Why root-prune when you repot?

tapla (mid-MI z5b-6a)December 13, 2005

I have spent literally hundreds of hours digging around in root-balls of temperate deciduous trees collected from the wild, nursery stock bonsai candidates, and trees that have already had some root work in preparation for bonsai training. The collected trees are a challenge, usually for their lack of roots, and have stories of their own. The nursery stock is probably the closest examples to what most of your trees are like below the soil line, so I'll offer my thoughts for you to consider or discard as you find fitting.

I've purchased many trees from nurseries that have been containerized for long periods. Our bonsai club, just this summer, invited a visiting artist to conduct a workshop on Mugo pines. The nursery (a huge operation) where we have our meetings happened to have purchased several thousand of the Mugos somewhere around 10 - 12 years ago and had been potted-up into continually larger containers since. Why relate these uninteresting snippets? In the cases of material that has been progressively potted-up, large perennial roots occupied nearly the entire volume of the container, plant vitality was in decline, and soil in the original root-ball had become so hard that in some cases a chisel was required to remove it.

In plants that are potted-up, rootage becomes entangled. As root diameters increase, portions of roots constrict flow of water and nutrients through other roots, much the same as in the case of girdling or encircling roots on trees grown in-ground. The ratio of fine, feeder roots to more lignified and perennial roots becomes skewed to favor the larger, and practically speaking, useless roots. Initial symptoms of poor root conditions are progressive diminishing of branch extension and lessened vitality. As rootage becomes continually compressed and restricted, branch extension stops and individual branches die as water/nutrient translocation is further compromised. Foliage quality may not indicate the tree is struggling until the condition is severe, but if you observe your trees carefully, you will find them increasingly unable to cope with stressful conditions - too much/little water, heat, sun, etc. Trees that are operating under conditions of stress that has progressed to strain, will usually be diagnosed in the end as suffering from attack by insects or other bio-agents while the underlying cause goes unnoticed.

Potting-up temporarily offers room for fine rootage to do the necessary work of water/nutrient uptake, but these roots also soon lignify while rootage in the old root mass continues to grow and become increasingly restricted. The larger and larger containers required for potting-up & the difficulty in handling them also makes us increasingly reluctant to undertake even potting-up.

I haven't yet mentioned that the dissimilar characteristics of the old soil as compared to the new soil when potting-up are also a recipe for trouble. With a compacted soil in the old roots and a fresh batch of soil surrounding the roots of a freshly potted-up tree, it's impossible to establish a watering regimen that doesn't keep the differing soils either too wet or too dry, both conditions occurring concurrently being the rule rather than the exception.

Most who read this would have great difficulty showing me a containerized tree that's more than 10 years old and vigorous, that hasn't been root-pruned at repotting time (Trees in extremely large containers excepted. Growing in very large containers is similar to growing in situ). I can show you hundreds of trees 20 years to 200 years old and older that are in perfect health. All have been root-pruned and given a fresh footing in in new soil at regular and frequent intervals.

Acers are one of the most forgiving of trees when it comes to root pruning. The process is quite simple and the long term benefits include best opportunities for plants to grow at or near their potential genetic vigor, and stronger plants that are able to resist the day to day perils that bring down weaker plants. Root-pruning is a procedure that might be considered borrowed from bonsai culture, but bonsai culture is nothing more than refined container culture and to restrict the practice of root-pruning to bonsai only, is an injustice to those of us who simply enjoy growing trees in containers.

I didn't mean for this to be so long. I haven't even touched on the methodology of the process yet. If I've presented my case adequately and there is interest, I'll follow with the short version of how to root-prune & repot. If not - I had a good time writing this anyway. It gets really boring around most of the forums at this time of year. ;o)


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Sure we are interested in the follow up of how to root-prune & repot. I have visited this forum at least 50 times since yesterday evening waiting for your post to show up. I have visited some bonsai sites that have articles on repotting and root pruning bonsai. In fact the attached link has how to videos on different topics including repotting.
I am very interested in learning root pruning and repotting techniques as applied to regular container grown japanese maples where you are not so much constrained on the container size, etc.


Here is a link that might be useful: Bonsaichannel

    Bookmark   December 13, 2005 at 10:12PM
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You have my attention. Actually you got that with your precursor statements in the Superthriver thread. I certainly will not argue with your ideas or justification, so all that leaves is the how of shuch a task.

I have grown maples in containers for the past 5 years. Some make it to the landscape, but the vast majority of them do and will reside in pots. I frequently undergo the potting-up process to give the roots room to grow--that little extra space you mention--but at some point, around a 20-25gal. can, the return on roots space in relation to the soil volume and weight greatly diminishes. Thereby, there is much less benefit the larger the container we use. So it makes great sense that we turn our efforts towards management of the roots at some point--if not all along.

To the extent I have pruned roots, it seems very easy and the plants have never adversely reacted. When repotting, it is common to have roots on one side of the plant (say the side that is shaded) grow in a more robust manner. When placing in a new pot is difficult to deal with a long stringy root mass, so I often even things up. The other scenario is whereby the roots are a tangled mess, and in sorting out the mess, roots can break or be damaged. I usually clean these up with sharp scissors.

Those tasks are maintenance during repotting, but do not constitue the removal of lignified woody roots to make space for feeder roots and to keep the plant vigorous and the pots size manageable.

How much root mass can be removed? Is it done in stages or sections? What time of year? I have seen prebonsai specimens after the inital topgrowth and trunk pruning. This can be very severe. Do we handle roots that way also? Is it necessary to cut back the top?

I hope that is a good start. I look forward to hearing more.

Best regards,

    Bookmark   December 13, 2005 at 10:48PM
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Embothrium(USDA 8 Sunset 5 WA)

For what could be seen as a related discussion...

Here is a link that might be useful: Myth of Fragile Roots

    Bookmark   December 14, 2005 at 12:26AM
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schusch(Luxembourg z7)

I join the others in encouraging you to share your analysis. Please do so. Your great experience in bonsai culture seems absolutely to the point.
The maples I planted were mostly containerized when I got them, and even when planted into the soil I often had to work quite a while on the rootball since it is so dense. Separating as much as possible the old soil so there are less problems with moisture once in the new media is daunting, if not impossible. I also hesitated when faced with larger roots that were circling about the best way to cut them without destroying a large portion of the feeder roots since it was impossible to evaluate how much feeder roots a given larger root led to.
A second problem I am struggling with is this one: I have planted older, bigger maples into large containers in the 30 -40 gallon range. I want have to replace the soil next year, even if the roots will still have sufficient space. You mention this in your initial email: 'Trees in extremely large containers excepted. Growing in very large containers is similar to growing in situ.' Can you elaborate?
Again, thanks for the info you provide, and please share your thoughts.

    Bookmark   December 14, 2005 at 4:31AM
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tapla (mid-MI z5b-6a)

Great! There's interest. I'm off to try to make a dollar so I can pay my bills right now, but I'll be back to post soon. I'm very glad for the link supplied by BBoy. It surely relates directly to what I'm discussing and hopefully lends additional credibility to my contentions.

Something to think about - wounded roots stimulate a chemical message that tells the plant the roots are in trouble, causing the plant to direct/redirect energy flow to root initiation and development.

Hopefully, I'll be able to write something tonight.


    Bookmark   December 14, 2005 at 7:22AM
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Embothrium(USDA 8 Sunset 5 WA)

There might be something for you here as well. A college library near you may have his books, at least two of them in my vicinity do (including a community college--although it does have a hort. department).

Here is a link that might be useful: Lacebark Inc. Horticulture Research

    Bookmark   December 14, 2005 at 3:02PM
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tapla (mid-MI z5b-6a)

Interesting. I've long used the Micromax granular nutrients as a/the source for the minor elements in my soils and often use containers that "air prune", especially for plants in which I'm looking for speedy development or plants in decline.

I have access to Dow Gardens' library, so I'm sure that they would have these books. The works of Dr. Alex Shigo have been a valuable learning tool for me, but two books that also really advanced my knowledge of trees & how they grow are:

Physiology of Woody Plants
by Kozlowski & Pallardy
ISBN 0-12-424162-X


Growth Control in Woody Plants
ISBN 0-12-424210-3

They were tough sledding for me, but everything you'd want to know about physiology & the impact of cultural conditions is in these volumes.


    Bookmark   December 14, 2005 at 4:21PM
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tapla (mid-MI z5b-6a)

The root framework of in situ trees is comprised of a network of large perennial roots and increasingly finer secondary and tertiary roots. The perennial and secondary roots make up nearly all the root biomass/weight - usually more than 90%. In length measurement, the >90% of large roots accounts for normally less than 5% of o/a root length. This ratio is impossible to maintain in containers w/o some attention to pruning of roots.

It's rare that we would ever get to see what goes on under the soil. Most of us would be very surprised. Most of the stringy roots we call hair roots actually do little of the absorbing of water and soluble elements - nutrients. They are primarily channels for the translocation of water & the nutritional building blocks plants use to make food and keep their metabolism running smoothly. The real workhorses are the microscopic thread and hair roots attached to and branching off of these visible roots.

Important: Fine rootage is in a constant state of flux. Fine roots are short lived, and if they live, they quickly becomes suberized and less efficient at water absorption. A more likely scenario is that they will die as a result of some unfavorable cultural condition. Heat, cold, too much water (lack of O2), too little water, are a few of the things that kill fine roots. The two things we need to guard against most diligently is too much water and its accompanying O2 deprivation, and heat.

If you use a soil that doesn't require daily watering, you are killing more roots than you need to. It's physiology. Roots deprived of O2 die within hours. If the soil is saturated and dissolved O2 is used up in root metabolism, fine roots begin to die. The longer the roots are deprived of O2, the larger the roots are that succumb. This is why I said that roots are in a constant state of flux. They die regularly & regenerate when cultural conditions return to favorable. This seesaw death/regeneration is taxing to plants & calls mightily on the plants energy production or reserves each time the roots regrow. This is also why I continually preach that use of an open soil along with more frequent waterings will be so much superior to a peat soil that goes days w/o drying. The more often your plants require water, the healthier they will be. Each watering pushes the trapped CO2, methane, and other gasses from the soil and brings in a fresh charge of air. This O2 rich root environment does wonders for evening out the seesaw effect of root death/regeneration from O2 deprivation.

I hope you can see why the preservation of the fine rootage and minimizing the seesaw effect by providing a root friendly soil environment allows energy that would be used in root regeneration to be directed to other plant parts. Your trees will have more reserve energy, will over-winter better, and will resist insects, pathogens, and other biotic and abiotic stresses more effectively.

I mentioned root-pruning/repotting in the opening paragraph. The procedure is simply a way of maximizing the amount of fine rootage and assuring cultural conditions that are conducive to their ability to function efficiently. I'll turn to the procedure in another thread.

I really hope those reading this found something they could use. I know I wish I would have had someone to encapsulate some of the info I put down here. I introduced a good number of trees to the compost pile while paying my dues & this knowledge might have reduced the mayhem I spread among our forest friends.


    Bookmark   December 14, 2005 at 6:06PM
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radagast(US east coast)

Very interesting information here, and I'd agree that bonsai techniques and "growing trees in pots" are closely related.

Maples respond well to such treatment, but is that all maples or just some? Japanese maples obviously can be root-pruned - how about red maples? Or silver maples?

What other trees work? I'd guess anything with a true tap-root, like oaks, hickories, etc, can't be root-pruned very well.

Thanks for your info thus far and we all look forward to more.

    Bookmark   December 15, 2005 at 9:56AM
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ron48(z6 Mass. Essex)

Maples respond well to such treatment, but is that all maples or just some?

If you ever saw how side walk repairs are done you would just laugh at the question.
They use a back hoe to remove the hot top or concrete. They reach out with the bucket and rip the roots off the tree. No saws or axes.

The trees don't seem to notice. But it scares my palmatums.


    Bookmark   December 15, 2005 at 8:42PM
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tapla (mid-MI z5b-6a)

Rad - I have probably done over 100 repot/root-prunes on various species of Acers, including quite a few different palmatums. I can honestly say that the only two trees (Acers) I lost were two bonsai trees (palmatums) that were growing in shallow containers. I overwinter some 200 temperate plants in my garage and I'm usually very careful to throw a little snow on the drier trees during weekend checks for soil moisture. I must have neglected the two palmatums. They were attractive and growing with very good vitality as they approached dormancy, but they never awoke in the spring. BTW, they were not repotted in the spring in which they expired. I'm sure it was a dessication issue.

I think red & silver maples could be chopped into pieces and spread on molten lava where they would quickly take root & prosper. Seriously - the genetic vigor of these species would allow aggressive root-pruning with little risk.

I probably should caution that trees entering dormancy in poor condition are going to be less tolerant of root-pruning in spring, but they will be less tolerant of any stress. Trees that are stressed, even strained and in decline can be bare-rooted and given a lighter version of root-reduction when repotting. After they have returned to normal vigor, repot before next spring bud movement.


    Bookmark   December 15, 2005 at 9:45PM
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While this thread primarily addresses root pruning, it brings up many valuable points specific to growing maples in containers. And since that topic tends to get very popular at this time of year (what kinds of maples for containers, when to repot, how to repot, what to use when you repot, proper fertilizing and aftercare, how and when to root prune, etc.), this seemed a very appropriate time to reintroduce this older thread that contains a wealth of information.

Read, learn and enjoy!

    Bookmark   March 28, 2009 at 11:36AM
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botann(z8 SEof Seattle)

Good thinking GardenGal! This is all good information.
I just spent a good part of my day at the Pacific Rim Bonsai Collection yesterday. I was with a tour given by the head curator, David de Groot. He said the same things as mentioned above. It takes a lot of knowledge and responsibilty to transplant and root prune a 400 hundred year old Bonsai plant on loan.
Botann....just lending a little credibility to the info in the posts above.

    Bookmark   March 29, 2009 at 12:33PM
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tapla (mid-MI z5b-6a)

Not being a name dropper, but I know Dave. When next you see him, ask him about the time they inadvertently procured bark from fir trees that had been ponded in salt water. It added considerably to the TDS in the soil solution and created plasmolysis (fertilizer burn) on many of the beautiful specimens.


    Bookmark   March 29, 2009 at 5:27PM
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Botann, I understand they are closing the collection. What's happening to all the plants?

    Bookmark   March 29, 2009 at 10:39PM
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tapla (mid-MI z5b-6a)

The collection will remain intact, and Dave will still care for them - they just won't be accessible to the public on any regular basis.


    Bookmark   March 29, 2009 at 10:51PM
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gomero(SW France, Z8)

When visiting specialist Japanese Maple Nurseries in the Tokyo area last November I noticed that fairly large maples were proposed for sale in relatively small containers. A quick inspection showed that the pots were not root bound and, after inquiring, I learned that, generally, they root prune the nursery stock rather than pot up. This approach makes a lot of sense in that area since real state is so expensive and,as a result, surface available at nurseries so scarce.


    Bookmark   March 30, 2009 at 5:07AM
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tapla (mid-MI z5b-6a)

Yes - You must attend to roots if the plant is to maintain vitality or grow at close to it's potential genetic vigor. As proof, you need only look at the art of bonsai and it's hundred year old, perfectly healthy trees in very small containers, then compare them to the trees that have had no attention to roots and at 5 years in the same soil are struggling and weak.


    Bookmark   March 30, 2009 at 5:46AM
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botann(z8 SEof Seattle)

Gardengal, as Tapla says, the collection is closing as of April 1st and will only be available on a very limited basis to the public.

Curator of the Pacific Rim Bonsai Center, David de Groot

    Bookmark   March 30, 2009 at 10:03AM
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Thanks Al and bt ;-) It's good to know the collection will remain intact - it is truly remarkable - but not so good that it won't be readily accessible to the public. That's a shame.

    Bookmark   March 30, 2009 at 11:40AM
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rooftop_rose(Harajuku, Tokyo)

I have maples in large containers (100 liters/26 gallons) that have been upgraded in container size from smaller gallons. I have had them from 4-7 years and I always thought about root pruning them because they look outgrown no matter if you repot them or not.

When is the best time to root-prune? How do you go about it? Can you do it in the spring after the leaves come out? If a link can be provided that would be much appreciated. Thanks in advance.

    Bookmark   April 1, 2009 at 9:32PM
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tapla (mid-MI z5b-6a)

There is a thread on this forum named Root Pruning/Repotting. If you do a search for it after the search function is restored, it will take you to the procedural info you asked about.

Best time to repot maples is 2 weeks prior to the onset of bud movement in the spring.


    Bookmark   April 1, 2009 at 11:41PM
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gomero(SW France, Z8)

Rooftop rose;

You may find useful information in this link:


Here is a link that might be useful: Root pruning maples

    Bookmark   April 2, 2009 at 9:21AM
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tapla (mid-MI z5b-6a)

I found the link, Rose.


Here is a link that might be useful: Root pruning/repotting

    Bookmark   April 2, 2009 at 10:55AM
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rooftop_rose(Harajuku, Tokyo)

Thanks Al and Gomero. Really appreciate it. My container grown maples seem root bound or pot bound and so will try and work with the roots more next time.

    Bookmark   April 4, 2009 at 6:56AM
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My apologies, I just realized I posted this in the wrong thread! I don't see a way to delete it. Sorry.

Hi Al,

First, thanks for all of your time and energy spent explaining things to us all!!!

I have just purchased a 10 gallon fiddle leaf fig that actually has two plants in it (may be one plant, I'm not very knowledgeable). They are currently about two feet tall--from soil level to top of plants. I have read and think I understand how to root prune. I would like to separate these two plants into two different pots and grow them both up tall and bushy on top (like in the attached photo). I would greatly appreciate your expert advice on how to achieve this in terms of pruning the canopy.

Are there any roots I SHOULD NOT prune if I want these plants to grow tall?

Also, do I understand correctly that the appropriate time to repot and root prune this particular plant is in the spring before it becomes active?

Thanks for your time!!!

This post was edited by e-w-031 on Fri, Nov 15, 13 at 15:44

    Bookmark   November 15, 2013 at 3:38PM
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tapla (mid-MI z5b-6a)

EW - The best time to repot F lyrata is around mid-Jun if you're N of the equator, and mid-Dec if you're south of it. If you're less than 25-30* N or S latitude, it's not particularly important when you repot.

If you want to achieve that look, and especially that look combined with that ht, you'll need to make some sacrifices. Trees thicken and trunks strengthen in a direct relationship with foliage mass, The more leaves, the faster the tree grows, thickens, and strengthens. To replicate that look, your job would be to try to make sure the terminal bud on the main stem doesn't get damaged or removed until the plant is about 3/4 as tall as you want it in the end. In the meantime, you should only tip-prune any branches that occur, so you maintain as much foliage on the tree as possible. As the tree approaches the desired ht, you'll start removing the lowest branches and working your way upward over time.

If you allow any secondary branching (branches of the main stem, to grow 2 - 3 leaves, then remove the apex (growing tip), more branches will be forced to grow from the leaf axils. You can then keep those oriented toward growth that flatters the plant, and remove those that spoil your vision for what you'd like it to be.

Often, growing is a catch 22. If we maintain a plant so it always looks its best in the immediate, we might be destroying its future. If we're serious about having our plants grow into something that is going to be pleasing to the eye at maturity, we usually need to make some sacrifices and stick with a plan as the plant moves through the formative stages. I realize how much that is to ask of a grower because of how long it took me to develop the ability to look ahead. The good news is, though, I didn't have anyone to help me learn to see into the future or explain all this to me, you do. ;-)


    Bookmark   November 17, 2013 at 3:25PM
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