Bonsai don’t need repotting very often; they can stay in one pot for several years at a time. If you have purchased a tree within the last half a year, it would be very safe to say that you won’t need to transplant this tree unless you don’t like the pot it was in, or for some other cosmetic reason.
Q: Is it like repotting house plants?
A: We have to assume that the person repotting a Bonsai has had some experience repotting house plants, but there are many differences.
Usually, when you are repotting a houseplant, you are trying to give your plant room to grow bigger and bigger by transplanting it into larger and larger pots. With Bonsai, however, you want the tree’s needles and leaves to continue getting smaller in size, so there’s no reason why the pot should keep getting larger. Therefore, roots need to be pruned to invigorate the tree and to generate new growth, and to be able to fit it back into a small pot.
Q: What is root pruning?
A: Root pruning is an important part in the repotting process. Root pruning is a matter of nipping the roots of the tree back, to reinvigorate the tree or to fit it into a new pot.
The time of year that the tree is the strongest is when you can prune the most roots without damaging it, since that is the time the new fibrous roots will grow back most vigorously. That is why we don’t suggest you do any root pruning just before the dormant season starts.
Q: What should you look for in repotting Bonsai?
A: In repotting a Bonsai, it’s very important to think about balance and placement.
The tree should be about a third of the way off to the side, never in the center of the pot, unless it is a round or square pot.
Look at photographs of specimen Bonsai to get a feel for how the tree should be balanced in its pot.
The size of the pot should harmonize with the size of the trunk and the expanse of the foliage.
Q: What is the purpose of moss in repotting?
A: We use moss to cover the root ball at the base of the trunk, but the function of the moss is mostly ornamental.
It does, however, keep water in to a degree; the tree won’t dry out as fast, and when it’s hot out, it actually keeps the soil a little bit cooler.
Q: What about rocks?
A: Again, rocks are an ornamental feature used to balance the tree. Rocks help to create the illusion of a tree growing wild in the mountains.
Q: Are there any dangers to watch out for when repotting your tree?
A: The most important thing is that the roots don’t dry out in the sun and wind. If you are repotting it in the summer, you will place it into the pot as quickly as possible, so that the root hairs are not exposed to the air for too long a time.
Q: Can you prune too many roots?
A: Yes, and that is a very common mistake. The tree will immediately show the stress by dropping leaves and wilting. That is another reason that we recommend very strongly that you bring your tree to a reputable Bonsai nursery for repotting instructions before you attempt it yourself.
Q: How often should I repot my tree?
A: Very seldom does being pot-bound affect a tree’s health; in fact, being pot-bound is one of the principle aspects of Bonsai: the roots have no place to go, and the tree stays small.
Although we recommend repotting every two to five years, many trees have been in the same soil far longer than that and are still doing well. As your tree becomes more pot-bound, you may need to feed it more, but repotting is not necessarily called for.