Nine Things You Need to Know About Decandling Japanese Black Pine

Nine Things You Need to Know About Decandling Japanese Black Pine

Decandling Japanese Black Pine Spring Work

One thing that most bonsai enthusiasts find fascinating when they first start to learn about Japanese Black Pine bonsai (JBP), is that all of JBP's new growth must be cut off in the summer. The technique, known as decandling, is frequently misused or misunderstood. Here are nine important things to know about decandling pine bonsai:

1. Decandling is the pine equivalent of defoliation in deciduous trees.

Decandling removes all of the new growth from a Japanese Black Pine. Trees are built to react to foliage removal by replacing the foliage, normally with more compact foliage. The plant typically goes more conservative in reaction to any type of pruning; this is to conserve energy and allow itself to re-establish the photosynthetic surfaces. In deciduous material partial or total defoliation allows lots of light into the interior branching. Pine decandling does the same thing, simultaneously removing the new growth and exposing the interior branching to more light.

The story goes that decandling was discovered as a viable technique at Daiju-en nursery near Nagoya, Japan when all the new growth on a tree was defoliated in early summer by an unusual insect attack. The owner was sure the tree would die. But, to his surprise, the new growth emerged during the rest of the growing season and was more compact with shorter needles than the spring growth.   The defoliation had resulted in exactly the type of reaction that people were looking for to make Japanese Black Pine into great bonsai.

Unlike deciduous defoliation where all leaves are removed, for pines only the new growth is removed.


2. Decandling can balance branching with wildly different strengths.

There are different techniques used for decandling black pines, and they can sometimes be confusing. Each is a system for balancing growth, and only one should be used on any given tree. Japanese Black Pines are inherently apically dominant – they grow strongly upward. So, the upper branches and the outer branches on the tree will always be stronger naturally than the lower and interior branches. The reasons for this are a combination of access to light and the distribution of hormone production in the plant. The balancing techniques aim to counteract this natural tendency by giving some advantage to the weaker growth while simultaneously giving some disadvantage to the stronger growth.

The common balancing techniques that I use when decandling are:
  • Cut candles in three stages. Divide the growth into four categories: very weak, weak, moderate and strong. Leave very weak growth alone entirely. Cut weak growth, wait ten days, cut all the moderate growth, wait another ten days, and then cut the strong growth. This technique effectively gives the weaker growth a head start in regrowing, which balances the size of the summer growth. This technique gives the most control, but requires keeping to a precise schedule. It can be hard to schedule multiple trees!
  • The “stub method” involves leaving a bit of the base of each candle to act as a “fuse”, which effectively slows down the reaction to decandling. As with intervals, divide the growth into four categories based on candle strength. Leave the very weak growth alone without decandling. For weak growth cut the entire candle off, leaving only 1-2 mm (1/16″) to ensure the node where new buds will emerge is retained. For medium candles leave 5-6 mm (1/4″), and for stronger candles leave up to 10 mm (1/2″). In all cases, it is important to note that there should not be any new needles left on the candle stub. This technique offers less control than the interval technique, and is harder in practice to execute because it requires precisely gauging of the length of each stub to effectively control the summer growth afterward. The advantage is that the tree can be pruned all at once.
3. Decandling also involves removal of older needles.

The point of decandling is to keep the tree compact; to stimulate back-budding; and to allow light into the interior of the structure. Thus needle pulling is also frequently performed on full trees at the same time as decandling. People will frequently count the number of needle pairs left on each branch tip in an effort to balance the vigor of the tree. Leaving more needle pairs on weaker growth (say 10-12 pair) while leaving fewer needle pairs on stronger growth (say 8-10) helps balance the reaction to decandling. I also frequently thin small buds that are clumped together or tips that have too many shoots in close proximity.

Some people say that needle pulling forces the tree to make more needles…while others say that needle pulling weakens the tree. So which is it? The answer to that is not so simple: needle-pulling is part of the regimen used on full trees that allows a lot of light and air into the freshly thinned canopy. It simultaneously weakens the tree by removing the foliar surface, and induces growth by allowing light to hit the areas that were previously shaded. Applied to a single branch apart, needle pulling will weaken the branch. Applied to a tree that does not need thinning it will only slow things down. Only perform needle pulling along with decandling for a dense tree. For trees that are thin or weak, leave all the needles intact.

For pines in development where the branching is immature, leave some 2-3 year-old needles along the tops of the branching. Thin the needles on the bottom of the branching and the 1-year-old needles.

A small older pine, decandled in two stages with some needle pulling. The tree was unbalanced because it was not decandled the previous year due to poor health. This year’s growth is vigorous with weaker lower branches. They were decandled 10 days ahead of the stronger upper branches. Left: Before decandling. Middle: weak shoots removed. Right: 10 days later the stronger shoots are removed and some needles pulled.

4. Decandling needle buds is not a good idea.

Needle buds are growth that has emerged from between a pair of needles. On JBP there are dormant buds at the base of each needle pair and in a cluster at the node points. Standard decandling technique primarily stimulates the dormant axial (aka adventitious) buds at the nodes, but secondarily will stimulate needle buds to grow back further, normally from any remaining two-year old needles. This is the reason that it’s important to leave some older needles on branches while developing young pines. (Uniform removal restricts the back budding to just the nodes!) In their first year, needle buds normally don’t get too big. Thus, in their second year when they really take off for the first time it is tempting, but imprudent, to decandle them.  

Small needle buds on the left, axial buds on the right. The needle bud will need to be left alone for another year before it can be decandled.

You can recognize the year-old needle buds by the lack of a rosette of mature needles at the base. A candle coming from a needle bud will normally only have a few weak year-old needles at the base. Normally, the result of decandling is either the death of the bud, or a severe weakening that is counter-productive. If the needle bud is strong or too long, take just the tip of the new growth off when decandling the rest of the tree. This will slow it down without potentially killing it; new buds will form at the point where you cut off the tip. Is possible, leave the entire candle intact; this will set a node which can then be used to decandle in the following season.

5. Decandling can be counter-productive.

Before decandling, ask yourself, "What are my goals for this tree?" Decandling as a technique is aimed at containment, needle-size reduction, and minimization of wood production. It also greatly slows root production during the summer. So if your goals are to increase the trunk girth on a young pine; to maintain a healthy well-ramified tree; or to care for a weaker older pine that has sent out minimal growth, then it's important to analyze the potential benefits against the potential risks before decandling. There may be more benefits to not decandling a particular tree. 

Move forward and decandle when:

  • Growth is healthy and dense
  • Controlling size is important
  • Growth is long or out of silhouette and the tree is healthy
  • Needle size is more important than trunk development
  • Branch refinement is needed.

Consider not decandling when:

  • The tree is weak
  • There is weak branching that needs to be kept healthy
  • Trying to build wood or grow trunk girth quickly (in this case, maybe decandle only a few small things…)
  • Branching is no longer needed. 
6. Decandling will weaken the tree.

This is important enough to repeat - decandling is an operation that is stressful for trees. In fact, most pine species can’t be decandled at all because they’re not inherently strong enough to recover well from it. Japanese Black Pine are a vigorous tree – so they handle it well when they are healthy. Regardless, this process is robbing the tree of a lot of resources. That’s why when in doubt, it’s better to leave a pine alone for a year to grow unfettered, building metabolic speed. Blindly decandling weak trees can lead to branch dieback or even the death of the entire tree.

To counter the drain of resources the tree will experience from decandling, be especially attentive both before and after the process to ensure that the tree maintains health. In the spring months leading up to decandling, use a system for fertilization that is both regular and aggressive. Weather plays a big role in the way that pines grow; make up for less-than-ideal conditions by fertilizing more and optimizing light. JBP can do okay with as little as 4-6 hours of direct sun, but they do much better with 10-12 hours during the growing season.

Traditional wisdom states that after decandling, stop fertilizing for 4-6 weeks.   This is meant to shorten the summer growth, but take it only as a suggestion and make individual decisions based on the strength of the tree and the summer weather. With cooler weather or weaker trees, consider fertilizing straight through; strong trees or hot weather likely means that it would be best to hold off on fertilizer for a few weeks.

7. Decandling can be performed on some branches and not others.

There are some times when decandling a whole tree might work, but decandling a few branches will work better. The result by fall will be that some needles are shorter than others, and that some branches are weaker than others, but properly applied, selective decandling can actually balance growth, just like the interval and stub methods.

Here are some scenarios where I would consider decandling only some branching:

  • For a tree in development with a large sacrifice branch, decandle the branching that will be used for design while leaving the sacrifice to grow.   Judge the balance between the two; decandle only when the lower branching is strong enough.
  • A tree left to grow for 2 or more years without decandling will likely have much stronger upper branching than the lower and interior. Decandle only the strongest half of the growth. This will give all the weak growth a huge advantage while the strong decandled branches catch up during the summer.
  • For a tree with certain branches that need to be elongated, leave them without decandling. It’s often the case that a lower branch needs to be bigger or longer than it currently is. Decandling will keep it about the same size year after year, so leave a few tips on the lower branch to run for a year while decandling the other branching on the tree. After just a couple years, the branch should be larger and longer.
8. Decandling timing is based on the weather and the amount of time left in the growing season.

Decandling is timed to a date somewhere near the middle of the growing season; the exact timing depends on how quickly pines grow in a particular climate. The best way to determine the exact timing is to pick a date between June 1st (20 days prior to the summer solstice) and July 1st (10 days after the summer solstice.) Decandle a tree and gauge the reaction for the rest of the year. In very hot areas the tree will grow back like nothing happened, while in climates that have cool summers the tree may only make buds rather than new needles. Here are a set of suggested starting points:

  • Areas with cool summer temperatures (like San Francisco and northern climates): Memorial day weekend - June 1
  • Areas with moderate summer temperatures (highs in the 70s-80s like the inner San Francisco Bay area): June 1-15
  • Areas with moderately hot weather (highs in the 80s-90s): June 15-30
  • Areas with consistently hot and/or humid weather (highs 90s-100s): July 1-15.

Evaluate the results from that first year to determine if the date was too early or too late. For shorter needles and more compact growth decandle later the next season. For more vigorous growth, decandle earlier. Generally speaking, decandle weaker trees earlier and stronger ones later. Also, decandle larger trees earlier and smaller ones later, which matches the size of the growth to the size of the tree.

A young pine in development. Not only was the tree decandled; different techniques were used in different places. The lower left branch was decandled behind the nodes to force needle buds; two strong branches on the upper part of the tree were pruned back; and the weaker buds were decandled normally. Needle buds in the second trunk section were not decandled at all. These will be important branches in the coming years.

9. Decandling is one of the best times to prune pine branching.

A healthy tree is going to bud back and respond to cutback because it's already had so much growth removed all at once. In many cases this is also the best time to prune branching on the tree. While decandling is cutting off just the spring growth, pruning is also cutting back older growth that is too long, too big, or otherwise unsuitable to the desired shape of the tree.

Cutback of sacrifice branching in stages is well-timed to coincide with decandling. The removal of parts of the sacrifice branch will stimulate more vigorous growth from the lower branching by removing shade and hormone inhibition. Also consider cutting back older trees where branching has become leggy. The tree will have the rest of the growing season to send out new growth.

Remember that whenever cutting back on a JBP, don't remove all of the green from the branch. Always cut back to growth that has at least a few needles. 

Now, time for some fun! Watch Eric decandle 49 Japanese Black Pines in quick succession!

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