Defoliation is the process of pruning leaves to induce a new flush of finer growth and is an important part of the upkeep of bonsai trees to keep them in proper shape. It should be performed only on healthy trees. Bonsai defoliation involves removing some or all of the leaves of a tree thereby forcing it to grow new leaves which are smaller in size. This also leads to increased ramification. The increased energy after new leaf formation encourages more branching, thereby causing thickening of the trunk line. Defoliation can be carried out on most deciduous trees but some plant species need to be handled more carefully.
The bonsai defoliation process can be broadly classified into two categories of pruning based upon its purpose:
- Structural Pruning : The main objective of this is to shape and style the bonsai. It is preferably done on young trees.
2. Maintenance Pruning : This is done to give a boost to the growth and vitality of the bonsai. It is generally done during the growth season and also helps in keeping the bonsai in shape.
How to defoliate a bonsai tree
Bonsai can be defoliated in four ways
- Removal of all the leaves from the plant .
This is generally done when wiring and shaping the plant so that the entire branch structure becomes visible. Complete defoliation should not be carried out
- On freshly potted plants.
- When the plant has pest infestation.
- On young or weak plants.
- During May and June when the temperature is high.
One can reduce the stress of the tree by defoliating it while doing some off-season emergency repotting , specially in summers. Since there will be no transpiration due to the removal of all leaves , the roots. instead of supplying water, use their energy in establishing themselves in the new soil.
2. Removal of outer Canopy
Extra growth is removed by defoliating the outer canopy only while keeping the inner canopy intact.
3. Removing about one third of the leaves
This is done mostly when the plant is young and cannot bear the stress of too much foliage.
4. Removing only the larger leaves
This encourages the production of new smaller sized leaves and involves the removal of big leaves while leaving the smaller ones intact.
The tree should be defoliated by cutting the leaf where it joins the leaf stalk so that the leaf-stalk remains intact and attached to the branch. The defoliated plant should be kept in shade or semi-shade and never be kept under direct sunlight.
Defoliation can also be used to reduce or boost growth of certain parts of a tree, for example, defoliate the top part of the tree while leaving the bottom part untouched, in doing so you restore balance in the tree. Selective defoliation is very useful if you want to strengthen a particular weak branch, which does not match the thickness of other branches in a mature tree. This can be achieved by defoliating all the branches except the specific one. Retaining all the leaves in the specific branch will encourage it to grow longer and thicker.
The vigour of a particular branch can also be reduced ( if it is thicker than needed in your design) by just defoliating that branch whilst leaving the other branches untouched.
When to carry out defoliation
In Delhi depending on the variety of tree, defoliation can be carried out twice a year. The first defoliation should be done in late August to early September, specially on Pilkhan and some varieties of Ficus, when there is a lot of growth during the monsoon. Weak branches which may be diseased or lanky because of lack of sunlight during rains can also be removed. This is also the time to restucture the tree or change its style.
The second defoliation is done in late February or early March when the tree is vibrant with lots of new growth but has too many unnecessary branches. Again at this time one can get rid of all unwanted branches after defoliation so as to give a neat look to the design of the bonsai.
Feeding tips after defoliation
After defoliation the tree should be fed a potassium and phosphorus rich diet. No nitrogen is added as nitrogen feed after defoliation results in longer internodes and larger new leaf size. Potassium is useful for enhancing the leaf colour, and when combined with phosphorus it takes care of the health of the roots. The addition of phosphorus also thickens the roots and helps them to fuse if they are already thick. Dried banana leaves or wood ash can be used as natural sources of potassium. Bone meal is a good source of phosphate.
The correct ratio of nitrogen, phosphorus and potassium at different stages of bonsai growth are essential for the development of proper roots, branches and leaves. For example. a young Ficus tree needs more nitrogen to mature more quickly than the one which is already established and mature. In a pot, if the concentration of one element is more than needed, the absorption of other minerals will be reduced. So it is necessary to maintain the NPK balance.
Fruiting trees should never be defoliated while they are fruiting. The needs of fruiting bonsai are different since they are not growing in the ground, and the concentration of nutrients required for the proper formation of fruit is also different. Phosphate along with potassium should be given to all flowering and fruiting bonsai in the spring. The nitrogen content should be kept low as nitrogen encourages vegetative growth in the tree