Repotting is an indispensable operation for the care of our bonsai and should be performed when the plant is healthy so as to overcome the stress caused by the repotting itself; generally the optimal periods are spring and autumn.
There are several reasons for repotting:
- depletion of the soil: over time, the nutrients of the earth are depleting;
- root growth: root growth causes the soil bread to rise, and we can also see the roots coming out of the lower holes of the pot;
- bad drainage: we do not see water coming out from the holes of the pot, the granular soils flake out becoming dusty and compacting leads to poor drainage;
- soil too compact: excellent for retaining moisture, but once dried the water struggles to connect again (an example is that of bonsai just bought).
Root cleaning and potting
When we repot our bonsai we must ensure that we are in a place in the shade and away from the wind, this to prevent the roots from drying out once out of the pot. Have a nebulizer at hand to moisten the roots if necessary. With a non-sharp object separating the soil from the edge of the pot, the plant should come out easily. If this does not happen, press a finger or unsharpened stick into the lower drainage holes of the pot.
Now it is time to clean the roots of the old soil. You can start with the sides of the bread and below, using your fingers to peel off the soil and a small stick to untangle the roots well. This operation could be very long and tiring if the roots have reached such a mass that they have covered all the bread with earth. Lay out all the roots previously untangled and shorten them as needed according to the new pot, cut the large roots leaving some capillary upstream of the cut. A proper root system should have large roots in the visible part of the plant, ie the foot (nebari) and under the ground many capillaries. To assess whether we have done a good job we should be able to insert the bonsai in the old pot without the roots bend and leaving about an inch of space from the walls of the pot. It is very important to remember to moisten the roots from time to time with a sprayer.
In the already formed specimens and with years of cultivation behind them, usually they try to utilize a smaller pot; on the contrary, in the growing specimens and in the phase of formation, they tend to utilize a bigger pot, but always rather low, in order to avoid the growth of the roots downwards. The holes for drainage must be large enough to allow a good air circulation and the outflow of excess water, you can widen them or make new ones if you think it appropriate.
Attach the nets for drainage to the holes, you can use, for example, the wire used for the ligatures of the branches. The meshes of the netting must not be too narrow, otherwise the water will not be able to drain off. Prepare the bottom with draining material (akadama, pumice, lapillus, etc.), if using akadama remove the excess dust with a sieve. Now add a layer of soil creating a small hill where we will place the nebari. Anchor the bonsai with wire for ligatures using by passing the wire in the holes of the pot and using as a handhold the trunk or a root firmly. Personally I do not like to weld the bonsai in this way, if the plant can be stable even without the anchorage with the wire jump this step. Add the other soil and using a stick push it trying to make it penetrate into all the spaces between the roots (be careful not to damage them!). Use this method throughout the pot until you feel that the stick is struggling to enter the ground. All that remains is to water abundantly but gently with a nebulizer in order to avoid holes in the ground.
Put the plant in twilight and away from the wind and freeze it for about twenty days. Do not fertilize for one month and limit defoliation as the lengthening of the vegetation corresponds to the lengthening and development in the roots.
Never repot plants in crisis, they may not overcome the stress of repotting and die.
It is always recommended to keep in the ground material such as sand or crusher gravel with measures from 1 to 4 mm. These facilitate the growth of new capillaries as these encounter an edge, a stone or a grain and tend to fork and/or turn around them.
In conifers you will often find a white powder or patina attached to the roots, put it aside, they are fungi that live in symbiosis with the plant and of maximum utility that should be repositioned as close as possible to the roots just before adding more soil.
Each plant has different needs, always consider what bonsai you have in hand and what it needs.