Let’s do some science – a very straight forward experiment. All other things being equal, what difference do a few weeks make in terms of when you choose to prune spring growth?
Bonsai teachings say we wait to prune new growth until the spring leaves are hardened off. This is a gray area at best for most species, so what is it that we are really waiting for? Or put another way, what does hardening off signal that makes this a good time to prune? Ryan Neil speaks in terms of the tree being energy positive, having regained the resources spent through the winter and the spring flush. If waiting to prune allows the tree to build energy, might waiting longer result in more energy stores that can be measured in stronger growth the following year?
The subject of the experiment will be two boxwood trees, genetically identical, very near the same size and planted in pots with similar dimensions. Growth on these boxwood clones was very consistent and well balanced in spring of 2021 with most spring extensions having 6 leaves. In the year leading up to this baseline, the two trees were treated in a similar fashion including the timing of spring pruning for both plants done within a three day window in May 2020.
In 2021 I applied the same pruning approach to both trees — most spring shoots were reduced to the first pair of leaves, some older leaves were plucked to allow more light and air into the foliage pads, and relatively few other branches were reduced or pruned to improve the structure. The only difference is that they were pruned 5 weeks apart — the twin trunk was pruned on May 16 and the single trunk on June 19.
Will the extra time with hardened leaves provide any notable benefit?
Over the year that followed these trees were treated consistently – water, fertilizer, exposure, winter placement, everything. Both trees are healthy and have shown no signs of pests or disease. As mentioned above, these are genetically identical. In fact, the two trees were split apart at the base from one plant over 15 years ago. Relative to my other bonsai experiments, this all adds up to some pretty strong controls.
Both trees pushed reliably in 2022. I let them both harden off with no miscellaneous pruning. Now it’s just a matter of measuring this year’s growth. What do you guess the results will show?
I randomly selected a couple dozen shoots to examine on each tree and counted the new leaves. The current year’s leaves are still quite easy to distinguish from last year’s growth which is a much darker green.
The twin trunk tree, the one that was pruned earlier in 2021, had eight leaves on the vast majority of new shoots with just a few having ten.
Almost every shoot on the single trunk tree, the one pruned later, had six leaves with just a few having eight leaves.
This is not what I expected! My hypothesis had been that more time before pruning would result in more stored energy leading to stronger spring growth. I would not have been surprised with no apparent differences, but instead I got the opposite result!
Obviously, there are differences to acknowledge which could explain the measured difference. The pots are both shallow rectangles, but they are not identical. The single trunk vs double trunk makes for easy visual distinction of the subjects, but more relevant is the exact amount of foliage on each tree. I estimate that the twin trunk example carries 20% more foliage just as a factor of its design. Whatever the differences, I am reminded that I had nearer equal growth in spring of 2021.
What could this mean? There are two possibilities that seem most likely:
- The differences between the trees are enough to result in the difference in spring growth and the time of pruning was not a significant factor.
- Pruning earlier positively contributes to the strength of next year’s flush by allowing the tree more time to build energy into the buds being set for the following spring.
Let’s consider that second possibility with a comparison to the second flush on decandled black pines. We know that JBPs decandled earlier have more time to grow a second flush before the end of the growing season resulting in stronger growth and longer needles on the second flush. JBPs decandled later will have smaller, shorter growth in comparison. Pine growers use this knowledge to time the pruning of trees based on size (larger trees earlier and smaller trees later) or to help balance growth on a single tree by decandling in carefully timed phases.
The boxwoods in our experiment didn’t push a second flush (though boxwoods sometimes can), but could it be that the timing of pruning impacted how much energy was stored in the buds being set for the next year? If bud development is happening between mid May and mid June, could it be that the twin trunk tree was able to store more energy in just two buds per growing tip (at the base of the two new leaves left on the shoot) while the single trunk tree was expending energy to build six buds per tip? …only to have four of those six removed a few weeks later?
This is only a theory, but I am inspired to continue making careful notes about the timing of pruning on these trees as well as other specimens and species in my collection.
Bonsai Iterate 