Answering Your Frequently Asked Questions About Re-Potting Bonsai

7th Feb 2018

While bonsai trees are generally quite low-maintenance, there will come a time (several times, actually) in a bonsai-owner's life that they need to re-pot a bonsai. These trees need room to grow, particularly in the first years of their life. Repotting becomes a necessity.

This is, of course, one of the most stressful events that can happen in a bonsai tree's life - and one of the few things you can do with a bonsai that carries a genuine risk of harming it. Knowing the proper ways to re-pot your bonsai is essential!

In today's blog, we wanted to answer some of the most common questions that come up during the re-potting process.

The BonsaiOutlet Bonsai Re-Potting FAQ

1 - When should I re-pot my bonsai?

In nearly all cases, spring is best for re-potting. There are rare exceptions for certain breeds, particularly those that don't go dormant in the winter, but generally speaking spring is re-potting season. This is when your bonsai will have the most energy - all ready for its spring growth season - and can quickly overcome any incidental damage done in the potting.

2 - How do I know it's time to re-pot?

There's no set schedule for re-pottings, and it will entirely depend on the growth rate of your tree. The best way to know if it's time to re-pot is to gently remove it from its pot, by carefully sliding a knife or repotting sickle around the interior edge. Then check its root growth. If the roots have hit the edges of its soil and are circling around, it's time for a re-potting. If not, put it back in place and check again next year.

3 - How big should the new pot be?

This depends on the age of the tree, and your goals for it.

For younger, fast-growing trees, you'll want a pot that's at least a couple inches wider than the one you're currently using. There's not really any downside to having a pot that's "too big," aside from being considered unsightly in some schools of bonsai tree training. A larger pot simply gives the tree more room to grow.

For older trees, or when it's reached your desired size, there's no need to increase pot size. You can even re-pot it back into the same pot it came out of. For these trees, re-potting is more about maintaining its health than encouraging growth. A mature tree can stay in the same sized pot indefinitely.

4 - What does re-potting achieve?

In nature, a tree's roots will grow indefinitely outwards as it ages. This can't happen in a pot, but the tree doesn't know that. Its roots will continue to grow and grow until they become a tangled mass which begin to choke the tree out because the roots aren't touching enough soil. So the main purpose of re-potting, beyond allowing the tree to grow larger (if you want), is to trim back its roots and give it more room to breathe.

Yes, this does moderately damage the tree, but it's necessary. This is why re-potting in spring is best. In some bonsai breeds, re-potting may slightly stunt its growth for the year - particularly if you have to cut away a lot of its roots in the process. This is normal, and not to be worried about. It will heal up, then won't need re-potting again for several more years.

This also gives you an opportunity to change out the tree's old soil for fresh, nutrient-rich soil.

5 - How do I know what roots to trim?

Before trimming, you'll need to comb out the roots with a root rake. This is surprisingly like combing out hair: be gentle and work any tangles out one-by-one. You'll also be removing any old caked-on soil in the process.

Once you can see the roots, you can start looking for the ones to trim. Basically, you want to avoid trimming the active roots - these are ones which have white tips. You want to trim the longer roots which lack the white tips. Don't go overboard, though. You don't want to trim off more than around 25-30% of the root mass except in extreme circumstances. Target the longest roots.

6 - What soil should I use for re-potting?

Just about any blend of soil designed for bonsai trees should be fine - we carry a full selection of soils including both general-purpose soils as well as breed-specific blends. We also recommend putting a layer of lave rock at the very bottom of the pot (to help drain moisture) as well as a layer of bonsai moss at the top. Moss is a bonsai's air-conditioning system.

Do you have other questions about re-potting your Bonsai? BonsaiOutlet is here to help! Just contact us and one of our experts will get back to you with advice.