Why Repotted Trees Need Additional Care From Freezing

Hey ya’ll, I’ve had a question that I’d love to have a horticultural answer for so ease my mind that kicks against the pricks so hard :smiley:

One of my main questions is why is it that trees that are recently repotted are more vulnerable to cold damage (roots mostly from what I understand, unless I’m wrong, which I’d love correction if so)?

As someone who runs a plant nursery, I understand that trees that might not normally have cold damage to roots, are susceptible when they’re in black nursery containers above ground. This makes obvious sense as the roots are exposed.

What I’m curious about, is that nursery soil (organic) holds more water and is significantly less porous, which is my understanding as to why I can leave trees (pre-bonsai) on benches in nursery soil, but not bonsai in bonsai soil. If I repot a pre-bonsai to work out the roots and adjust it before it grows out some more, is it just as susceptible if I put it back in nursery soil?

Better put, is the reason for the sensitivity to cold after repotting horticultural, or cultural? Is there a physiological change that removes the cold tolerance, or is it that we remove the moss, and empty the container of any broken down particles that close air spaces within the pot? If it is air spaces, then are Shohin less susceptible after repot as there is less airspace between the soil particles?

For reference if important, I’m in Southern California in an area where our winters are routinely in the low 30’s with occasional mid/high 20’s a few times a month from Nov-Feb, so I’m not talking Colorado cold, which I assume might be a different answer?

I’m also asking as I have plenty of work in the winter and I’m seeking comfort in my early repotting practices to accompany my work :smiley:. ATM I have the more important recently repotted trees in an unheated garage.

Nate - My understanding is that the freshly repotted trees do not store the sugars and starches necessary to handle the colder weather. The sugars and starches reduce the freezing point of the water in the cells and protect the cells from damage due to the formation of ice crystals. A freshly repotted tree uses those sugars and starches supplied by the foliage to grow more roots rather than storing them in the cells. Therefore, it is more prone to root-zone damage from the cold.

An interesting observation this fall was that we had a hard freeze (14F, -10C) mid October before the trees had fully pulled the sugars and starches from the leaves. We had lots of dead, brown leaves on trees. One group of maples had nice red color within an inch (2.5 cm) or so of where the petioles attached, but dead and brown outside that. My hypothesis is that the tree was in the process of reclaiming the sugars, starches, and other mineral from the leaves, but the sudden freeze happened before the process was complete so the outer edges of the leaves died, while the inner portions survived the cold snap and had a more or less normal fall.


Marty makes some good points and one more thing is the tree isn’t in an energy positive state. Keep in mind winter in SoCal (I live here in the Orange County) is not as cold as most of this country. We have a Mediterranean climate which as you stated can get into the 30’s but doesn’t stay there long and we have had 80F days in January which isn’t all that uncommon. Our ability to repot is a little different. I asked Ryan on a Q&A one time about repotting one of my trees (Coastal redwood) in January because it lost is water/oxygen balance in late Summer and wasn’t doing well. He felt good that I would be successful. This is not a practice I am advocating for but I have had better success repotting trees in January then I have repotting in June…FWIW

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Hey Nate,

The main reasons i am aware of are as follows:

  1. During the repot you should be trimming the roots to stimulate new finer root growth in the fresh soil. In order for the tree to produce new roots it requires two main things:

       a) A sufficient amount and intensity of sunlight for photosynthesis to provide the sugars and 
           starches to grow new vascular tissue.
       b) Warm temperatures. Depending on the species trees need warmer temperatures to maximise 
           root growth. Pines can produce new roots in slightly cooler temperatures but junipers (for 
           example) require much warmer temperatures for root growth.

Neither of these are present during the winter time and even if you have a heated greenhouse to maintain higher than ambient temperatures, you cannot control the amount and intensity of sunlight. If you cut roots they require moisture to callous and then lots of oxygen and drier conditions to produce new roots. If you cut the roots and don’t have conditions that favour root growth (temperature, sunlight and, depending on your climate and available shelter, too much rain) they will not grow and may even rot which will weaken the tree.

  1. As Marty suggested, the roots are a storage centre for sugars and starches and the tree uses these to reduce the freezing point of the fluid in its cells to prevent them freezing and being destroyed. By removing roots and exposing to freezing temperatures, you reduce the trees ability to do this and increase the risk of damage to the tree when it freezes.

  2. Whatever major work you do on a tree, whether that be repotting or major styling, foliage reduction, big bends etc, you want to do it when the tree is metabolically active so it can repair the damage you’ve caused and replenish lost foliage. All trees, even conifers go through winter dormancy when they are less metabolically active and able to repair and replenish.

Now some of these points can be worked around if you have a tree that is really sick and needs an emergency repotting. I have had to do this at the beginning of December with a JBP. I built a heat bed so i could maintain the root temperature at 27C (80F) and i have also built a heated cold shelter to prevent freezing temperatures. If you live in a tropical/sub tropical area it may be different. I’m UK based so have no idea what that would be like!!! :joy: Must be nice!! And you may get away with repotting in Fall and Winter once, twice, maybe several times but it is not a sustainable practice to get and keep healthy trees in a temperate climate that experiences temperatures below 40F during the winter.

Hope this helps mate!

Some good additional comments. I was thinking of trees repotted in the early fall when they still had enough light and warmth to get the new roots started, but not enough time become energy positive.

I believe that Ryan and crew start repotting in January with the trees that can handle cooler root zone temperatures as they recover. They keep the trees in a greenhouse and perhaps on a heat bed. I built a seasonal greenhouse and plan to start to do some repotting in mid to late February this year. That is about 4-6 weeks earlier than I can do without extra protection. The greenhouse is currently set for 34F (1C) and I will increase the minimum temperature to about 38-40F (4C) when I start repotting.

My Japanese and European larch are usually ready for repotting from the start of February and although I have a cold greenhouse I haven’t found it necessary to place them inside. That being said, although we might have frost in the UK it is nothing as severe as some of you get, it may hang around at 0 C for a day or two or even drop down as far as - 5 C, but I’ve never found this a problem for them.

After a repot AND the temps are going to be low, protect the roots. Especially if you habitually repot early.
You might dodge the bullet, but why chance it… Especially pricy/older trees.
Same concept for summer heat / dry. Just protect those trees! Shade and more / frequent water.

Hello everybody,
This question is simple, but also difficult. Everything I read is good, but just a repetition of what we hear from Ryan or read somewhere. So far there is no definitive answer.
For example, I repot a juniper. The strength lies in the leaves, I don’t touch the leaves when repot. So there is no need to protect.

I see only one reason to protect. If I break the winter protection with the repotting prozess, every tree has to protect, if not I only have to protect the trees with strength in the roots.

I hope everyone is now confused like me! :fearful:
regards Bernd

Try an experiment. Repot two trees of the same species. Leave one unprotected and the other protected. See what happens.

A step further. Do this with sets of species that store strength in their roots, a set that stores strength in their foliage, and a set that stores strength in their vascular system.

I’m not confused. I prefer to do work when the tree is going to recover the quickest. I don’t have so many trees that I need to be spending half of the year repotting. I’ve got maybe 9 trees to do this spring. I can easily do that across a couple of months. The species vary and some need to be done early spring and others can wait until late spring.

You really need to do groups of trees under each condition - perhaps 10, but 20-30 would be better. It might also be a good idea to control the freezing to -5C (23F) or so since it is fairly easy to kill even heathy, well established trees with root zone temperatures in the -6 to -15C (20 to 5F) range.

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True, but I was trying to keep it semi-reasonable lol.

But what would you conclude if the one that did not freeze died and the one the froze lived? I might actually run the experiment sometime in the next few years. I will most certainly have enough seedlings available.

Bernd, there are so many variables to your question. Obviously there is the harshness of your average winter. Time of repot and scope of work done. Then throw into the mix the hardiness of the individual tree (not repotted) and it can get pretty confusing.
Where about in the world are you? There are Mirai members from every climate zone who would be able to help you in depth.

Hi Keith,
I am looking for an answer as to whether repotting triggers something in the plant that affects winter hardiness. So far we have only talked about what to do if there is such an effect.

I am not a native English speaker and apologize if I do not always express myself correctly.

I think you are looking for something that has most likely never been investigated. Bonsai folks are the only ones who do the type of repotting where a fair portion of the roots are removed. As a result, the horticultural industry has probably not put the money into conducting such a study. I imaging this type of study would require a fairly sophisticated lab in order to understand all of the various chemical changes that might result from bonsai type repotting and then how those changes impact survival over the winter. There is probably a lab out there that can do the study for US$ 500,000 or so over a couple of years.

Until then I think we have to rely on what folks have observed over the years and some of the small scale, backyard experiments that the bonsai community seems to be doing more frequently.

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My take on the whole process is that no tree likes to be repotted and have its roots pruned. When we do it we do put the trees at risk of all sorts of problems. It is only because we know the correct aftercare that the risks are reduced.
The tree is ‘shocked’ (for want of a better word) after the process and immediately goes into recovery mode. With junipers the foliage is left alone and can sustain its energy levels (for a period) with a reduced root system, until it can grow enough roots to supply the foliage needs. (As with all trees (even junipers) if too much root is removed it will seriously weaken a tree what ever the time of year)
If however the repotting process is not done early, or is done late in the year, the tree will try to regrow the roots and will not be able to build up enough winter reserves, or will use these reserves to grow roots, which will seriously weaken the tree and with less sun and daylight for growth for recovery, it will compromise its winter hardiness and subsequently will require more protection than usual.
If the repotting is done late and the tree is left outside during the winter when wet or freezing conditions prevail, the roots will not have had time to harden off and subsequently the soft root tips will die rapidly followed by the rest of the roots and new roots will not or cannot grow.
That’s why more and more bonsai people are investing in heat beds so roots can grow happily while the top remains in winter stasis. Especially those where winters are long and extremely cold.

In addition to what others have said, I think the other big variable is that healing requires energy.

So when a tree is damaged such as pruning the branches or pruning the roots, it will use some of its stored sugars and starches as energy to heal the wound and possibly begin growing more new tissue. This lowers the concentration of dissolved solute and reduces frost tolerance.

Ryan has talked about this primarily in the context of branch pruning, but I think it would be similar with root pruning.