Chapter 11 Growing Space

Trees need sufficient growing space just as much as they need sufficient light and water. How much space a tree needs depends on where it is situated and what your goals are for its future growth.

For example, trees being grown in the ground to fatten their trunk and primary branches need space for the root mass to expand without competition, and for the ends of the branches to extend freely without being shaded by other trees.

Space requirements for trees in pots on the bench is a bit more complicated. First, all trees need sufficient space between them for air to circulate freely. Poor air circulation increases their susceptibility to fungal and bacterial diseases, particularly molds and mildews. Poor air flow keeps the humidity high around the leaves, producing conditions in which molds and mildews thrive. Once an infection begins, slow-moving air makes it more likely for spores to land and germinate rather than being blown away from uninfected leaves nearby.

Spacing on the ground or a bench also has a direct effect on branch growth patterns. If trees are crowded too close together on the bench, the foliage from taller trees can block sunlight from reaching the smaller trees, slowing down growth or causing uneven growth on different sides of the tree. You can use this to your advantage by intentionally placing smaller, more sun-sensitive trees so they are shaded by larger sun-tolerant trees during the hotter parts of the day.

Bench spacing can affect growth even when there is ample sunlight reaching the trees. Mature trees in natural stands show varying degrees of crown or canopy shyness, meaning their branches will not extend far enough to overlap. As a result there is a clear space or gap in the canopy between the individual trees.

The spaces between the branches of individual trees are an example of crown shyness. Link to original image

Species that display the strongest shyness include Sitka spruce, lodgepole pine, black mangrove, several species of eucalyptus, and some species of larch, schefflera, and hackberry. Exactly what causes crown shyness is not known for certain. Some researchers have proposed that it is caused by physical injury of growing tips as they rub against adjacent trees. Others think that crown shyness occurs because of a photoreceptor-mediated shade avoidance response. A third possibility is a natural hormonal response in plants called thigmomorphogenesis: basically, touch-induced growth inhibition. When branches from adjacent trees touch each other, both branch tips produce hormones that inhibit lengthening, and instead make the branches thicken. My personal opinion is that it is a combination of all three causes. Regardless of the mechanism though, we can use

Quick Tip: if you have a tree with fine, weak branch tips that you want to thicken, give the whole tree a shake every week or so. You do not want to shake it hard enough to disturb the root mass, just hard enough to bend and move the weak tips. The vibration triggers thigmomorphogenic hormone

11.1 Growing Out Versus Growing Up

Production nurseries use the canopy response to control how young stock develops. Pots are placed in a grid with branches from adjacent pots just barely touching. This makes the existing branches thicker and stimulates heavier foliage growth. After 1-3 weeks the pots are moved slightly further apart to encourage the lateral branches to extend. Clipping back the tip of the dominant vertical leader further stimulates this process. As the tips grow and start to overlap, the same thickening response occurs. Repeating this process several times over 1-2 seasons can dramatically increase the thickness of branches and the foliage density.

If the goal is to promote vertical growth of young stock trees, the tips of the lateral branches are clipped off and the pots are set out close enough that more of the branches overlap. This inhibits side branch growth and most of the energy goes into lengthening the main leader.

If the goal is to extend lateral branches, the apical leader is clipped back and trees are spaced so they are well separated from one another.