Apical Dominance

Apical dominance is why trees and many other plants end up tallest in the middle, but there’s more to it than that.

Conifer (PIXNIO) Free Use

Oak tree (k_r_craft) Pixabay

Imagine, if you will, that every bud on a young tree grew into a full-sized branch, and that each twig from each branch also grew into a full-sized limb. You can see how convoluted things would get rather quickly. Apical dominance is what keeps that mess from happening.

Apical dominance refers to the way that central stems are dominant over other stems. It also describes why branches are dominant over their twigs. It makes sense. If the twigs were bigger than the branch, it would eventually break. If side branches were bigger than the central trunk, trees and shrubs might not get enough sunlight. Let’s see what makes plants behave this way and learn how we might use this behavior to our advantage in the garden.

Apical dominance and sun exposure

Plants need sunlight to grow. The tallest plants get the most sunlight. By growing upward first and then outward, a plant’s chance of survival is increased. For conifers, which often grow at higher latitudes where sunlight is lower on the horizon, the triangular Christmas tree shape provides the greatest amount of sunlight. For deciduous trees, growing at lower latitudes with more overhead sunlight, the rounded canopy provides the most sun exposure. In both cases, the woody structure supports the leaf canopy that allows for photosynthesis. Photosynthesis is all about sugar production. And, contrary to popular belief, sugar is the reason behind apical dominance.

Apical dominance and terminal buds

The buds at the ends of stems are called terminal or apical buds. This does not mean they are waiting on a visit from hospice. Just the opposite. This is where meristem tissue is found. Meristem tissue is new, undifferentiated life. This is where growth happens. Even though growth is happening at all the buds and twigs along a stem or trunk, the ones at the top and/or ends have priority. We used to think that this happened because of a plant hormone, called auxin, but research has shown that it is the growing tips’ demands for sugar that robs their neighbors of enough sugar to grow equally fast.

If apical buds are removed, lateral buds are free to demand more sugar and grow more quickly. This is where pruning, coppicing, espalier, pollarding, and tree training come in.

Apical dominance and pruning

If you pinch off the central stem of a basil plant, just above a pair of leaves, it will grow to be bushier and produce more delicious leaves.

Pinching basil (Kate Russell)

Pinched basil (Kate Russell)

If you apply production pruning to your nectarine tree, it will produce significantly more fruit. Different fruit and nut trees benefit from different types of tree training, but the results are the same: healthier, more productive trees.

Espalier uses the same concept but in two dimensions, rather than three. Apical buds are pinched back to stimulate side growth.

Pollarded trees in Spain (Kate Russell)

Pollarding and coppicing apply the same principle but on different parts of a tree. Coppicing refers to the practice of cutting small trees and shrubs back to ground level regularly to harvest several thin stems that are useful for basket-making, firewood, and wattle-and-daub fencing. Pollarding is the same practice but done higher up in a tree to promote new overhead growth.

How are you putting apical dominance to work in your garden?