In bonsai, “pruning” is a catch-all word that covers several related plant actions.
On a bonsai, a scissors could be considered a pruning tool. Then there’s the periodic hard cutback into old wood, using concave pruners. Then there’s extreme pruning, applying a saw to every last trunk and large branch, leaving little or no foliage.
Last month I saw a good example of growth following extreme pruning.
Yews just after collection. These are big yews, taller than a person, and these are not pots, but tubs. The plants in the background were not extreme pruned the year before collection. The one in front was. This early extreme pruning gives the yew in front a big advantage—the large in-the-ground root system flushed many low shoots that can support the plant in its first year in a container. The ones in the back will have a harder time this first year.
Not all plants respond well to this. Some plants can be extreme pruned before collection and some should not be.
You may have noticed that few recently collected ancient conifers from the mountains have pruning scars on them. Most of these trees are compacted to begin with, with little to cut off. Secondly, these old plants need all the photosynthetic surface area they can get to reestablish roots.
Some common bonsai plants do not respond well to extreme pruning—leaving little or no foliage—even if strong: Pine. Juniper. Others usually do well with extreme pruning: Azalea. Boxwood. Most deciduous trees.
Timing is the last consideration. Plants such as yew, azalea and deciduous may be extreme pruned in the spring as growth starts. That gives the longest season of growth for the plant to rebuild its resources.