Defoliating is exactly what you would suspect: Removing the foliage of a tree. One can distinguish full and partial defoliation. For full defoliation, all leaves on a plant are removed. And with partial defoliation only specific parts of the leaves are removed.
Why defoliate bonsai trees
Naturally, as with anything you do in bonsai, there are specific things you want to achieve when defoliating a (pre-) bonsai. Each of them has a specific moment in the development of the tree. The following objectives play a role when defoliating:
Increase ramification of bonsai branches.
Under normal circumstances several buds on the branches are kept dormant by hormones produced by the leaves and growing points on a tree. By removing the leaves and growing tips on branches, the branch is stimulated to create side-branches. For the best effect, the whole tree needs to be defoliated.
Balance energy in the bonsai canopy.
Trees typically are apically dominant. As a result the growth in the top of the canopy is much stronger than on the lower branches. Therefor branches in the upper section of the tree thicken much quicker, threatening to ruin the balance of your tree. (After all, you would like to have the thickest branches low in the tree, and very fine branches in the upper parts). By removing foliage from the stronger branches on top, and leaving more leaves lower in the tree the tree will invest in the lower branches. When trying to balance growth, it is important to simultaneously cut back the top 1/3 of the branches, removing the growing tips; This will boost the weaker areas.
Reduce foliage density of bonsai to stimulate back-budding.
A well-developed crown is nearly completely closed and will shade the inner & lower branches. As a result the tree will invest less in the lower branches which will not increase in thichness, nor increase ramification. The branhes higher up in the canopy will not not create foliage on the inside. Slowly only the upper outer canopy will develop and the tree will continuously grow bigger, and heavier on top, without any chances of cutting back to inner branches.
By removing part of the leaves in the outer canopy layer, more light enters the tree, stimulating back-budding and building strength on the lower branches. This will create branches in the inner canopy, to which can be cut back in winter, assisting in creating taper & movement in branches.
Facilitate wiring & carving.
Particularly with evergreen species such as ficus spp. it may be hard to make styling decisions. Foliage would normally block the view on the branch structure. Removing all foliage will stimulate back-budding, and at the same time provide the opportunity to wire & style the tree.
Reduce foliage density of bonsai to reduce fungal infection.
A well-developed crown is nearly completely closed and airflow is reduced to nearly zero. As a result fungal infections can easily spread causing large mildew infections. By opening the canopy, airflow is increased. Air moisture level is reduced and foliage dries up more quickly after raing. This reduced the suitability for fungal infections to spread.
When to defoliate bonsai
Defoliation is done only after the plant has had a change to replenish reserves. Deciduous trees invest a lot of their energy reserves in creating new leaves in early spring. Allow the tree to grow and develop a dense canopy before thinning. Once the first leaves have matured, and the growth has slowed down after the initial spring-flush, allow the plant several weeks to re-build reserves. Normally early summer is the period for removing leaves & trimming back the canopy. (In North-Western Europe this would be around June, depending on the spring temperatures.)
How to defoliate
If you look at a leaf carefully, you will notice that the point where the leaf attaches to the branch is slightly wider and thicker than the rest. Depending on the species you can see a tiny bud exactly at the border between the branch and the Petiole (leaf stalk). This is your future side branch. Therefore, care needs to be taken to not damage that connection. Defoliating therefor is done by cutting the petiole, rather than pulling the leaf. Alternatively, one can opt for cutting lager leaves in half, which is an effective way for partial defoliation. Light can more easily penetrate, yet light is dappled, reducing the risk of sunburn. Over the weeks after defoliating, the leftover sections of petioles and/or cut leafs will dry out and fall off, as the side-branches develop.
In some cases the tree has grown so dense, that full removal of branches is needed. Often this is the case by young trees in early stages of development: In winter, the canopy looked OK. However, when in srping the your branches emerged, the canopy become too dense. Removing secundairy brnaches may provide a good way to improve the structure and increase openness of the canopy. A positive side-effect: Wounds seal quickly in summer, and scars may therefor be less visible.
Risks of defoliation
It is important to realize that defoliating plants has a big impact on the tree. If done at the wrong moment it can cause a loss of fine branching, and in extreme cases full primary branches or even the whole tree can die. Furthermore, many trees are very sensitive to full sun. The leaves protect the delicate bark of the trees against sunburn. Removing all the leaves will leave the bark exposed with potential sunburn as a consequence. Therefor it is best to avoid full mid-day sun exposure until the new foliage has emerged. To avoid the main problems, as a rule of thumb:
Never defoliate more than once a year
Only defoliate trees that have had their foliage for more than a month
Only defoliate strong, healthy plants. So do not defoliate:
Newly collected plants (Within the first 2 growing seasons of collection)
Plants with heavy pest infestations
Recently styled plants
Plants that do not show overall strong growth
Defoliation will weaken the plant. Therefor it is important to first allow a plant to regain the energy spent in spring on new foliage. But equally important: To not defoliate to late in the year, and allow the plant to recover before winter. New branches that grow too late in the year will not have time to mature before winter. This may result in a loss of branches over winter, and a lack of reserves will cause a slow start the next year. I would recommend to not defoliate after about 8 weeks before fall starts, so in North-Western Europe, after mid-July.
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