Hopefully you know that late winter/early spring is the right time to prune most trees and shrubs. (Early spring flowering shrubs like Forsythia and Lilac should be pruned after flowering.) Here is the scientific explanation of what happens to the plant tissues when we prune.
It is hormones that dictate the plant’s response. There are two types of hormones that control plant growth: cytokinins and auxins. Cytokinins are produced in the roots and move up the branches. When the weather is warm enough that water can travel up through the plant via transpiration, the cytokinins will stimulate bud break.
Auxins are concentrated in the shoot tip at the very end of the branch. Auxins move down the branch. When auxins are at higher concentrations, they suppress cytokinins in most of the lower, auxiliary buds. This is called “apical dominance.”
Some plants produce more auxins and have stronger apical dominance, thus they have one central trunk. A Balsam Fir Christmas tree is a good example of strong apical dominance. Apical dominance can help a plant grow taller or wider, which is especially important when competing with other plants for sunshine.
When pruning, we remove the growing tips, changing the ratio of auxins to cytokinins. The cytokinins then cause the buds lower on the branch to open, so the plant is fuller or bushier than it would be without pruning. Over the course of the year, auxins again concentrate in the shoot tip and again suppress lower bud break the following spring.
Hormone production requires energy in the form of carbohydrates. Plants spend all summer producing carbohydrates with the help of water and sunshine. Waiting until the plant has produced leaves and then pruning costs the plant a lot of energy. This is one reason it is best to prune in late winter/early spring. NOW is still a good time to prune shrubs, but if you have fruit trees, you should wait until next winter to prune them.
For more information on pruning methods, see my previous blog posts:
These CSU fact sheets are excellent as well:
