March 20, 2020

When to Repot

Source Material: March 2020

          It is crucial to prune roots in their optimal window because if roots are pruned at the wrong time, this is one of the easiest ways to kill your bonsai (aside from simple watering neglect). The repotting season also is a vague window that changes from year to year based on the weather, adding to the risk of confusion for beginners and tree death. For the bonsai beginners out there, after today you'll be able to repot with much more confidence that your tree will make a healthy recovery. If you're already a bonsai expert, I put some pretty bonsai pictures in that I think you'll find worth scrolling for either way.


1. Optimal Temperatures for Root Repair
2. Swelling Buds
3. Advanced Exceptions, Caveats, & Disclaimers
4. Bonsai Bud Gallery

Japanese Maple/Acer palmatum waking up at Elandan Gardens.

When is it safe to repot and root prune a bonsai?

          For the newly inducted bonsai addicts among you, you should know that it is generally safest to prune roots and transplant temperate species of trees in late winter/early spring. In this ideal window, temperatures are warm enough for the roots to regrow and repair, but the tree has not begun pushing out of dormancy yet with its full enthusiasm. A healthy tree should have stored energy from the previous growing season to use for creating new spring growth; this stored energy can also be put towards repairing the root system once temperatures are warm enough. Problems can arise though if you wait too long to perform this operation, as once the tree is fully active and temperatures are higher, the demand for water is higher as well (especially in summer), so it becomes more of a gamble whether your bonsai tree can recover from our pruning on the tree's vital water-collection system (the fine root system).

The rest of the above maple. Collected, styled, carved, and owned by Dan Robinson.

1. Trim roots when low temperatures are above 32°F / 0°C

          The line of "late winter" varies wildly based on your location, climate, and species. (I'll put a few caveats at the end of this section for those of you with more advanced aspirations.) The best truly universal advice I can give you is to find a local bonsai club and ask them for advice. I have no idea what it's like to do bonsai in Italy or Japan or even in Eastern Washington! But, lacking those experienced connections, the common thread you will find is that it is safe to repot your bonsai and trim roots once you expect low temperatures will be consistently above 32°F / 0°C. Like leaves, new roots do not have as much protection to them and will be killed by frozen temperatures. If that happens to your freshly repotted or freshly collected tree, the tree loses its stored energy investment in those new roots and has to start all over! You better hope it has enough stored energy to try again. This is why prominent bonsai collectors like Randy Knight or why large scale seed propagators like Mark Comstock use heated mats under their plants in some cases. Putting a freshly collected tree on a heated mat that keeps the roots above freezing in winter will allow that tree to grow more roots! Likewise, putting a heated mat under your pine seedlings will tell them it is safe to germinate earlier, which means your seedlings will make more progress faster.

Larch/Larix is waking up!

2. Look for buds swelling and changing colors

          Another theme - check the buds of your trees for them to tell you if they're ready! I always look for my larches and snowdrops to activate first in my garden before I know it is safe to repot - some experience in your local environment can help you figure out when your safe window begins. For any given species, if you watch the buds towards the end of their dormancy, the buds will begin to swell and change color when your bonsai is ready to be repottedThis is right before the leaves emerge. But, if you wait too long after this, you might miss your chance for optimal recovery!

Full view of Dan's seed-grown larch (notice that it is growing in the ground to speed up trunk thickening! A subject for a blog post in the near future.

          Ideally, you would do your repotting and root pruning somewhere in between the timing of having temperatures above 32°F / 0°C and when the leaves start to emerge. This will allow your roots some time to recover before much foliar growth (and therefore demand for water) is occurring. Although, if you have more trees than you can get to in that time, generally you can push the limits of the second guideline and repot a bit after the leaves have already opened up. I'm not sure how late is too late to repot though, I kind of go by gut feeling - seeing how much new growth has occurred already, how hot the weather is lately, and how healthy the tree is overall. I asked my friend Julian Tsai who apprenticed in a bonsai garden in Japan for a few years for his thoughts on the closing of the repotting window. Julian said,

"Definitely bud swell is generally the best time [to repot] and it's a bit late to repot after
leaf break. You get some more leeway with junipers and it is possible to do some
repotting in the fall time on pines but it depends on the weather, health of the tree,
and nature of the repot."

Thanks, Julian for your insight! With that preview of the more advanced exceptions, let's carry on.

Plum flowers opening up. Some species have flowers that activate before their leaves.
Dan's flowering plum.

3. Advanced exceptions, caveats, variations, and disclaimers:

          So far, I've been talking about advice for repotting temperate species that live outdoors, but our world is diverse and so different species and regions of the world have different optimal timings for root pruning.
  • Tropical plants: Tropicals behave very differently than temperate trees! Instead of repotting while they are dormant, they should be root pruned as they approach the peak of their growing season - whatever that is for your climate. I do mine in June and July here in Seattle so they still have some time to recover before I bring them indoors at the end of summer. If you grow your tropical bonsai indoors year-round, any time will work, but your bonsai will be slower to recover than if they are able to enjoy the warmth and heat outdoors in summer.
  • Late summer/Early Fall: I have had some success with light root pruning and yamadori collection in late summer as temperatures begin to lower, rain returns to Seattle, and there is still some growing season left for the tree to make new sugars and repair its systems before winter comes. This is also very dependent on your local region and in some regions with harsher winters, it may not be possible. In any location, you run the risk of cutting roots too early and having a heatwave come afterward to kill your plant, or of cutting too late and the tree not having enough time to repair before a harsh winter. These risks make it less safe in my mind than the early spring window, however, Walter Pall in Germany is a big proponent of this method (especially with Mugo Pines) if anyone wants to seek advanced reading.
  • California & other mild/Mediterranean-esque climates: In climates with mild winters like San Francisco, Southern California, and Florida, some people repot all winter long. I'm really not knowledgable in these regions, but I can point you to some professional artists such as Jonas Duplich and Boon Manakitivipart in San Francisco, or Julian Tsai in Los Angeles, or Adam Lavigne or Seth Nelson from Florida if you have questions about those areas.
  • Flowering plants: Julian also pointed out an interesting idea to me which I had not considered. For plants like the flowering plum above and like Chinese quince and cherry blossoms which all open with their flowers first before their leaves there is a little more gray area on the repotting window. If you have a tree in development and don't care to appreciate the flowers, you can repot at bud swell before both the flowers and leaves emerge. However, if you have a more developed flowering bonsai, you can repot after the flowers emerge, but before the leaves emerge. Again, this only applies to those species which have flowers that emerge before any leaves do.

4. What do buds look like when they wake up?

          Now that we are through the meat of the article, I'm going to show what a variety of species buds look like at the edge of their dormancy/when they just start opening up, as well as the full tree just for fun.

Dan's famous "Acer palmatum baobab" or baobab-inspired Japanese Maple.

Creeping hydrangea buds. You can see they have a protective layer they're shedding.
Creeping Hydrangea

Creeping Hydrangea
Korean hornbeam buds
Boxwood buds sometimes lead with flowers. Yes, flowers on boxwoods are very small and strange.
Boxwood needs a trim!
Small larch. I think for a unique pot like this, a smaller, less powerful tree is a better match.
Larix/larch buds
Pines are a little trickier to see as they activate later in summer. the bud in the middle is what we are watching. This tree must have been de-candled too late last year as those most recent needles (which grew last year) are a bit shorter than we usually aim for.
Dawn redwood buds.
Dawn redwood in full.
Mountain hemlock buds not moving much yet, but they will soon.
This mountain hemlock is the first tree I ever learned to carve on!
Quaking aspen buds lead with a small flower (inflorescence, technically).

Quaking aspen
Dan's first bonsai - a mugo pine from nursery material.
Mugo pine. New pine needles tend to emerge later in summer.
Wisteria buds.
Deciduous azaleas can build up huge buds. This is a difficult species to ramify.
Yamadori rhododendron from southern Oregon's Klamath mountain region.

Trident maple ~10 years from seed.
The trident maple is moving!
Hemlock buds tent to turn white just before they open.

A yamadori mountain Hemlock that Dan styled at the NWFGS last year.

Trident maple in lava rock.

A little carving tease with the buds.
Dwarf rhododendron previously featured on my Instagram.
Amur maple.
Apple buds are ready to go.

I'm still figuring out how this camera works. I have a better picture of this apple bonsai on my Instagram.
The light green on this azalea is all new leaves. Everything is moving a little early this year.
Gary oak buds
Not the best backdrop, but I had to include this Gary Oak since they're quite rare in bonsai culture.

Crabapple buds waking up.
Crabapple in a lava rock.
Apple buds
Apple in a lava rock.

Tall European beech getting air-layered to produce two trees with all that extra trunk.
European beech buds.
I found in my own garden that my rose was the most eager to get growing - even more than my larches! Here's what those fresh leaves look like on a rose bonsai.
This rose bonsai was recently acquired by Dan from a local artist who passed away. The branching needs work, but it has a great trunk to start with!

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