Decandling is essential to the development of Japanese Black pine (Pinus thunbergii) bonsai. The goal of decandling is to contain the growth of the tree, force more compact growth, and create back-budding. Decandling removes all of the new growth from the pine, which like any tree, is built to react to foliage removal by replacing the foliage, normally with more compact foliage.
After any type of pruning, a tree typically reacts prudently to conserve energy and allow itself to re-establish the photosynthetic surfaces (e.g. needles). In deciduous trees, 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.
There are three main factors to consider before you decandle a Japanese Black Pine bonsai tree: 1. is the tree healthy; 2. is it the right time of year to decandle; and 3. how compact do you want your tree to be?
Is my Japanese Black Pine healthy?
The first step in decandling and summer cutback doesn't involve even touching the tree; it's to assess the health of the tree and determine exactly what work needs to be done. In decandling you are trying to contain growth and force your tree to make more compact growth and to back bud as close back to the trunk and inner branches as possible. However, only healthy, vigorously growing trees can be decandled if you expect good results.
There are alternative techniques to decandling that you should use if your tree is not vigorous and growing strong: 1. reduce some older needles to reduce density; 2. cut off only half of each new candle, leaving the smaller ones while reducing the size of the larger ones; or 3. do nothing but water and fertilize, and wait until the tree is healthy and vigorous.
Is it the right time to decandle my Japanese Black Pine?
Decandling is generally done around the summer solstice - near June 20-21st in the Northern Hemisphere, or December 21-22nd in the Southern Hemisphere. Using this as the baseline date, plan to decandle earlier if you want longer needles (which are typically for larger trees), or decandle later if you want shorter needles (which are typically for smaller to medium trees).
The primary factors that drive growth after decandling are temperature and fertilization. If you live in an area with cooler summers, you should decandle 1-3 weeks earlier than the solstice, and if you live in an area with hot summers then you may decandle 1-3 weeks after the solstice. Adding more fertilizer can help push things along; conversely, to reduce the size of vigorous summer growth, remove fertilizer for 4-6 weeks after decandling.
If you remove candles too early your tree will produce growth that resembles spring growth - it will be too large to make a good bonsai composition. If you remove candles too late, the tree will create buds but no new needles, making it uneven.
How compact do I want my tree to be?
Large trees will have larger branches and longer needles typically than small bonsai compositions. For trees under 14-16" you will want to maintain a very compact needle structure. For trees above this range, you can allow a larger growth pattern and longer nodes in some cases. How large you want your composition to be can help determine the exact decandling technique to use.
Decandling healthy pines with budding in mind
There are two types of buds: adventitious and needle. How your pine will grow after its been pruned and thinned will differ depending where you focus your decandling efforts.
Adventitious buds are at the base of a new candle, called the node point. The node is where the bud forms during the late summer-to-fall growing season and where the new growth elongates during the spring and sends out new needles. To stimulate the buds at the node point, cut just above the base of the new growth; leave 1/8"-1/4" of new growth. This will look like a bright green stub but should have no new needles remaining. The buds at the node will emerge relatively quickly; you should see buds starting 10 days later; and give shorter needles than the spring growth as they grow out the rest of the season.
Needle buds can be stimulated by decandling behind the node. Each pair of needles on a JBP has a dormant bud in the center. Find the base of the spring candle, then cut back below the ring of needles that are around the base. This removes all the buds at the node and forces the tree to bud back further. The tree will send out buds from the center of some of the pairs of needles below the cut. This technique provides less reliable placement of new growth and the emergence of the buds takes longer by about two weeks. The advantage is that you can create a tighter growth pattern than by decandling above the node points. Note that once needle pairs have been removed, that it is much less likely that a dormant bud will emerge at that location.
Key tool for decandling: scissor recommendations
It's important to have a good pair of bonsai scissors when decandling. The scissors need a relatively narrow tip that can be inserted between needles, and that can cut right at the tip of the scissors. The scissor needs two cutting blades, not one; most bypass pruners (like Felco) are too large to use for decandling. 1. ARS Trimming Scissors are made for grape cutting. They're readily available and inexpensive, but with a somewhat dull tip. These are a great beginner's tool; Eric's has lasted 10 years and he shares that one of the best ways to get a sharper tip is just to grind down the outer edges. 2. A regular bonsai shear is great for bigger hands or if you prefer a scissor with a longer reach. 3. Many bonsai growers choose a full metal handle Japanese scissor from makers such as Kaneshin.
Removing, reducing, rewiring
Once you have completed removing the new candles, it's time to thin the older needles also. For Needle buds, leave the pairs of needles that you want to create buds. The closer they are to the base of the prior summer's growth, the more compact growth pattern you can create for a small tree. For adventitious buds, remove some of the old needles from around the base of the bud, but leave some of the needles lower on the year-old growth to allow for more budding. Once decandling and needle pulling are done the tree should look relatively thin. You are forcing the tree to generate more buds and needles. Use this opportunity to assess the structure of the tree, pruning longer branches or anything that is not needed.
You can wire branches right after decandling. It's a nice time to wire because there are fewer needles which means more space! Wiring at this time allows you to set the branches in your desired position and get the tree to look really natural for the next winter. Don't do this more than 7-10 days after decandling because once the buds start coming out from the decandling sites, it's really easy to accidentally damage them.
Decandling Japanese Black pines in middle development is different
The decandling techniques above can be applied to refined (e.g. mature) healthy trees to maintain balanced branch strength. Young trees are generally not decandled, or only some branching is decandled. If you are still developing the trunk and nebari of your tree, or selecting primary branching, your tree is in middle development. With trees in development we often want to allow one or two large branches to escape to help with thickening the trunk or nebari.
Accomplishing a nice mature tree is your ultimate goal, but that might mean that your tree looks like a lanky teenager for a few years. During development it is not important to have uniform, short or even needles in the tree (like you would for a tree you're preparing for a show) - your purpose is different, so the technique is different.
You can decandle branching selectively to maintain growth close to the trunk so that you don't have to later graft or do heavy cutback. Decandling behind the node will create a lot of nice buds at the top of the tree. This technique gives you a lot of smaller branching to work with which is useful in forming the crown of the tree. Try to contain the length and size of the branching on the lower portion of the trunk.
Establishing a sacrifice branch requires you to NOT decandle at least one bud/branch for a few years. You can reduce the hormone and shading that a sacrifice branch creates by removing the side branching on the sacrifice. Allow just the central strong bud to grow each year.
Decandling to adapt existing coarse growth
With field-grown Japanese Black Pine or trees you purchase from nurseries or bonsai growers, you may have to reduce and/or remove a lot of larger growth in favor of creating smaller branching and compact dense structure. In many cases the existing branching on this type of material is too chaotic and large to use.
In the first year you own this type of coarse material, focus on understanding the tree's health. Remember, only healthy and strong trees should be decandled; no decandling should be performed on weak trees. At decandling time, use large pruners or branch cutters to cut back large unusable branching. If the branches are too large to bend effectively they are usually not considered usable. These larger cuts are pruning cuts and can be performed either in summer or early fall. Performing cuts in summer leads to a more immediate reaction from the tree, but normally fall cuts will also stimulate new budding the following spring.
Watch Eric decandle a large field-grown Japanese Black Pine!
Keep the interior growth and small buds on the trunk and near the base of the branches. Thin them to 1-2 in any single location. Allow these to run if they are small or decandle them if their vigor is sufficient. Normally the first 2-4 years of working with nursery material will be devoted to transitioning the tree from course to refined through this process of removing large branching and beginning the refinement process.
Want to learn more about decandling Japanese Black Pines? Check out this article, "Nine Things You Need to Know About Decandling Japanese Black Pine"!