strong central leaders under utility lines, and to start training trees near lines before they are 15 feet high.

  • CG: In your books you frequently encourage people to touch trees. What is your intention with that recommendation?

Alex Shigo: The point is that touching trees is making a connection with nature and with life. Without this connection, communication is not possible. God wrote two books: the Scriptures and Nature. In order to connect with and learn from our greatest living systems, we need to touch trees.

To learn more from Dr. Alex Shigo, consult his books: Modern Arboriculture, Tree Pruning, A New Tree Biology, A New Tree Biology Dictionary, 100 Tree Myths, and Tree Pithy Points.