Overwintering - A Discussion

[dbonsaiw]

Well, it's that time of year again and the threads on overwintering ideas are multiplying. Rather than simply trade ideas of WHAT we do to overwinter, let's discuss WHY we do it. Personally, when I'm thinking about caring for my trees over winter, my two main concerns are (i) preventing temperature fluctuations in the roots; and (ii) preventing desiccation on the top part of the tree. And I believe the two are inter-related. Depending on what we do, we also need to consider water, humidity, greenhouse effects, air flow and early budding.

Unlike a tree in the ground, trees in pots run the risk of their soil completely freezing solid. In addition, because these trees don't have the benefit of being insulated by the ground, they are subject to temperature fluctuations. There are multiple solutions to the problem. First, the trees should be placed on the ground to receive the benefit of the heat emanating from the earth's core. Pots can be placed near or in structures to "borrow" some of the heat retained in the structure. Lastly, and my personal favorite, the pots can be mulched in.

MY OCD just doesn't allow me to leave trees on the ground with nothing more. I also don't place any trees in the garage as I can't really water in there (and it's a filthy mess). Based on my very limited experience, I found that mulching does a better a job at preventing the entirety of the pots from freezing than placing them near a structure. I have also found that the mulch is excellent at maintaining a more constant temperature.

Which brings me to the above-ground portion of the tree. Assuming the roots are safe and sound, the top part of the tree, IMO, should just rough out the winter the way trees do. Of course, I realize that my opinion is largely based on living in zone 7b and having really only deciduous trees. That is, the biggest concern with the top of the tree is desiccation - and desiccation is a function of water loss getting ahead of the tree. If the soil isn't a block of ice because it is mulched, for example, I simply assume the tree will be able to get enough water, especially the deciduous trees (how much of a risk is desiccation with deciduous trees anyway?).

Again based on limited experience, I have not found that plastic coverings for the trees are effective at preventing freezing or maintaining temperature. As it doesn't seem to be helping the roots, I'm not sure how much assistance it is providing against desiccation. It would provide additional protection from wind (and give you full control over watering), but query whether wind or frozen roots is the main culprit here. Although I am still debating covering the mulch pile in a giant tent, I view it as more belt and suspenders than anything else in my situation (also not excited to spend $250).

This view is based on a guiding principal I use in bonsai - Trees belong outside and in the ground. When we change one or both of these, accommodations need to be made depending on the task at hand. My trees are outside and, once mulched, are "essentially" in the ground. So I kind of view the situation as nothing has been changed to the tree's natural living conditions and they should be able to use evolutionary skills to get them by without my interference (which likely just contributes to a more unnatural environment).

Hey I could be dead wrong and al my trees will die - hopefully y'all will chime in before then. I'm curious to hear other people's view points, and especially to hear from those who live in zones 5 and the like to understand WHY folks do what they do to get their (non-tropical) trees through winter.

P.S. I was checking out Walter Pall's thread on his garden and saw pics of his trees outside, covered in snow, on pedestals, in the middle of his yard with absolutely no winterizing. Hmmmmm. Making me rethink the need for a tent.

 

[Leo in N E Illinois]

I'm in zone 5b. I've lived in the same house for over 42 years. It needs paint. Any tree hardy in zone 5 or colder, and in plastic training pots, I would just set on the ground for the winter. Normal form of Chinese elm, does just fine set on ground. Most winters we get a couple nights between -12 to -17 F. All my trees pull through including JWP, bur oak, Amelanchier, shimpaku, mugo, Scots pine, hornbeam, ostrya, and other hardies.

I do bring into my well house Satsuki, and princess persimmon and other less hardy trees. These are all zone 6 or 7 or 8.

 

[newby]

I have raised vegetable beds, empty now gardening season has finished. I decided to dig my pots into theses beds for the winter, and will make an enclosure with burlap to protect them from dessicating wind. We have had 6 inches of snow on unfrozen ground right now so things are well mulched! It will melt next week, so I'll do a proper job with chopped straw then.
Last year I left my trees in an unheated greenhouse with door and window cracked, lost several trees- I hope this year will be better.

 

[Deep Sea Diver]

Protecting a bonsai’s roots (depending on the species)will go a good way towards preventing trunk split, frost cracks, sun scalding or southwest injury. Yet if a potted tree’s roots freeze and and the tree is exposed to prevailing winds it’s Katie bar the door. This is also covered here. Despite its billing as azalea focus, the physiology and methods shown are the same for most every tree.

Plastic sheeting on cold frames serves a purpose, to provide wind block and moderate temperature changes, given good venting. Very few structures in a freezing winter environment, unless heated (which brings its own challenges.) or underground will prevent freezing. But likely can prevent freeze thaw damage in the fall and spring bridge times, given the trees are properly maintained.

@Leo in N E Illinois makes a good point about knowing what his trees can survive.

I’m a bit more conservative, having seen a number of bonsai survive, but be damaged, drop branches or slow to develop as spring unfolds. So we use multiple storage methods and yard locations for different tree species, from huddling & mulching trees in the ground in discrete areas around the property to protecting very young trees in cold greenhouses. Similar to what @Leo in N E Illinois is doing.

In your situation it might be wise to experiment a bit. I particularly like your example shown in another thread with your trees mulched in against a fence under bushes.

Cheers
DSD sends

 

[Colorado]

Out here on the front range of the Rocky Mountains, trying to prevent freezing roots outdoors is a futile exercise. It doesn’t matter if the pot is mulched in or not, bonsai kept outside are going to freeze during the winter when it’s 0F outside. And that’s fine. I’ve overwintered numerous species that have frozen solid for days at a time to no ill effect.

There is a lack of discussion about species. Obviously a ponderosa is much more cold hardy than an azalea. But to suggest a blanket statement that freezing roots kill the tree is not accurate. Some species can handle it, some can’t.

Personally, I don’t do all that much winter protection for native species and other zone 5 or less species. I put them on the ground, huddled together a bit. Location is between a fence and my garage, so there’s good protection from the wind. North side, so very little direct sun during the winter.

More sensitive species and shohin I put in an unheated garage when temps are forecasted below about 25 or so. I do move those in and out throughout the winter.

 

[Paradox]

Its pretty simple, I do it because some of my trees could die if I dont.

We can and do get some pretty serious cold snaps here but we dont reliably get enough snow to bury and insulate trees.
So I have the cold frame as my reliable winter protection for those of my trees that need it

 

[newby]

Out here on the front range of the Rocky Mountains, trying to prevent freezing roots outdoors is a futile exercise. It doesn’t matter if the pot is mulched in or not, bonsai kept outside are going to freeze during the winter when it’s 0F outside. And that’s fine. I’ve overwintered numerous species that have frozen solid for days at a time to no ill effect.

There is a lack of discussion about species. Obviously a ponderosa is much more cold hardy than an azalea. But to suggest a blanket statement that freezing roots kill the tree is not accurate. Some species can handle it, some can’t.

Personally, I don’t do all that much winter protection for native species and other zone 5 or less species. I put them on the ground, huddled together a bit. Location is between a fence and my garage, so there’s good protection from the wind. North side, so very little direct sun during the winter.

More sensitive species and shohin I put in an unheated garage when temps are forecasted below about 25 or so. I do move those in and out throughout the winter.

No 'sensitive' species outside, just first year air layered trees and seedlings , all very hardy, and some collected trees also very tough

 

[JudyB]

Honestly I love all my trees too much to risk outdoor storage. A purpose built storage for them is essential to the way I feel about bonsai. It allows me to enjoy them in the leafless phase, to get some work done on them and not worry about the weather. Stress isn’t part of my bonsai winter life. I have species that require protection, but some that would not need it. but they all go into the cold house and enjoy the easy life!

 

[Ngidm]

I have maples, spruce, mugo, mulberry, buckthorn (yes, I know, terrible for bonsai, but I have a REALLY pretty trunk that will not ever be anything more), river birch, boxwood, and a few others. Have a raised bed garden that is made of 2x12's -- it is 12 foot square, with two 12'x4' rows of dirt, so there is a 4' row in between the two. It is normally 1/4 full of cedar chips. I scrape the chips away (so that I'm 12" down from the top of the bed), stick my trees in the middle shoulder to shoulder, and then backfill with the chips. Usually I add another bag or two of chips for good measure. Buries the pots and a little of the trunk (depending on the pot size). I use a double layer of floating row cover (frost fabric?) over the top and staple it to the bed wood. Zone 5. I have not lost a tree yet. I have some weak Japanese maple airlayers that I'm not convinced will survive this winter, but if they don't make it I think that's not an error in the winter protection -- I think I started them too late.

I like the idea of having water around the roots from the wood chips, which keep heat in and give some air circulation at the same thime. The cloth buffers a little against temperature swings for the branches, but also lets light in -- which I've heard is important for even deciduous trees.

Seems like a good balance of protection and sun/rain/air to me.


N

 

[Carol 83]

I have WAY to many tropicals and they have to be inside under lights for the winter here. It is a pain bringing them in and setting up the lights but it gives me something to do during the winter and I get to see some flowers. My satsuki's, crapes, beautyberry's ,Nippon daisies and one little JBP seedling go in my detached, unheated garage.

 

[Scorpius]

I keep mine in a heated and cooled greenhouse. I care about them too much to chance the winters in my area. I would be heartbroken to lose them.

 

[JudyB]

I keep mine in a heated and cooled greenhouse. I care about them too much to chance the winters in my area. I would be heartbroken to lose them.

Same! Do you have a good humidifier? I found one that's amazing a couple years ago...

I'm looking at my remote thermometers while having morning tea, the outside is reading 12. Greenhouse air is 32 with 68% humidity and probe is at 37. Pretty happy sight.

 

[IzzyG]

I had gone back and forth with both local practitioners and other more experienced professionals about over wintering and I’ve -personally concluded- that there’s a few main factors that dictate winter protection decisions.

1: Ability to minimize temperature fluctuations while staying between sub 40F but above X temperature. X is dictated by the other factors below. Wind protection is also a major factor.

2: Species and state of tree(s). Pretty explanatory and I don’t see much decision variation between practitioners. Cold hardy species/healthy/larger/more developed trees are given less protection vs the more sensitive species/recent work/smaller/younger trees. When protection is given exactly, is dependent on the next two factors.

3: Risk tolerance. There’s practitioners that will keep their appropriate trees(see point 2) out all the way down to 0F or even lower with no ill effects while some will keep it above freezing but below 40F. Even within the professional group, there’s a very obvious difference despite other factors being similar. Speaking with Bjorn, he leaves his (appropriate) trees out all the way down to low 20s and only worry if it goes sub 20F(which Nashville don’t often experience). Todd on the other hand, leaves his appropriate trees outdoors, on the ground through winter(Denver, CO) and there’s definitely days of single digits experienced. Ryan seems to be the most conservative. If you follow Mirai’s social, they’ve started moving some trees into winter protection a couple weeks back(most were the more cold sensitive ones but there was definitely some of his more valuable conifers moved in as well) and their lows was still only in the 30s. The recommendation we had discussed was also along the lines of “protected with temperature maintained between 34-38F”.

4: Availability of resources. I’ve definitely seen practitioners who are early to protect their trees if they have an easily ready/available resource to do so.

Personally I decided to take on a hybrid of all of the advice I’ve gathered. Early to protect the recently worked on/smaller trees while leaving the larger/older/healthier conifers out to experienced 20s through Fall but moving them into protection once teens were forecasted. All of them got moved into the climate controlled garage yesterday(we are expected to see a few nights of low teens here and winds are also expected to pick up). Admittedly, there were a few more valuable trees that I moved when low 20s started becoming more common.

 

[yashu]

Out here on the front range of the Rocky Mountains, trying to prevent freezing roots outdoors is a futile exercise. It doesn’t matter if the pot is mulched in or not, bonsai kept outside are going to freeze during the winter when it’s 0F outside. And that’s fine. I’ve overwintered numerous species that have frozen solid for days at a time to no ill effect.

There is a lack of discussion about species. Obviously a ponderosa is much more cold hardy than an azalea. But to suggest a blanket statement that freezing roots kill the tree is not accurate. Some species can handle it, some can’t.

Personally, I don’t do all that much winter protection for native species and other zone 5 or less species. I put them on the ground, huddled together a bit. Location is between a fence and my garage, so there’s good protection from the wind. North side, so very little direct sun during the winter.

More sensitive species and shohin I put in an unheated garage when temps are forecasted below about 25 or so. I do move those in and out throughout the winter.

My experience is similar and I use a similar method. I’m zone 5/4 and just to give an idea the frost line in Maine is 74”. If your trees are outside, the roots will freeze and as @Colorado says, no amount of healing in or cold frame protection will prevent roots from freezing. Also similarly, I have found that my zone appropriate pines and junipers are not bothered by this and experience this in the wild anyway. Those trees stay outside against the foundation sheltered mostly just from wind as that definitely can be detrimental to limbs and buds. Wild trees don’t much care about some dropped twigs but for a tree that you’re working on that can be a lot of work lost.

*Just make sure there is moisture available in the soil when the temps rise and replace it when the soil begins to dry.

My azaleas, cherry, shohin and JBP go into my garage (insulated) where the temp is regulated to not dip below freezing as all of my well and water filtration equipment is in there. The azaleas get supplemental lighting in there as well. I have a Modine heater that kicks on at 32°F but it rarely comes on due to a gas boiler (my water heater) that sheds enough heat while running to keep temps in there right around 34° all winter. Due to the dry nature of my garage and the fact that the cars are in and out daily there is enough air circulation that I’ve never had to deal with overwinter pests or fungal issues with the trees stored in there.

My acers, temperate elms and other deciduous go into the stairwell that goes to the garage attic unless there is an extended period below 0° forecast. The stairwell and attic are uninsulated so the temps in the stairwell will usually be just a bit above outside air temp as there is a bit of heat that comes through the door and bumps the temp up a couple degrees. We occasionally will get a snap of around -20° for a couple days to a week and when that happens I’ll move these trees into the garage itself just to make sure the plants don’t desiccate. The air gets REALLY dry when it gets that cold.

So yep, there are trees all over the damn place as my better half likes to remind me and I do some shuffling when we get the occasional deep freeze but that all keeps me interacting with my trees through the cold season which is fine by me.

 

[Deep Sea Diver]

Personally I decided to take on a hybrid of all of the advice I’ve gathered. Early to protect the recently worked on/smaller trees while leaving the larger/older/healthier conifers out to experienced 20s through Fall but moving them into protection once teens were forecasted. All of them got moved into the climate controlled garage yesterday(we are expected to see a few nights of low teens here and winds are also expected to pick up). Admittedly, there were a few more valuable trees that I moved when low 20s started becoming more common.

Decent summary, although I got somewhat lost in #1 and 2 each of which are complex subjects. However it seems you are pretty much on track.

Methods: Comparing professionals is interesting, yet may not be of much value unless one knows the species kept, age and work/goals for each tree and these and their microclimate matches one’s own.

Most professionals generally work with large specimens more aged to get the best return on their money. In contrast many bonsai hobbyists have a plethora of specimens, generally much smaller and younger.

I believe over the years Ryan Neal has become more and more a member of the, “I’d rather be safe then sorry camp“ vs the “My trees survived the winter“ camp. Money talks.

For an example of a place that is perhaps the gold standard on wintering over trees safely, I do know at PAC Bonsai Museum the trees in display are in open front boxes, that are closed/heated during the winter if temperatures are forecast to dip below freezing for longer then overnight, or if drastic dips below freezing are forecast. All the rest of the trees are kept in inside storage… cold or hothouse. The Curator was the assistant at the National, wonder it they do the same there? Anyone privy to this information?

Timing: In our case, everything gets an appropriate winter home pretty much at the same time, mainly as we don’t like running forth hither and yon with trees in freezing weather. Things are more susceptible to damage, being brittle and freezing hands and ground make for accidents. Been there, done that!

cheers
DSD sends

 

[kale]

I'm in zone 5b. I've lived in the same house for over 42 years. It needs paint. Any tree hardy in zone 5 or colder, and in plastic training pots, I would just set on the ground for the winter. Normal form of Chinese elm, does just fine set on ground. Most winters we get a couple nights between -12 to -17 F. All my trees pull through including JWP, bur oak, Amelanchier, shimpaku, mugo, Scots pine, hornbeam, ostrya, and other hardies.

I do bring into my well house Satsuki, and princess persimmon and other less hardy trees. These are all zone 6 or 7 or 8.

Im also in zone 5 and been contemplating taking my european hornbeam, zelkova, and a crabapple out of their wooden growboxes and put them in the ground. Seems pretty obvious! They get winter protection since theyre in growboxes but you’ve got me thinking thats not even necessary.

 

[Colorado]

Of course, all of us would love to have the resources to utilize the “Gold Standard.” For most of us, that is not an option due to financial and/or space constraints.

If I had a greenhouse like Mirai, I’d put most/all of my trees in there!

 

[Cmd5235]

Just have to mulch them in at this point. Probably going to use chopped leaves from the yard. I have one additional bed to move down tomorrow

 

[Lorax7]

I put my cold hardy trees on the ground all huddled together along the north side of the garage. Putting them on the ground places them in thermal contact with the earth, providing great temperature stability. Keeping them on the north side of the garage provides some wind protection and, importantly, avoids the freeze-thaw-freeze cycling that can happen in early spring when there’s a sunny day but nighttime temperatures remain below freezing.

The most sensitive of the trees that I keep outside stay closest to the wall of the garage so they receive the maximum protection. These include a shore pine, azaleas, and trees in small containers. I’m also conducting an experiment with some coastal redwood seedlings that I grew from seed this year that I am keeping outdoors. They were grown from seed collected in a high elevation area and are supposed to have better genetics with respect to cold tolerance. So, the experiment is to see how they fare in a Michigan winter. The most cold hardy species form the outer phalanx of the winter storage area (Amur maples, ponderosa pine, junipers, Korean x Japanese maple hybrid).

Tropicals go indoors under lights since they would not survive outdoors. My tiny azalea stayed outside for most of the fall but was brought inside before we got down to freezing temperatures. Prior to this summer, the azalea had only ever known indoor growing conditions (it belonged to Jack Wikle before I bought it). It comes indoors because the volume of the container is way too small to have any thermal stability whatsoever, its watering must be watched carefully to ensure it doesn’t dry out too much, and I don’t know exactly what cultivar it is, so I have to assume it’s not cold hardy to be on the safe side. I also have a giant sequoia and a coastal redwood that come indoors under lights. They’ve spent the whole winter outdoors before and survived but did not do well under those conditions. The sequoia would get a little bit of die-back on the branches, presumably from desiccation. The coastal redwood lost most of its branches the one year that it was left outside. So, they come inside now.

 

[SC1989]

My fenced-in area : trees are nut to butt and lightly mulched with leaves anything on the outside of the fence to break wind. Pallets, empty bins and cinder blocks. Plenty of oak leaves surrounding and piled a foot or two around. All sides are already surrounded by snow via shovel- high enough to break wind, but not let rabbits over the top when we get lots of snow. I usually just bury everything in the snow. Have used the garage , but buried in snow is best. JBP and japanese maple pulled through zone 4 . I have had no issues completely covering everything with snow