Balancing Branches and Roots?

Every spring, in repotting season, this question comes up:

“When I prune the roots, should I prune a roughly equivalent amount of branches to balance the roots?”

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It’s true that chopping a roughly estimated amount of branches to “balance” the pruned roots at the same time, can reduce or stop the remaining foliage from drying and dropping, and indeed that’s the traditional view.

But modern bonsai horticulture views the overall energy level in the tree as more important for its long-term health. 

Question: What is the problem with chopping roots and branches at the same time?

Answer: When we remove a significant amount of roots and branches, we seriously deteriorate the tree’s energy reserves. So to develop and protect the long-term health of the organism, you shouldn’t do that unless it is a super-vigorous grower in top health.

In Spring all the sap – transporting up the stored energy – goes to pushing out new buds and leaves, and flowers and fruit/seeds. So if you prune significant branches in spring you are removing a significant amount of energy. And then it can “bleed” sap too.

But pruning roots in spring is not a big deal as all the energy is up in the trunk and branches.

“Pruning roots in spring is not a big deal, as all the energy is up in the trunk and branches.”

When leaves dry and drop, that doesn’t normally mean the tree is dying; it means the tree is conserving water by throwing off some of the older leaves.  Indeed this can be to compensate for a depleted root system unable to supply enough water for the transpiration that happens in leaves. But for most tree species, leaves are disposable, they can be dispensed with temporarily while the tree recovers. And letting the tree ‘decide’ which leaves to drop is a natural process, its usually the older and bigger leaves that get thrown off, so it could actually help to keep the appearance of small leaf size.

In conifers that could be bad if a whole branch dries. So for some coniferous species it is necessary to limit your root pruning to perhaps 1/3 of the root mass, or maximum 1/2, and keep at least half the roots in order to avoid the loss of a significant branch. You can do some branch pruning to avoid this risk, but again, this is likely to make a dent in the energy reserves and take a lot longer to return to vigorous growth again.

But in deciduous and tropical trees, dropping the leaves doesn’t cause a branch to die (unless it’s for other reasons like fungal infection).

Question: Is there some way to prune roots and avoid any loss of foliage due to the depleted root system?

Answer:

In all these cases, I have tried and tested the following successful method: in spring, chop roots but not branches, then stop the transpiration in the foliage by putting a clear plastic bag over the entire tree for a week or so, keeping humidity at near 100%, so the leaves don’t need to draw up water for their transpiration.  If you live in a hot climate with many hours of bright sun, keep it in semi-shade for week or two. After a short time, new root hairs are grown from the remaining roots and the tree starts supplying more water again to the leaves. 

Put a clear plastic bag over the entire tree for a week or so, keeping humidity at near 100%.

For deciduous trees in early Spring, there’s no need to cover with a plastic bag even after you prune off a significant proportion of the roots. This is because just as the leaf buds are swelling or starting to unfurl, they are using the sap in the branches for all the millions of cell divisions; but they are not yet drawing up a lot of water from the roots. That’ll happen later in the season once the leaves are big and photosynthesizing.

When the foliage is ample or the leaves are very big, that’s when the clear-bag trick is most effective. If you leave a repot until early summer, this trick is very useful to keep the leaves green in the otherwise hot, dry air. But as previously mentioned, it is not a big issue for a deciduous tree to throw off many of its leaves in summer, as long as the tree was previously vigorous and healthy. They’ll grow back.

• Note: Don’t prune roots in late summer or autumn. The tree is attempting to pack the energy down into the roots at that time. In my early bonsai days, I lost a pomegranate like this in autumn; after repotting, the young tree dropped all its leaves to retain moisture, and then started budding out again in mid-autumn. The resultant leaves were small and dropped in mid winter. It didn’t come back to life the following spring… or ever! I conclude this was because after root pruning in autumn, the limited remaining energy all went to leafing out again, too late in the growing season to generate significant photosynthates needed to grow back sufficient root mass needed for winter storage.

I use the clear-bag trick for broadleaf evergreens – indoor and outdoor. Examples include: azalea, boxwoods, carmona, holm oaks, cork oaks, olives, privets, sageretia, serrissa, etc.

Question: Why refute the teachings of traditional bonsai practice?

Answer:

When the traditional practice says “balance” the root pruning with an equivalent amount of branch pruning… how can we possibly know what is balanced? How do we know how much water the foliage or the branch stems draws up, compared to how much root mass is needed to supply that water? Is it ounce-for-ounce roots for branches? (I doubt it). Or cubic inch for cubic inch? Probably not. It’s impossible to measure in terms of water supply and demand.

Also, it is certainly different at different times of year. And for different species. There is such a lack of exact science here, that following a broad statement to “balance” the tree parts is at best unpredictable and unmeasurable, and at worst is wrong for some species and circumstances, and could jeopardise the long term health.

In summary:

For these reasons, and using the clear bag method, I always wait to do major branch pruning for several weeks after root pruning, at least until I see new leaves have formed and hardened.  I haven’t lost a tree due to repotting for many years now.  Sure, I’ve lost some leaves, but they grow back.

And vice-versa for indoor evergreens: If I first prune the branches in spring, then I wait until I see new buds forming all over the tree before I repot and prune the roots.

Notice I wrote “major branch pruning”. It’s perfectly ok to trim a few minor twigs, leaves, or growing tips to keep the silhouette of the tree in shape at the same time as repotting and root pruning. Perhaps I should also say “major root pruning” for the kind of practice I’m talking about – removing more than half of the root system; sometimes up to 90% of the root mass. You’ll know if you have a trident maple or a crab apple. They root very vigorously because the leaves generate so much energy during the growing season. Even so, I would prefer to keep them vigorous and healthy, rather than tiny and unhappy!

Notable exceptions

Some species are so vigorous that it seems you can do almost anything to them without harming them. It could be considered that significantly reducing the energy in such bonsai is necessary to keep them small. A bit like defoliation… but that’s another topic in itself.

Ficus species (F.retusa, F.microcarpa, F.benjamina, F.religiosa etc.). These are tropical broadleaf evergreens and don’t follow the same energy calendar as deciduous or coniferous trees. It seems not to matter at all whether you prune branches and roots at the same time. You might be reducing energy but they just alway bounce back. It seems to me that the only way to let these trees die is to leave them unwatered for months. Reducing branches and roots seems to have no ill effects.

Portulacaria Afra (dwarf jade): These are succulents; not trees. In any case, you can chop anything off these; branches, roots, trunk chops, etc., then put the remaining plant in dry soil for a week or so, and it will come back to life. Might drop a few dry leaves in the meantime, but it’s the same mechanism, to retain water in the stem ready to push out new growth.

Elm species – to some extent. In general elms are both vigorous and very resilient. This is why Chinese elms make such great starter bonsai, as well as their naturally small leaves. It is possible to chop roots and branches together and see the tree bounce back unscathed within weeks. But personally I treat my elms as the same as any other deciduous trees: repot in spring; prune branches in summer and winter.

Olives – these trees are also very resilient and with enough light are extremely vigorous. They can survive top-and-tail pruning without problem. Having said that, I still use the clear-bag trick in semi-shade for a week or so after major root pruning.

One final exception: repotting without pruning much root mass. If you do this in summer, then it’s fine to prune the branches at the same time. In this video, I call it “Pot Refresh”: