Once your tree has grown in the general direction and shape you want, you can refine it through pruning. Cutting into a tree can affect its health & vigour, so it’s helpful to understand exactly what happens to a tree when you do this. A really excellent paper explaining the effect of pruning is available from Purdue Universityref but to summarise, pruning has the following effects on a tree:
- it removes photosynthetic material (leaves) thereby reducing the tree’s ability to generate energy
- it reduces transpiration (the evaporation of water from the canopy) and the rate of water transport up the tree
- it disrupts the pathways of plant growth regulators, causing regrowth but also consuming stored energy
- if the main xylem vessels in the trunk are cut, it causes embolisms which reduce the water carrying capacity of the tree
- it exposes the internal vascular system to the environment where bacteria and fungi can enter (by causing a wound)
- on some conifers, pruning the shoot or branch removes options for future bud growth because dormant buds and meristem tissue is often concentrated in the more recent growth
Minor or leaf pruning is used in bonsai to keep the shape of a tree according to a design, but also to create ramification and reduce leaf size (or, keep leaves small). As per point 3 above, pruning leaves drives the tree to refoliate and it does this by activating dormant or suppressed buds. In deciduous trees there is usually a bud in every leaf axil and this will go on to produce at least 2 shoots, so you also get increased ramification. With only stored reserves to use for refoliation, shared across twice as many buds, leaf size will be reduced. Read more in: ramification of branches and foliage.
Major pruning which involves cutting off branches or significant parts of the foliage may have more impact on the tree. The first thing is that removing large amounts of foliage will reduce the tree’s ability to generate energy. It will also reduce the tree’s energy requirements but not by as much as is lost (since leaves are working for the whole tree and not just to sustain themselves). See this article: Defoliation.
Major pruning is often required to get the design you want for a bonsai. So is it better to grow out then cut back, or cut back then grow? Growing first generates lots of energy but also lots of wasted growth, which is eventually removed. Cutting first saves energy by directing it all to the places you want to develop on the tree, but it reduces the total amount of energy available for growth.
To test this look at the following calculation. If you start with two identical 50-leaved plants, and the goal of reaching a particular level of refined foliage in 5 years time, you have two options. Scenario 1 lets the plant grow unpruned all the way to the end of the period then has a major prune back down to the target level of foliage. Scenario 2 prunes every year, gradually building up to the target level. Although they start and end in the same place, the first plant has generated a whopping 195,250 ‘leaf units’ of energy for growth – 12x what the second plant has generated.
As much as 80% of the energy created by leaves is exported to the other organs of the plantref. These energy units could have been used in places that don’t eventually get removed in the ‘Cut’ scenario, such as thickening the trunk, storing reserves for stronger budding or refoliation.
The most obvious risk with major pruning is the fact that you are effectively wounding your tree. Read more about how it responds in repairing (?) damage.
What kind of pruning tools should you use? Learn about the difference between carbon and stainless steel bonsai tools here.