By Jerry Meislik
Definition and goals of reduction
Reduction is the cutting back of a bonsai or bonsai material to a shorter trunk line. This is also commonly referred to as a “chop”, "hack" or the process as chopping. The reduction achieves several goals including the decrease in height of the tree as well as an increase in the taper of the trunk and can introduce directional change or movement to the trunk.
Often beginners complain about their tree’s small size but most often the trunk size is not deficient but it is the lack of taper that causes the tree to appear young and immature.
Not all trees require dramatic or extreme taper often called a “Sumo “design. Many designs use long, linear, slowly tapered trunks and they can be quite elegant. One design is not intrinsically better than another.
Another possible goal with reduction is the opportunity to build in trunk movement and directional change to the trunk’s flow. Do you wish a trunk that is straight or one with lots of movement ? Reduction can give you that choice.
Redesign of a finished tree or new bonsai material
Young trees can be improved by reduction cuts introducing taper and directional change to the trunk.
Reduction is often needed in an older or finished bonsai. Bonsai trees tend to grow most strongly at the apex and over time the top outgrows the bottom. Over time trees "lose their way", get longer than desired and upper branches get too heavy. The original design gets lost. The taper set by the lower trunk is then not matched by the upper trunk.
To recover the design and to force lower branches to get healthier and thicker the top must be reduced back and not infrequently the whole top must be removed and the top re-grown.
Not all plants can be reduced back in size. Most conifers cannot be cut back leaving no foliage or needles below the reduction; conifers (most often) will die and not sprout back from the trunk.
Generally, reductions are fine on many deciduous plants. However, all plants considered for reduction must be healthy. A sick or weak plant will lose branches or die completely after a reduction cut.
Since all plants will not tolerate reductions check with experienced bonsai growers to see if that specific species will sprout back. Some trees sprout back but do it quite unreliably. Some species sprout back all over the trunk and will give you lots of choices for branches and for apices while others are so erratic in sprouting back that reductions are always a problem.
Time of year
Reductions ideally are done when the tree is actively growing. Making large injuries when the tree is inactive is an invitation to invasion by fungi, bacteria, borers etc. These wounds cannot callus or heal until the tree is growing.
Speed of recover
The time that it takes to re-grow a new apex varies with the material and the diameter of the reduction. Some materials grow slowly and this of course slows the development of the new apex.
Roughly, if the trunk is two inches in diameter than the new segment will need to be approximately 1 inch. And the next segment will be approximately half an inch. These are rough guess-timates of the proper transitions. Transitions going from a 3” trunk to a new 1” apex often look peculiar.
Calculate how long it takes for the tree in a bonsai container to grow that 1” trunk. The more foliage that is left on the growing apex the faster that 1” thickness can develop. Trimming, and keeping the new apex well-groomed slows down the process. The more rampant the growth on the new apex the faster the growth to the 1’’ diameter and the faster the next reduction to 1⁄2 inch can occur.
Shohin or small sized bonsai can be developed in a much shorter time as the re-growth of the smaller apex material is much faster; the larger diameter trunk will require longer time to re-grow a suitable apex.
Allowing the tree to recover in as large a pot as is feasible and not in a small bonsai container.
Appearance and types of reduction
“Blind” reduction no existing branch is available to become the new apex at the desired reduction point and a new sprout will hopefully develop or a graft will be placed to become the apex grafting becomes necessary if there is no growth suitable for the new apex
"V" cut reduction shape suggested for the reduction point to prevent over-swelling of reduced broom style trees
Tapered/slanted reduction a slant away from the front to produce taper and to hide the reduction point can be done primarily at the time of reduction or at a later date after the apex is healthy, strong and growing well advantages of the slant is immediate visual improvement disadvantages are more potential die-back at the apex, and committing to an incorrect apex as new branches grow out in other areas
Flat reduction cutting back with a flat cut vs a “V”, or tapered cut
Cut Paste or wound sealants
Some experts use sealants while others do not.
Design of the new tree
Where to do the reduction depends on how tapered you wish the tree to be
Before doing any reduction establish where your root line/tree base really is. Discovering later that the new apex is too high since the true root line was discovered later to be deep in the soil of the pot
New branch breaks are often stimulated by the reduction - the reduction will stimulate latent buds in the trunk, providing new branch choices that did not exist before the reduction
Plants that fail to break properly
Some materials are reluctant to bud back and if they do, they bud back in a weak and unpredictable fashion
In unreliable species make a blind reduction point higher than you want the new apex to start and then once the apex has sprouted and is growing you can cut off the excess apex and reduce to a lower point
Do wounds on this species heal?
If wounds heal well - no worries about the scar
Non-healing - incorporate the scar into design or conceal the reduction scar on the back
How to handle the new apex
Do not wire until it is slightly woody or you may break it off
With blind reductions the apex may grow at the back and it is not at the right position or it may be at awkward angle to trunk this may necessitate re-orienting the bonsai and choosing a new front
Allow rampant growth do not trim or wire excessively
Determine what style of tree you wish - tapered or not and how strongly tapered.
No one design is superior to another they are just different.
Determine where to cut depending on the future design and the root/base.
Cut back less or more depending on the vigor of the species.
Use a large growing container or the ground.
The larger the trunk the longer time to re-grow the apex.
Decide on whether to hide or use the scar or try to callus it over.
Know your material before you reduce it and how it responds.
Make sure that the material is very healthy.