Since we are now officially in autumn — a great time for planting hardy trees — I want to mention a common problem, which never seems to go away. In part, it has to do with our wishes, as thrifty consumers, to get the most for our money. It also has a lot to do with nursery container-growing practices.

The problem: A tree that looks big and beautiful in the nursery fails to thrive or just falls over in the landscape. The cause is literally in the roots. Trees that are kept too long in containers become “pot bound.” They develop tightly compacted root systems that repel water and girdling roots that interfere with anchorage and the uptake of water and minerals.

With annual bedding plants, some perennials and smaller woody plants, the problem can often be corrected at planting time but, too often, trees are permanently damaged goods before they are planted in the landscape.

Here’s a typical scenario using an oak as an example. An acorn is sprouted in a small tube-like container. The radicle (taproot) grows straight to the bottom and out the drainage hole. In contact with air, the tip of the root dies. So far, so good. The air-pruned root responds by growing branches — lateral roots — behind the dead tip. The lateral roots grow quickly to the side of the tiny container where they are deflected by the impervious surface.

 Here’s where the trouble begins. The tube-bound seedling is shifted into a one-gallon can. Some of the lateral roots manage to spread to the bottom and side of the pot, where they are again deflected. They continue to grow along the path of least resistance, which is circular in a typical nursery pot. The longer the tree is left in the container, the more the roots form a woody coil. The process is usually repeated in shifting to progressively larger containers: 5 gallon, 15 gallon and larger. The root system can end up like a set of those carved wood Russian nested dolls. But with trees, the result is not pretty!

A landscape inspector for a nearby city once made it his mission to combat this problem for the long-term health and safety of trees for which he was responsible. At a seminar, he displayed the badly coiled root systems of some of the failed trees he had removed from landscapes. It was a disturbing sight to me, having spent so many years working for nurseries. He described one unstable tree that he had to remove. He had noticed it actually rotating in the ground. One day he drove by it and noticed that a branch that used to be over the road, was now parallel to the road!

The old saying “as the twig is bent, so grows the tree” applies to roots as well. They can be straightened out fairly well before they become woody. It’s much more difficult, or impossible, later. 

Girdling roots may also form when trees are planted too deeply or when an outward growing woody root is cut near the trunk, and a lateral root at right angles to it expands near the trunk. Studies have shown that certain genera of trees are more prone to this than others.  They include some of the maples, oaks, elms and pines. At least one study has shown that girdling roots that are pruned back tend to grow back in the same direction. The tips of horizontal woody roots often display a habit known as plagiotropism. When deflected, they tend to continue growing in their original orientation and they tend to keep growing at their original depth. How do they “learn” or “know” how to do this? I don’t know.

It is certainly a big disappointment, a waste of time and money and a potential hazard when a good-looking tree grows for a year or more in the landscape, only to tip over or lose vigor.

Avoiding the problem involves nursery growing practices as well as care on the part of the buyer.

One interesting and successful method is bag-growing, whereby trees are grown in special fabric containers in the ground or in raised berms. Rather than being deflected, roots that encounter the porous fabric start to grow through the tight pores, which then constrict their growth, causing the roots to branch and to store starches near the point of constriction. The result is a densely branched, nutrient-rich root system. A couple of growers I know of in our region are using this technique, but mostly for native oaks only: Main Street Trees, right here in Napa, and Specialty Oaks in Lower Lake.

 Buyers should look closely at the root collar  — the base of the tree at the soil line — and, if possible, slip the tree out of the container and look at the roots before buying a tree. Soil above the first woody root can conceal serious problems, like girdling roots. Buttress roots should spread directly away from the base of the trunk, like the legs of a tripod or a pedestal. Ideally, the face of the root ball should show plenty of fine, non-woody absorbing roots and no thick, woody roots that circle around the pot. In some cases, minor problems can be corrected by a little root pruning before planting.

A tree is both a living, changing organism and a large, heavy structure that relies on good engineering to keep it standing. Even if the tree looks small for its container, it’s better to start with a vigorous young tree without structural defects. It’s a better value.

Bill Pramuk is a registered consulting

arborist. To contact him,  visit his Web site, www.billpramuk.com , e-mail him at info@billpramuk.com, or call him at 226-2884.