Overwintering out of the sun?

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So, I keep seeing folks say to overwinter trees out of the sun and wind. The wind part makes sense, and so does the sun part, at least to the extent that we want to keep trees dormant during the winter, and the sun, in theory, risks creating too great a temperature differential--or does it? I ask because I see places like Eisei-en (in Nashville!) overwinter many trees on benches in full sun, the very same benches used in the summer. Yet, elsewhere, I see folks from way up north, near the Canadian border, strongly advising to keep trees out the sun, and these are places where the average daily winter temperatures during the day are much, much colder.

Is there something I'm missing? I know the day/night temp difference can play a role, but, if anything it would seem that a place like Nashville has a greater day/night differential, and, thus, a greater incentive to keep trees in the shade, and yet the opposite happens there.
 

A. Gorilla

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This seems too full of assumptions to answer as phrased.
 

Forsoothe!

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OP is right, some do and some don't. Everyone has very good, solid scientific reasons why their way is better, which says something about words. Words mean things, except in bonsai and and every other human endeavor, wherein they mean things and we stretch meanings to fit. @Walter Pall overwinters on benches where the snow get as high as an elephant's eye, and I see pictures of @William N. Valavanis trees snow-covered on benches, too, in upstate NY, but most of us put them on the ground and mulch them with leaves. Some do it the shade, but I do it in the sun so I don't have to re-introduce full sun plants to direct light and fry the new foliage in spring. Everything has full sun, including Japanese Maples, until June 1st when the sun becomes too intense. All my trees are perfect :rolleyes: so I'm right and everything that anyone else says is baloney. That should set the stage...


(Next, ask about soils!)
 

Scorpius

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My winters can be very mild or brutal. Some years we get lows in the -20 Fahrenheit range and other years it hardly snows and it never gets below 20 Fahrenheit. I overwinter all my stuff in a greenhouse that gets light and I've also wintered my tress in the garage with very little light and all my conifers and deciduous trees do fine in both.
 

Bonsai Nut

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This risk in winter is mostly drying out.
^^^^^
This. I'm not sure that light or dark matters so much as long as you can keep your trees from drying.

In Chicago I overwintered my cold hardy trees in an unheated garage without light. Here in NC, I overwinter them outside on benches. The main difference is that many days in NC it doesn't even get to freezing, and the air in the winter is much more humid here. Between the rain and the humidity, last winter I'm not sure I touched my trees outside from late December until late February. Early March I brought my tropicals outside and that was winter.
 
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rockm

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You have a lot of underlying assumptions here that are confusing you. Zone, species and local conditions are critical in overwintering.

For instance, leaving trees on benches in Tennessee USDA zone 7a is not an example to follow in Indiana (which encompasses zone 5 and 6). Conditions in your area are colder for longer than in Tennessee. What works there may not work where you are.

Additionally, tree species is a critical part of this. Western conifers, ponderorsa pine, Rocky Mountain Juniper, etc. are completely winter hardy in Zone 7. They require no winter protection there I've had ponderosa pine for years here in Va. Zone 7. I leave them on the benches through the winter. Protection for them here is counterproductive since they are hardy to much lower temperatures and storage can weaken them. Deciduous trees are more vulnerable to overwintering errors--once their buds show leaf margins and open up, they can be killed by freezes.

Keeping overwintering trees out of the sun is a good practice here in the East, given the sometimes roller coaster temperatures and unpredictable snow cover. Sun in late Jan. and Feb. and into March can heat up the pots--even under significant mulch or other cover on the rootmass. That rising heat can spur growth, which is particularly dangerous for deciduous trees. New growth on maples in early February means the root masses have lost 95 percent of their winter hardiness. Once trees begin growing, they go all in on growth and largely abandon their abilities to withstand below freezing temps. In other words, if you have leaves emerging, and you get a hard freeze, the tree can be killed.

Winter storage is not really about keeping trees "warm." It is about keeping them as cold as possible for as long as possible into the spring. Shade helps that. Sun doesn't.

Also something to keep in mind--southwest disease...https://www.uaex.edu/yard-garden/resource-library/plant-week/Sunscald-southwest-injury-07-08-2016.aspx
 

rockm

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FWIW, I overwinter most of my trees outdoors under heavy mulch out of the sun. Here's pic of some of them after a blizzard a few years ago. Cold and snow can be your friend.
 

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penumbra

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Simple answer is that there is no right answer. We can all reply as to how we winter our trees and we can state our success and our failure. But even then it boils down to a personal choice which is based upon both the science espoused by some bonsai keepers, the experience of most, and the gut feelings (often called pseudo science by some, but I consider it para science) of others.
All my temperate zoned plants stay outside in a manor of speaking, as some are under hoops or in cold frames (both use white plastic), some are simply mulched in and some left upon the bench. BTW, my cold frames and hoops are used for very young plants just rooted, or plants like satsuki that have a real tendency to dry in winter. They are all in natural light because it feels natural to me. The only place I attempt to limit the light is in the placement of cold frames and for my young Japanese Maples which tend to leaf out often before last damaging frost.
I know a lot of people, particularly my northern neighbors, keep plants with great success in garages, basements and so forth. For me, my basement is for all my tropicals.
 
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This seems too full of assumptions to answer as phrased.
My apologies, Mr. Gorilla! I know it's difficult for your species to understand, but we humans have trouble formulating our inquiries (especially in informal settings, such as Internet forums) with perfect scientific rigor. I promise to try harder in the future. When I grow up, I hope to represent my species as a dignitary before your Supreme Leader. Obviously, I have a lot of learning to do before that happens!
 
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Thanks, all! A lot of info to unpack here. I didn't quite realize this question was such a hot topic. I genuinely just found it strange that some do, some don't overwinter in sun and figured it best to ask! Glad I did.
 

A. Gorilla

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My apologies, Mr. Gorilla! I know it's difficult for your species to understand, but we humans have trouble formulating our inquiries (especially in informal settings, such as Internet forums) with perfect scientific rigor. I promise to try harder in the future. When I grow up, I hope to represent my species as a dignitary before your Supreme Leader. Obviously, I have a lot of learning to do before that happens!
Monke strong together.

We will divide and conquer. When you are isolated and scared, we rope you and keep you as chattel slaves.

Day of monke soon!
 

A. Gorilla

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My area can be 50F at Xmas or in the teens.

February could have a thaw to 40F or have daytime highs of -15F.

Snow is not reliable.

I never considered day vs night as a big issue unless it was spring where new growth might be nuked by a very hard and very late frost.

Its about keeping things that can deal with the cold and keeping things stable from December to April, not so much from day to night.

Bjorn obviously knows nashville. I do not.

The big chicago area bonsai nursery just says screw it, and keeps it all inside at 35F from December to April. They are taking no chances with all yamadori. They know Chicago.

FWIW
 
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rockm

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Thanks, all! A lot of info to unpack here. I didn't quite realize this question was such a hot topic. I genuinely just found it strange that some do, some don't overwinter in sun and figured it best to ask! Glad I did.
It is really not a "hot" topic. Overwintering is all about protecting the root mass of your trees and knowing the capacity of individual species in cold weather.

This is pretty much the most on-target video I've seen about overwintering. The temperature limits in it, as well as the duration of the cold and size of the bonsai container are the keys:
 
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Deep Sea Diver

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Thanks @rockm ! That video is a really good primer for folks looking for general guidelines. As mentioned earlier each species are a bit different in response to freezing. I’m constantly experimenting on azaleas in this regard.

Satsuki azalea as well as most other Evergreen azaleas are especially prone to bark split and sun scald especially during extended freezing cold snaps in winter. Or a shorter very sharp drop in temperature with extended high winds and bright sunshine…. Such as happens during/after a cold front passage. I have also seen this phenomena affect other thin barked species too, like Stewardia.

Basically these two phenomena are thought to be caused by the roots being frozen so water cant be uptakes by the tree. Then wind and/or bright sun hits the trunk of the trees. The roots are able to uptake water through the roots to replace the water lost due to drying from the wind or sun. At that point the side exposed to the wind or sun can become damaged with splits, cracks or scalded spots. Often the extent of the damage doesn’t become evident until late winter or early spring when the trees become active. Then one sees a dead branch, a trunk split or scald marks. Trunk splits can especially affect a tree in a bad way.

(@rockm ’s resource at this link accurately describes this phenomena) Note it calls trunk split as tree blast and mentions young trees as susceptible)

There are simple measures one can take to protect these trees. A hoop house, like @penumbra , other cold frame, cold greenhouse, placing the trees on the ground against a structure, while mulching them in with sifted bark nuggets (including above and below the pot - below to assure drainage) and building a windbreak. Watering lightly before and after these events also helps provide an edge to protect the trees.

If one does notice a split trunk, it is possible to help if it is noticed early on. Cut paste the wound and wrap with Parafilm or grafting tap…. and cross your fingers!

cheers
DSD sends
 
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rockm

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Might also help to mention that water can protect against shallow freezes, like at night. Watering trees on benches ahead of nighttime lows predicted into the low 30's high 20's can act as a kind of shield against roots freezing--the water around the root has to freeze first before the root itself. That can give enough "lag" to prevent freezes from killing roots. It's also another reason not to let trees exposed to prolonged cold to dry out...
 

Deep Sea Diver

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Good stuff!
Might also help to mention that water can protect against shallow freezes, like at night. That can give enough "lag" to prevent freezes from killing roots. It's also another reason not to let trees exposed to prolonged cold to dry out...

Yep. It takes a lot of cold to go from 1C to 0C and freeze water…. It takes 80x the energy loss to lower the temperature of water from 1C to 0C as compared to what it takes to lower the temperature of the exact same amount of water from 2C to 1C. Hence the time lag to freeze water

Consequently the water forms an insulating blanket, at first, around the roots.

Cheers
DSD sends
 

rockm

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Good stuff!


Yep. It takes a lot of cold to go from 1C to 0C and freeze water…. It takes 80x the energy loss to lower the temperature of water from 1C to 0C as compared to what it takes to lower the temperature of the exact same amount of water from 2C to 1C. Hence the time lag to freeze water

Consequently the water forms an insulating blanket, at first, around the roots.

Cheers
DSD sends
Thermodynamics, don't leave home without it! 😁
 

leatherback

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It takes 80x the energy loss to lower the temperature of water from 1C to 0C as compared to what it takes to lower the temperature of the exact same amount of water from 2C to 1C.
Almost..

It takes 80 times the amount of energy to turn water at 0c into ice compared to what it takes to lower the temperature from water 1 degree.
 

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