Summer Repotting

[Brian Van Fleet]

Last year, this thread sparked discussion about the merits of late-summer repotting. Instead of replying there again, I thought I’d start a new thread.

While that thread’s OP calls it “late summer repotting”, often that process is described/shown as shifting a tree from one vessel to another without any root-pruning, and minimal soil removal. In this particular thread, the soil was changed out, but no root pruning was done, and the OP issued these caveats:

“Warning: all this refers to the climate in the south of Germany. ... Mind you: we have not cut off a single living root nor have we cut off a single little branch. This is very important. it helps the tree to grow immediately and establish a healthy root system in the new pot. This was August 5 and the weather forecast is mild with some moisture within the next two weeks.”

My view on repotting, is that the roots below are as important as the tree above. A good bonsai is built from the bottom-up. That is why I named my site “Nebari Bonsai”, as a constant reminder to myself that the best bonsai are built on a good nebari, and it’s something that I’m always chasing. With that approach, I see repotting as a critical and technical window of opportunity to develop and improve the roots. I recognize not everyone shares this view, and that my trees don’t all have perfect nebari.

During the course of the thread, this challenge was issued by a new enthusiast, which got me curious to try summer repotting with a couple commonly-used species to see if it would work:

Anyone else in the last three or four weeks that passed since this thread started did any repot of a deciduous broadleaf tree? Even more so, did any of the vocal nay-sayers here if they are based in locations that allow for a late summer repot tried to expand their mind based on facts and their own experimentation? I hope so but I doubt.

Below are the results of my experiment. This may not be a large enough sample size for some, or scientific enough for others. But I will say the trees were repotted just as I would have done it at my usual time of March, and they were sited and treated exactly the same as the other bonsai of the same species.

 

[Brian Van Fleet]

First up, Japanese Black Pine. A seedling 6-8 years old, in that pot for the last several years.
Repotted on 8/11/19:

root-pruned, and arranged as I would do at the normal March Repotting:

Wired in, and new soil worked in:

5/20/20:

Still alive, but not healthy; severely stunted, and quite yellow. Compared to another seedling from the same batch:

Compare the growth in the late spring:

 

[Brian Van Fleet]

Second, a Japanese Maple. Also a seedling which I have largely neglected for the last 8 years. It was repotted in March 2019, and again for this experiment on 8/11/19:
Before:

Light root pruning complete:

A month or two later, and it seems to be responding well:

By April, it wasn’t looking good:

On 5/20/2020:

Most of the tree has died, so I lost 2 sections of trunk, representing several years each.

 

[Brian Van Fleet]

Full details are here:

Late Summer Repotting experiment

As an experiment to test the viability of repotting in the summer, when leaves aren’t growing, and roots are, I repotted a Japanese Black Pine, and a Japanese Maple. The pine had been in that…

nebaribonsai.wordpress.com

Who else has real summer repotting results to share? Photos would be appropriate to support anecdotes. In some climates, I can see getting away with it, but in most cases, I don’t see any advantage to repotting in summer vs. Spring. Interestingly, Michael Hagedorn addresses this topic in his book, “Bonsai Heresy”. Had a chance to start reading it while on vacation this week.

 

[coachspinks]

Brian,
I don't have any photos to share, just a few thoughts. I think your last post hits the nail on the head. The op was in southern Germany, that climate is quite different from Birmingham, Atlanta or other parts of the southern US. In my earlier years I worked for a wholesale plant grower/nursery. We grew everything from gallon shrubs to large tree stock. We sold to landscapers and to large developments. Once late May hit we would really discourage large projects from planting. We had a couple of acres set aside for purchased plants that were waiting for fall planting. Likewise, any repotting or planting of material was done before it got hot or later in the year when things cooled off, and we did no real root work. Without near perfect aftercare it is just too hot to summer repot.
I looked at your website after you posted. Nice job! The side by side pictures really illustrate how stressed summer repotted trees are.

Yesterday I was out and saw a crew planting dozens of 3 gallon shrubs and 20' in the median of a divided roadway. These plants are going to get full sun all day. I made a mental note to go back and see how many were still alive a month later.

 

[Wires_Guy_wires]

I don't take pictures of corpses, but I'm not a fan of repotting anything when it's hot outdoors. Not even mugo pines. Those do fine over here with spring repotting, they don't seem to skip a beat. The later I repot them, the worse they look.

I read a couple articles about conifers in general, mainly on US dry soils. It seems that the root growth in summer is minimal; plants either hit a water table below soil in spring, or go semi-dormant in summer by shutting most activity down to minimal levels. I honestly don't know about broadleafs.
This makes me assume that those three dead trees I ended up with after summer repotting, didn't have a chance due to the repotting.

With not working the roots at all, and just providing a bigger/equal container or just lifting the tree out and adding a coarse bottom layer, the issues are less severe but still present in my experience.

 

[Starfox]

I repotted 2 Callistemon yesterday, they really needed it as the roots were like a mat of carpet and had to struggle to remove the old plastic over the drainage holes that were embedded deep inside. Pics aren't great as I wasn't really documenting the process though.


That said it's a Callistemon and they should be fine(famous last words). I have repotted them in summer before with no issues so I'm confident giving the roots some growing space will be beneficial for them but had they just needed an annual repot I still would of waited for summer.

From what I have seen and done in the past I'm happy to repot my broadleaf evergreens any time from spring through to mid October as long as they are actively growing. I probably have more to do if I can rustle up some appropriate pots.

Can't speak for conifers and deciduous though and to be fair not overly keen on that experiment just yet.

 

[Cadillactaste]

I guess, I'm confused...the two techniques were not identical. The poster didn't really touch the roots. Where you did major root work. I'm not sure that is the same procedure with end results as you are showing...had the process been the same. I could grasp it more being a comparison.

 

[Ohmy222]

In the past I alway repotted Satsuki azaleas after flowers. This is the first year I didn’t do that as I now plan to do repots in March. I only lost one tree out of about 10 over 5-6 years. Either way, I can’t imagine GA climate is good for summer repots. May be possible if plants have a summer dormancy which I know nothing about. Lastly, despite how hot it is, it is not summer yet.

 

[Forrestford]

I stumbled across this video the other day. Not sure when he repots, but the trident maple is in full leaf and he explains(vaguely) you must defoliate the tree before hand. Is this a tried and true technique?

 

[Starfox]

I guess, I'm confused...the two techniques were not identical. The poster didn't really touch the roots. Where you did major root work. I'm not sure that is the same procedure with end results as you are showing...had the process been the same. I could grasp it more being a comparison.

Nothing to be confused about, major root work in summer is no problem for these. Even less of an issue when you barely touch the roots.

Brian asked and that's what I did yesterday. Had it been another tree it would likely of been a less invasive procedure. Not all repots are equal.

 

[markyscott]

I guess, I'm confused...the two techniques were not identical. The poster didn't really touch the roots. Where you did major root work. I'm not sure that is the same procedure with end results as you are showing...had the process been the same. I could grasp it more being a comparison.

Why would anyone advocate a preference for repotting in a season where you can’t touch the roots?

This is the best explanation that I’ve seen Walter give:

Repotting big maple

Please note that this is not a full on bare rooting re-pot but more of a root pruning and partial soil change... big difference between the two.

www.bonsainut.com

He also points out that you can be more aggressive on root pruning younger material:

Repotting big maple

And then my own knowledge and style still changes. It is not helpful if you quote me what I wrote in 2007. I would have never done late summer repotting with precious maples then. Only fools never change their minds. I've changed my mind on many aspects of bonsai cultivation : the more you...

www.bonsainut.com

He discusses carrying out a light root pruning, no top pruning and no defoliation (from my perspective, that’s what Brian carried out in his experiment). The big difference is climate - Alabama in August is significantly different than Germany or England in August. I think Brian is just cautioning that there is risk in extrapolating what one can get away with in one climate to be a universal truth for all, a caution that I’m sure Walter has stated also. That Brian obtained less than satisfactory results performing the technique in a much less forgiving climate is a result that should present zero surprises. So the lesson is not that Brian should have cut off fewer roots, it is that one should exercise caution when distilling universal truths. I think that’s a valuable reminder.

S

 

[Cadillactaste]

Why would anyone advocate a preference for repotting in a season where you can’t touch the roots?

This is the best explanation that I’ve seen Walter give:

Repotting big maple

Please note that this is not a full on bare rooting re-pot but more of a root pruning and partial soil change... big difference between the two.

www.bonsainut.com

He also points out that you can be more aggressive on root pruning younger material:

Repotting big maple

And then my own knowledge and style still changes. It is not helpful if you quote me what I wrote in 2007. I would have never done late summer repotting with precious maples then. Only fools never change their minds. I've changed my mind on many aspects of bonsai cultivation : the more you...

www.bonsainut.com

He discusses carrying out a light root pruning, no top pruning and no defoliation (from my perspective, that’s what Brian carried out in his experiment). The big difference is climate - Alabama in August is significantly different than Germany or England in August. I think Brian is just cautioning that there is risk in extrapolating what one can get away with in one climate to be a universal truth for all, a caution that I’m sure Walter has stated also. That Brian obtained less than satisfactory results performing the technique in a much less forgiving climate is a result that should present zero surprises. So the lesson is not that Brian should have cut off fewer roots, it is that one should exercise caution when distilling universal truths. I think that’s a valuable reminder.

S

Oh, well I was going on what Brian had shared in his first post. He didn't mention the second link you shared. So that is where I paused. But, as you state...Location/Climate is always a key factor in knowing what works for you.

 

[Brian Van Fleet]

I guess, I'm confused...the two techniques were not identical. The poster didn't really touch the roots. Where you did major root work. I'm not sure that is the same procedure with end results as you are showing...had the process been the same. I could grasp it more being a comparison.

Scott is correct, this was not an apples-to-apples comparison in any way. This was an experiment to see if I could get away with repotting (by my definition) in the summer; as many here advocate. Unless I’m simply shifting a tree to a different pot (which presents no risk, in my opinion, and is also not truly repotting), why would I choose to repot in the summer? By these results, I wouldn’t do it.

 

[Cadillactaste]

Scott is correct, this was not an apples-to-apples comparison in any way. This was an experiment to see if I could get away with repotting (by my definition) in the summer; as many here advocate. Unless I’m simply shifting a tree to a different pot (which presents no risk, in my opinion, and also not repotting), why would I choose to repot in the summer? By these results, I wouldn’t do it.

Okay, I understand now. I was just...confused. Thinking it was a comparison. I personally, nothing against anyone and their process that works for them. After as long as I've had success with what I already do. Would never change a thing. But...I get now what you were doing. I do early spring...just as buds swell. I've passed on repots when things moved to fast and I didn't get to them fast enough. But...I'm just plain vanilla...I don't like pushing boundries...I like tried and true methods which I've done. Over anything new...A bit OCD possibly in that aspect. Location as always stressed is key to factor in. That was taught to me early on. This post, goes with that aspect of thinking. Thanks for explaining.

 

[Adair M]

Why would anyone advocate a preference for repotting in a season where you can’t touch the roots?

This is the best explanation that I’ve seen Walter give:

Repotting big maple

Please note that this is not a full on bare rooting re-pot but more of a root pruning and partial soil change... big difference between the two.

www.bonsainut.com

He also points out that you can be more aggressive on root pruning younger material:

Repotting big maple

And then my own knowledge and style still changes. It is not helpful if you quote me what I wrote in 2007. I would have never done late summer repotting with precious maples then. Only fools never change their minds. I've changed my mind on many aspects of bonsai cultivation : the more you...

www.bonsainut.com

He discusses carrying out a light root pruning, no top pruning and no defoliation (from my perspective, that’s what Brian carried out in his experiment). The big difference is climate - Alabama in August is significantly different than Germany or England in August. I think Brian is just cautioning that there is risk in extrapolating what one can get away with in one climate to be a universal truth for all, a caution that I’m sure Walter has stated also. That Brian obtained less than satisfactory results performing the technique in a much less forgiving climate is a result that should present zero surprises. So the lesson is not that Brian should have cut off fewer roots, it is that one should exercise caution when distilling universal truths. I think that’s a valuable reminder.

S

I agree with your assessment, Scott.

On a similar topic, even though I love pines, I never give advice on Mugo pines. I’ve never had any success with them because they’re really just not suited for my climate.

That aspect of bonsai (proper matching of species and local climate) is not stressed enough. There’s a number of forum members here who live in the Pacific Northwest who apparently don’t understand that species that grow well there don’t like the environment 2000 miles to the Southeast! Lol!!!

 

[Meh]

Thank you for sharing this experiment.

The professionals that I have seen discuss summer repotting typically mention avoiding freezing temps the following winter. Did you consider doing anything different for these trees, or was this more about seeing if you could successfully do summer repot while treating them the same as your other trees?

 

[TomB]

Who else has real summer repotting results to share?

Dwarf Norway spruce, Picea abies 'Little Gem', which I'm developing as a shohin.
I'm in SE England so relatively gentle climate. The tree gets winter protection (cold greenhouse) though that is to keep it out of the rain rather than being concerned about cold.

Purchased from a garden centre 22nd August 2017. Immediate major root reduction (about 60%) bare rooting one side. Major top reduction (about 90%).






Reasonable growth (for a very dwarf variety) in 2018.

Repotted 26th August 2018 - bare rooted other half. Tried to remove as few roots as possible, coiled in pot. Some new root growth evident.





Have not touched roots since then, though there have been further top reductions. Tree seems to be healthy and growing well.


You can see the vigorous fresh growth in the picture. It has recently been pruned.
Note that I don't think this was a case of 'getting away with it' - the tree responded very well. Whether it would have without the massive top reduction I don't know. This is the first spruce I've had in a long time - I had another one of these about 15 years ago, and I was probably similarly aggressive to it - so I don't have any points of comparison for this process. I chose the summer timing for this based on accounts of experienced Spruce growers (Walter in particular). The only other thing I'd repot through choice in the summer is Mugo pine (other than opportunistic collection of juvenile decidous material).

 

[coachspinks]

Bonsai rewards patience. If I miss the optimal repotting window I try to be patient enough to wait. The more valuable the tree the more patient I am!

If I had a shaded a grow/greenhouse I might be more aggressive.

 

[sorce]

Since going against BVF is my MO...

Just throwing that sarcasm in for folks who won't get passed it .....

Across the board, this winter seems the worst to go thru in a while, and get accurate results.

Other....on "Sorce's Center-Point Bonsai Calendar"(coming soon) summer repotting in Bama should take place well after August's full Moon. For the record, these repots were done 4 days before the full moon which is good.

I'll have some data soon. WT...people been blowing fireworks like they been free, so repotting to the sound of leftover fireworks is going to be like repotting in spring this year.

Sorce