The potting and repotting of bonsai can be tricky for the beginner learning how to do bonsai. There are several bonsai tutorials online that you can follow if you are thinking of repotting bonsai. But below I'll give you some tips you can follow once you are feeling comfortable about repotting a tree all on your own.
Juniper rock planting via via Bluprint instructor Colin Lewis
From my experience, the process of repotting bonsai causes a lot of stress in beginner bonsai growers. More so than bonsai design techniques. The first thing a new bonsai owner wants to do is repot the tree because they think the tree needs it. The reality is that bonsai trees are so mass-produced now that it is unlikely your tree was sitting on a shelf in a garden center long enough to become root-bound.
Chalk it up to our desire for immediate gratification, but we seem to want to stick a tree in a bonsai pot the moment we get it home from the store. Yes, bonsai look better in a nice pot, but it should only be planted in a bonsai pot when the tree is worthy of the pot. What does that mean? It means that until you are satisfied with the styling of your tree, it should remain in a bonsai starter pot. That could be a terracotta pot, a bonsai growing box or even the pot the tree may already be in.
When's the right time for repotting bonsai?
You need to know what type of tree you have and what its growth habits are. Some trees grow so fast that they must be root pruned and repotted every year. Depending on the size of the pot, fast growing trees may be repotted twice a year. On the other hand, some trees grow so slowly that they may only be repotted every 3-5 years. Generally speaking, older trees grow slower and need less repotting than younger trees.
Tropical and "indoor" bonsai should be repotted during the hottest part of the year when they are actively growing. If you live in the north and decide to repot a ficus during the middle of February, chances are that you'll have a dead ficus bonsai by April.
Temperate climate trees (deciduous and evergreens) should be repotted and root pruned just before their buds break in the early spring when they are coming out of dormancy. Some tree species can be repotted in the fall.
Never repot a bonsai tree in direct sunlight. Do it in the shade or indoors and assemble all of your bonsai tools and supplies so they are within reach to lessen the chances of your tree dying because the roots dried out during the repotting process.
Bonsai larch grove via Bluprint instructor Colin Lewis
How much should you prune?
If you are root pruning to fit a tree into a pot, you will need to remove as much foliage as roots. For example, if you pruned 1/3 of the roots of your tree, you have to remove 1/3 of the foliage from the tree. Why? Because there are not enough roots now to feed the leaves above after pruning. So you need to create a balance between the roots and leaves.
What about bonsai soil?
Do not reuse old bonsai soil. Every repotting should be accompanied by a replenishing of the soil. Add in new fresh bonsai soil so the new roots have something to grow into. Whatever you do, resist the urge to fertilize your bonsai tree right away. Fertilizing after repotting will result in burned roots, which will then result in a dead tree.
There's no big secret to learning how to repot bonsai. Yes, it can seem a little tricky for a beginner just starting out and learning bonsai techniques and fundamentals, but once you've grasped a few tips and tricks, it should be really easy. Get comfortable with the idea of your bonsai being in the pot you bought it in for a few years. Remember, a tree should deserve the nice pot you are planting it in. Learn about growth habit of your tree before you go chopping off roots. And don't think about repotting bonsai trees outside of the right time for that particular species.
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