Here is a problem so common and troublesome one would expect it to get more attention: Structurally defective roots systems in trees.
A recent tree and garden visit brought it to my attention. Two purple leaf plums in a group of three showed sharply leaning trunks. The trees looked vigorous and one of them had put on so much new growth this spring the weight of it was making the lean worse.
A quick look at the base of one tree revealed a glaring problem: a woody root growing oblique to the trunk – crossing rather than growing radially away from it. The trunk leaned directly away from the crossing root. And just a few inches away there was another troubling sign: A slightly raised step where the root system behind the lean had lifted. It is easy to miss this if you are not looking for it.
The tree is small enough, only about 4 inches in trunk diameter, I could push on it and easily move it.
How do problems like this originate? In some instances, roots cannot spread out because of obstacles. I once saw a large Douglas fir that had fallen on a house. The turned-up root system showed a lack of anchoring roots where a boulder near the trunk had prevented root growth.
Most often, it starts at nurseries where trees are grown in containers. As trees become pot-bound, roots are forced to grow around the inside of the container. What begins as a fine absorbing root becomes a woody root growing in a circle near the trunk.
Think of a wine glass with its base spreading out in good proportion to the height, weight and width of the glass. The base needs to be of good proportion for stability. In a way, trees are similar but they grow larger, so the base must grow too.
Trees growing from seed in a forest or woodland are more likely to have the benefit of unrestricted root growth. Looking at mature native valley oaks, you are likely to see about five buttress roots radiating outward from the flare at the base of the trunk, providing a sturdy and fairly symmetrical base. One of the reasons they live so long is their ability to keep standing by virtue of a well-structured base.
This is one reason I am such a fan of the Napa Resource Conservation District’s “Acorns to Oaks” project. Direct-seeding into the ground bypasses the pot-bound problem. Those that survive are likely to develop well-structured anchoring roots systems.
Sometimes, trees lean because of crowding and shading by the adjacent trees. Growing in response to available light on one side, the tree develops a lean. Such a tree can be stable. If so, the top may sweep upward to vertical once it has grown into full sun.
Warning signs of instability include leaning tops that are not self-corrected to vertical, movement of the lower trunk in gusty winds and changes in the shape of the soil around the base of a tree as I noted with the little purple plum.
When selecting a tree from the nursery, keep in mind the biggest one in a group of trees in same-size containers is not likely to be the best deal. It is more likely to be pot-bound. A young tree, just well-rooted in its container, is likely to grow faster and to be more stable than a pot-bound tree.
One way to check this is to slip the tree out of its container before buying it. Look for root defects at the base of the trunk and woody roots growing around the outside edge and across the bottom of the root ball. Avoid buying such a tree. But if you do, it is better to risk killing it by pruning away any circling roots.
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