Last week, a worried homeowner called me about a mature tree that had fallen and was leaning on her fence. She was concerned it might fall farther and cause more property damage.
It was a mulberry, not the locally common fruitless white mulberry, but an unusual one, probably black mulberry (Morus nigra). And probably a male tree, as the owner said it does not bear fruit.
It was one of three similar trees, providing shade and structure in a lovely backyard garden.
This one had fallen against the fence in recent heavy rains and wind. A second one was leaning sharply but the lean appeared to have begun many years ago. The third tree stood straight.
Looking closely at the fallen tree I could clearly see, where the base had lifted out of the ground, there were no anchoring roots on the side that lifted up. On the leaning tree there was a large root partially circling the trunk behind the lean. The third tree — the vertical one — showed good flare at the base of the trunk; outward taper where the trunk expands into the buttress roots.
Having seen many such tree failures and having worked in nurseries for many years, I recognized the problem as a common one originating when trees sit too long in nursery containers.
Roots need to elongate. When they are in a circular container, they are forced to grow in a circle.
In some cases, if the defect is not too severe, circling roots can be pruned at planting time to help trees generate new outward spreading roots.
Too often, the defect goes unnoticed. Roots perform their function of absorbing water and minerals, allowing the tree to grow and appear “healthy” but meanwhile, their other function—providing anchorage—is gradually compromised.
Here is a good way to visualize tree anchorage: Look at a wine glass. At about eight inches tall, it requires a base about three inches wide to provide good stability. The proportions are visually elegant and, not just coincidentally, just right for good stability.
For trees, good anchorage is provided when the trunk flare spreads out into a radially symmetrical root plate with a diameter equal to about six times the trunk diameter, with branching woody roots extending far beyond. A few buttress roots and their branching roots form the root “plate”. Deeper “heart roots” and “sinker roots” also provide anchorage, but the wide root plate is critical.
The fallen mulberry had a trunk diameter of 15 inches. Multiplying by 6 shows it needed a 90-inch wide base for stability. That is primary anchorage to a radius of at least 45 inches from the trunk.
The failed side lifted out of the ground at about one foot from the trunk, where there were only a few small diameter absorbing roots.
Looking at the leaning tree, with a big woody root partly encircling the base of the trunk, I imagined the probable scenario perhaps 50 to 75 years ago.
The trees had been growing in nursery containers. Two developed circling roots while the third somehow did not. The customer — the original homeowner — saw the trees as a good deal: quite large for the low cost of large trees in small containers.
The trees grew well enough through the intervening years, but trouble was developing just below the soil surface. One tree began to lean, but the owners did nothing about it.
They sold the place and the new owner had the misfortune of seeing the results.
A couple of take-home messages:
1. Do not take home pot-bound trees. The better bargain is a tree that looks healthy and rather small, but firmly rooted in its container.
2. When planting, examine the woody root structure. Spread or cut woody roots crowing in a circle. It is better to risk losing a young tree because of root pruning at planting time than to risk a large tree failure later on.
3. Inspect trees, especially leaning trees, with a critical eye.