My take on this is that it depends on container size. I may be wrong and I hope Al will correct me, if necessary, before I lead you and others too far astray.

In very shallow/small pots (not necessarily bonsai pots but certainly bonsai pots are good examples) there is minimal soil and therefore minimal area for roots to grow. In these pots the tree will likely need all of the available soil area for nutrient uptake roots. As such it is common practice in bonsai to remove all larger roots and provide support for the tree by wiring the tree to the pot. By doing this the tree needs no support roots and the container arborists can dedicate all available roots space to nutrient transport roots by pruning all larger roots (larger being subjective).

In larger pots where the tree has enough soil, container arborist can allow both larger support roots and smaller nutrient transport roots. Assuming of course the soil has nutrients to transport.

In the 1-1-1 mix, the soil itself has almost no nutrients to give to the tree. Some nutrients will become available as the bark breaks down, but generally this soil is thought of as "sterile". The nutrients come almost entirely from the fertilizers we feed.

As to your question of the size of "larger vs smaller" roots, I either was told by Al, read on here somewhere or read in one of the books Al suggested that roots over 1/8" really provide little use in container culture.

Actually, I make a distinction between support and nutrient transport roots as though they were seperate types of roots but in reality only the very fine "hair" roots are capable of taking nutients and water from the soil. I think in time hair roots will become larger support roots, only transporting nutrients in their cambium layer and not actually capable of taking nutrients or water from the soil. Because the tree can get many more smaller diameter roots in the same space as one larger root and therefore more "cambium" or space to transport nutrients and grow the xylem and phloem, subsurface roots over 1/8" can be removed in container culture. Also removing larger roots (which may have many hair roots attached along it's length) will encourage the tree to send out many more smaller roots each terminating in hair roots.

Probably more indepth and too far astray in understanding from your original question.

Short answer, small pots fewer roots over 1/8" and may need outside help for supporting the tree (ie wiring from underneath like bonsai culture) larger pots can keep some roots over 1/8" for support but the tree can still only support itself against being blown over by the wind to the extent that the inertia of the weight of the pot is not over come by the energy of the wind blowing against the top of the tree.

Blake

Good info, Blake.

Your goal is to have a few large roots attached to the trunk for water and nutrient transport, with the rest of the roots being very fine. The best way to achieve that end is to systematically prune the large roots back to just beyond a very small root that branches off of the main root, leaving the fine root as the new root 'leader', so to speak.

The first thing you prune off are roots growing upward or back toward the center of the root mass, and roots growing downward directly under the trunk. Eventually, you want your roots to spread evenly away from the trunk in a horizontal growth pattern before they start to branch. Then prune off roots growing directly downward, and truncate all the large roots near to the trunk, leaving a fine root to extend & colonize the soil.

I think the only part of root pruning that really requires much experience to make a good decision is how much rootage you can safely remove before you risk the plant shedding branches because it can't move enough water to keep a branch viable. Fortunately, there is something of a safety mechanism built into most deciduous trees, in that if you root prune before they begin the spring flush, they will only activate buds they can support, WHEN they can support them. This doesn't mean you can cut off all the roots and treat the stump as a cutting, but if you also prune the top, you can come pretty close to that in some plants - trident maple is one that comes to mind, but I wouldn't press things too much as a beginner.

One tip: If you have a 12" deep pot and the roots only occupy the top 3-4" of the soil, you might need to water every couple of days to make sure the soil is moist where the roots are. Let's make it 2 tips: Don't allow the roots to dry out while you're working on them. I usually work with a hose or a tub & dip or spray the roots very frequently until I get the plant repotted. It's those very fine roots that do all the work that hasten the plant's reestablishment.

I just (top) pruned about a dozen maples this weekend and today - still a little too early for me to be repotting. I have a really large trident maple forest that needs repotting. I'll tackle that this weekend - it's going to take a while as there are 20-25 trees in it. ;-)

Al

I don't envy your task Al, but would love to be a wall flower watching you at work to see what and how you prune. You have some beautiful trees from the pictures I've seen.

Blake

Thanks PeaPod and Al. I understand it better now. Al, your comment about pruning a large root until just after a small root branches from it helps a lot. For some reason, I couldn't get this concept from previous posts. Now that I understand this, I realize that I was probably overzealous in my root pruning of my first JM. I cut off some roots that were probably 3/4" in diameter. The tree hadn't yet opened its leaf buds, but sounds like I may have caused it some real problems. Yikes!

My Orange Japanese maple has grown thru the planter hole and is deep into the soil beneath the pot. This causes the pot to hold water. In the rainy season the tree is standing in water. I don't want to destroy the pot (it is large and expensive) . Would I kill my tree if I cut the root back and either planted in the ground or back in the pot? Or what if I emptied the water from the pot and covered the tree and pot with plastic until spring?

Any suggestions?

Sylvia

You're almost certainly dealing with more than one critical issue here. That the pot has no/ almost no drainage means roots will be unable to function normally/ efficiently while inundated. From your description, I think it's safe to assume root congestion is robbing the lion's share (by far) of your plant's potential for growth, vitality, and likely eye appeal. For growers who want to maximize the opportunity for plants to realize as much of their genetic potential as possible, regular repotting is essential. Unlike potting up, a half measure, repotting includes bare-rooting, root pruning, and a change of grow medium. Repotting ensures entire relief from root congestion while habitually potting up ensures the plant will have to deal with the stress/ limitations imposed by that root congestion, even if the tree is planted out in the landscape.

The best course would be to sever the root(s) growing through the drain hole immediately after the the tree has shed its leaves. Then, at the very first sign of budswell in spring, do a full repot and return the plant to the same pot (or smaller) in an appropriate grow medium; or, correct the root congestion and all problem roots before planting in the landscape. Here is a before/ after sequence of a repotted maple:

This tree was layered off of its old roots. It's old roots were ugly and one-sided, so I girdled the tree with a tourniquet (notice the heavy wire in the bottom image) to force a new set of roots to grow above the tourniquet. In the 3rd image, you can see most of the old root system growing under the new roots as I hadn't separated the tree from the old roots yet. No need for you to be concerned or bothered with the layering process.

Let me know if you have questions you think I might be able to answer.

Al

Hi, i have a question regarding that technique that i have seen on multiple youtube videos as well. When you severe the bottom inthe Fall, but then repot in the Spring, how do you set the tree in the pot for winter? the severed roots will free space. do you just our some soil under the tree and then change all in the Spring? I have a feww rootbound japanese maples in pots and i am zone 8. np real buds yet but i do see some foliage coming out of some other more early trees. I'm guessing trimming the roots now is not a good plan?

thank you for all the info and the visuals !

IME, it is a little late for root pruning your JM's now. I grow my extensive collection of Japanese maples solely in containers and I only root prune in winter when the trees are still fully dormant. Mine are just beginning to leaf out so I would consider repotting or up-potting now but there is less stress on the trees and a faster recovery to root prune in January or early February.

Thank you and we are in the same zone. i am in Vancouver, Canada. the japanese maples here havent started to leaf out yet but we have had summers so dry that i don’t want to risk stressing them out indeed.

OMG. Thank you so much for such a detailed answer and yes I meant once you have removed the bottom of the roots, your tree would basically sit well under the rim of the pot, so I was wondering if you add soil under and then still do a full repot in the Spring. So for JM in containers, here in BC we definitely are at the onset of budswell, you would still recommend to repot (cutting the bottom roots, adding new soil et replace in the same pot) now. I use a well draining medium because it normally (not this year) rains a lot here over fall/winter. I'm also always very afraid of pruning the wrong way but I will put my big girl pants on and do as you suggested :-)

Again this is so very much appreciated.

... here in BC we definitely are at the onset of budswell, you would still recommend to repot (cutting the bottom roots, adding new soil et replace in the same pot) now.

Yes, the timing is great. The purpose of performing full repots (includes root pruning, bare rooting, and a change of decomposing grow medium) is to remove the largest roots not attached to the base of the trunk, cutting large roots back to a shorter side shoot with plenty of fine roots.

Repotting before leaves are emerging will temporarily stall emergence of leaves. The plants chemical messengers will advise plant central (via hormonal balancing) when the root system will be unable to support a full canopy of foliage. When the size of the root mass is inadequate to meet the canopy's water demands, the leaves will stop opening. As new roots grow, the leaves will start to reopen at a rate the root can readily support; whereas, if you repot/ root prune while leaves are already partly emerged, the plant will shed the leaves it finds itself unable to keep hydrated, which represents a notable measure of wasted energy. If one tries repotting a maple in full leaf during the summer, it would be essential to make sure the tree was healthy enough to survive a complete defoliation to prevent the entire organism from collapsing due to the inability to move water to the canopy.

There is an amazing difference in growth rate, development from one growth phase (from seed to seedling to a juvenile state to sexual maturity) to the next, vitality levels, and the plant's ability to defend itself when a tree repotted at regular intervals is compared to a tree that has only been potted up or left to languish in a rootbound state. I'd say that root-pruned plants will exhibit up 5 - 10X the growth rate of trees only potted up.

Al

The current and former Michigan winters are/were so warm that my maples have all started to come into leaf, so I've been repotting for 2 weeks already this year - much earlier than I expected. I guess the bright side is it will take some of the pressure off at normal repot time and not cut so deeply into my fishing/hiking adventures in Apr/May.

@tapla (mid-Michigan, USDA z5b-6a), as you can see (first three photos), this ficus benjamin is starting to put out many new leaves. Keep in mind my climate (Italy, Naples). Could I therefore already carry out a repotting with pruning of the roots? Is there no need for me to wait for June as you generally indicated? Can I proceed now even if the plant has some signs of distress as you can see from the bare lower branches (the fourth photo) or should I wait for it to recover in spring? Among other things, I want to prune the roots because the plant has been in that pot for several years and I think that the suffering it undergoes every winter is partly due to root congestion. For the record, it's the same plant we talked about months ago here: https://www.houzz.com/discussions/6377512/ficus-benjamin-and-yellowing-leaves

Should I be root pruning my yuzu trees like this?? They are in large pots.

@michelerossi Could I therefore already carry out a repotting with pruning of the roots? The answer isn't an easy yes or no. Obviously, you can do whatever you want when you want, but that isn't always the best approach. If I had to make the decision, I would weigh a number of factors, the plant's current level of vitality, light levels, current temperatures, and whether of not I felt it was an emergency situation (whether the plant is likely or unlikely to survive (w/o repotting) until the most appropriate time to repot. You might also have other factors like your schedule or commitments to consider. If the plant is not in danger of succumbing to root rot or the congestion, I'd definitely wait until the optimum time relative to your geography and weather.

Is there no need for me to wait for June as you generally indicated? It's not a matter of hard, fast rules re the timing. Instead of "can I" or "should I" repot earlier than June, the question should be what's best for the plant. In our previous discussions I outlined why repotting close to the summer solstice is best. The plant will respond most enthusiastically and recover faster when days are longest/brightest and when the plant's level of reserve energy is at or near peak. Even if the plant is doing poorly now and you feel its level of vitality won't increase between now and mid-June, I'd still wait. I think the only thing that would make me repot now (here in MI) would be if I determined the plant to have a fungal root infection.

Can I proceed now even if the plant has some signs of distress as you can see from the bare lower branches (the fourth photo) or should I wait for it to recover in spring? I think I pretty much answered this question in what I wrote above.

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@kitasei Should I be root pruning my yuzu trees like this?? They are in large pots. When I suggest what you should or shouldn't do, it's always from the perspective of 'what is best for the plant'. Root congestion is a far more insidious robber of growth, vitality, and the plant's ability to defend itself, than 99% of growers realize. Potting up to a larger pot is not an effective strategy for managing roots because there will always be congestion at the center of the root mass. A full repot which includes bare rooting, root pruning, and a change of grow medium completely relieves root congestion and allows the plant to return to its normal rate of growth, level of vitality, and ability to defend itself (against disease pathogens and insect herbivory). A tree that is regularly repotted will normally grow 5-10x faster and exhibit a higher level of vitality, eye appeal, and it will be more productive.

If you're objective is to provide your tree with the best opportunity to maximize its genetic potential, repotting (full repots) are essential to that end. Root congestion is limiting and will always hold a plant back.

The best time to repot citrus is in the early spring - just before you notice the plant is getting serious about its spring push brought on by warmer temps and brighter/ longer days. Make sure you understand how to go about the repotting process before you dive in head first. Most loss of viability subsequent to repotting is due to overlooking something that seems minor but is important. One important factor is, the roots need to be continually wet during the process.

Al

Updte on the 2 JM that had their roots trimmed in early Spring. As predicted, they put out less leaves in the Spring. but now one of them is growing new leaves all around im guessing it developped more roots and is now ready to do that.

That is acer palmatum Kuro Hime with the new growth coming out now.

that is the root ball of my acer palmatum Heartbeat. Just to have an idea of how much was trimmed..

and this is acer palmatum Chantilly Lace. it was trimmed the same way and it’s looking good i keep seeing Chantilly Lace with green leaves in the Summer in pictures but this one is never green. Even though he is now in a new pot under a bigger maple tree. so it should not be sun stressed.

Thank you again for the advise and i think it did them good.

Looking good!

Al