USDA zone 2 overwintering advise

[trying-in-canada]

Greetings,

First time posting here, excuse my format. I am new-ish to Bonsai, did quite a bit of research on YouTube and Bonsai-U and Mirai Live among other websites. I live in Saskatchewan Canada, where the hardiness zone is around 2 USDA. The temperature plummets in November through March; it is pretty much permafrost during those months. And in late November through early March, temperature at night could reach -45C (-49F) without windchill; on some days we have temperature colder than the Arctic.

For trees that requires winter dormancy, I currently have a
1. Japanese maple (USDA 4),
2. common 'Green Velvet' boxwood (USDA 4),
3. Nanking cherry (USDA 2),
4. small Juniperus procumbens 'Nana' (dwarf Japanese garden juniper USDA 4).
I am planning on taking out the 8'x8' sand box in my yard that receive full sun and putting in around 12 inches of top soil, and moving all of the trees there, so they could get full sun next growing season.

I am building a cold frame with south-facing windows, I will add approximately 1.5 - 2 inches (R7.5 - R10) of spray foam insulation (around the sides). I will mulch around all the trees prior to winter, and putting the cold frame over them with the trees planted in the ground.

With the trees being in the ground, the mulch, the insulated cold frame, 3.5mil plastic cover over top of cold frame, and however much snow is around and on top of the cold frame.

1. Would you recommend getting supplemental heat source for inside the cold frame to keep the soil temperature slightly higher? For example, I am thinking heat wires to put in the soil, or heat mats on top of soil, perhaps heat lamp or incandescent bulb? With the lamp I am afraid that the tree might wake up early while the roots are still frozen, and the supplemental heat will only be turned on when the temperature is below -25C or -30C (-13F or -22F).
2. Should I leave the windows clear of snow for the sun's heat? I have read that on hot days you might need a black tarp over the windows to keep the temperature low to avoid waking up the trees.
3. Any other advise/suggestion to keep my tree safe in zone 2 rigid winter?

Much thanks in advance.

 

[hemmy]

Bonsai Heresy by Michael Hagedorn has a table of root and top hardiness temps based on an unpublished list from university research and used by a large nursery.

Japanese Maple and Boxwood are listed at 15F root hardiness and -20F top.

You might consider digging your cold frame lower to take advantage of slightly warmer temps. Most traditional bonsai species should probably stay above 20F at the root zone. But you also want them to stay frozen and not go through multiple freeze/thaw cycles.

I think you will want to tarp the cold frame to keep the sun from warming it too much. During our brief periods of “Artic” blasts and subzero F temps, I cover my cold frames with blankets and black plastic.

My cold frames are a wooden framed enclosure with R10 sheet insulation and a hinged top. The other is a stacked concrete block enclosure with wood mulch in the cavities and lined with R10 sheets and a R10 removable roof. Both against the north side of the house. Probably overkill for zone 6, but better safe than sorry.

 

[hemmy]

I enjoyed the Up North Bonsai podcast on iTunes. One of the few to talk about bonsai life in colder climates.

You should also reach out to any local or regional bonsai club for wintering advice.

 

[Potawatomi13]

Personally would suggest Temp control to maintain air temp 28 degrees F at a minimum. Roots might be a bit cooler than air temp but with decent ground insulation should not be much.

 

[trying-in-canada]

Thank you Hemmy and Potawatomi, I think I will put a small space heater in the cold frame and control it with a thermostat to avoid air temperature below -20F. I think with the insulation, the mulch and the radiant heat from the ground and the heater, I should be able to keep the root relatively warm. The trees I have are not expensive and we will learn from our mistake.

I do have one more question, once they experience a few frosts, I will begins heating them to a relatively consistent temperature, but at what temperature will they start waking up from the winter dormancy? I want to keep an eye out for that temperature.

-Hemmy, I will check out that podcast you mentioned, thanks. And unfortunately, there are no bonsai club within 8 hours drive from my area.

 

[Carol 83]

Maybe @Canada Bonsai can offer some advice about growing in Canada.

 

[hemmy]

do have one more question, once they experience a few frosts, I will begins heating them to a relatively consistent temperature, but at what temperature will they start waking up from the winter dormancy? I want to keep an eye out for that temperature

To be clear, I would let them freeze solid and stay that way until they thaw in the Spring. If you have consistent snow packs, you can cover the entire cold frames and I bet you would keep the root zones above that potentially killing temp of 20-25F. Most trees need temps below 30/40/50F for a certain number of cumulative hours over the course of winter before warmer temps bring them out of dormancy. I’m sure the temps to break dormancy are different for different species.

I would just try and keep everything consistently frozen, but out of the drying wind and not in a spot where the sun is going to warm your enclosure above freezing.

 

[Potawatomi13]

Thank you Hemmy and Potawatomi, I think I will put a small space heater in the cold frame and control it with a thermostat to avoid air temperature below -20F. I think with the insulation, the mulch and the radiant heat from the ground and the heater, I should be able to keep the root relatively warm. The trees I have are not expensive and we will learn from our mistake.

I do have one more question, once they experience a few frosts, I will begins heating them to a relatively consistent temperature, but at what temperature will they start waking up from the winter dormancy? I want to keep an eye out for that temperature.

-Hemmy, I will check out that podcast you mentioned, thanks. And unfortunately, there are no bonsai club within 8 hours drive from my area.

I have a question whether or not there is permafrost down below making ambient ground temp colder rather than warmer?

 

[Canada Bonsai]

1. Would you recommend getting supplemental heat source for inside the cold frame to keep the soil temperature slightly higher? For example, I am thinking heat wires to put in the soil, or heat mats on top of soil, perhaps heat lamp or incandescent bulb? With the lamp I am afraid that the tree might wake up early while the roots are still frozen, and the supplemental heat will only be turned on when the temperature is below -25C or -30C (-13F or -22F).
2. Should I leave the windows clear of snow for the sun's heat? I have read that on hot days you might need a black tarp over the windows to keep the temperature low to avoid waking up the trees.
3. Any other advise/suggestion to keep my tree safe in zone 2 rigid winter?

The tree don't need light during winter (the coldest parts of the winter), so feel free to close your shelter off completely with insulation. However, you will want the ability to start letting light in as the plants wake up in spring - when you start doing this, things can overheat surprisingly quick and ventilation becomes key not only for this, but also to prevent disease. Also make sure you can get in during winter to water. A thermostat that will automatically turn on a small heater can be a life safer.

Check out this document:

https://cdn.shopify.com/s/files/1/0283/4877/6541/files/Canada_Bonsai_Winters_in_Canada_4a599b94-e14a-498b-80d4-b7ff69d95b45.pdf?v=1603896022

And I have a list of bonsai clubs here. There are non in Saskatchewan, but other provinces might be able to help:

Resources

Below are a list of resources you might find helpfulCanadian Bonsai Societies, Groups, Nurseries and Shops Educational Recommendations Winters in Canada Seed Stratification Japanese Maple Propagation Canada Bonsai Propagation ScheduleThis schedule should be used as no more than a rough estimate...

www.canadabonsai.com

 

[hemmy]

I have a question whether or not there is permafrost down below making ambient ground temp colder rather than warmer?

Yikes! I hadn’t considered that. Hopefully, they live south of the permafrost line. But they might have frost depths that reach several feet deep. However, the ground temps should still be warmer than average air temps in the winter and moderate temp swings.

 

[ShimpakuBonsai]

I remember some videos of Tane Bonsai using a "muro" for winter protection.
These are made under the benches and can be closed during winter.

Muro​

Chamber. It refers to facilities to protect bonsai trees against damage from frost, freezing or snow in the winter season. A prefabricated house, plastic greenhouse or any other closed chamber is fine for such ”muro” facilities. But attention must be paid to maintain proper indoor temperatures in such facilities. The suitable indoor temperature is said to be around 15 C (or 59 F). The indoor temperature should be adjusted through ventilation. It is important to put a bonsai tree into a ”muro” chamber as late as possible and take it out as early as possible. A bonsai tree should be managed in a natural environment as much as possible. Excessive protection should be avoided.

Source: https://bonsai.shikoku-np.co.jp/en/word/2009/02/3751/

 

[MaciekA]

Someone above mentioned root kill temperatures (RKT) from Hagedorn's Bonsai Heresy book. That book is absolute gold and you should still buy it, but you can also get a table of RKTs from this Oregon State U paper: https://agsci.oregonstate.edu/sites...-greenhouse-and-christmas-trees/onn120509.pdf

Not a ton of lab time has been dedicated to this topic so you may have to extrapolate a bit based upon plant genera/families or common geographic areas to deduce the RKT for a species not listed. If you're growing Japanese Maples, that cold frame along with whatever other solutions you use (say, warming a bed of pumice up with some warming wires) will have to keep the roots above -10C, as an example. Here in Oregon one can still achieve that with a maple sitting outdoors on the ground even if it's -12C for several days, so it's not a clean mapping to ambient (i.e. cold frame interior could still reach -12C for days and not necessarily kill maple roots), but it gives you a sense of where to concentrate efforts (the floor of the cold frame basically).

 

[trying-in-canada]

Thank you all for commenting.

-Potawatomi, I guess "permafrost" is a bad choice of word, in the depth of winter, frost line is about 10ft deep.

-Canada Bonsai, the cold frame will have south-facing window that I can lift up, so watering and ventilation wouldn't be too much of a problem in the spring. I read through your guide on over-wintering in Canada, I will follow your advise and put a heater in the coldframe on a thermostat. I will however put it at 2C for power saving and because The thermostat will be on the other side of the cold frame from the heater. Don't want to overheat one side haha.

-ShimpakuBonsai, I am not sure how a muro is different than a cold frame set up to be honest, it seem pretty similar to a cold frame from what I have read.

​

-MaciekA, I will follow up with my method and the result in the spring, I don't think many people grow bonsai/pre-bonsai in zone 2, so maybe if others stumble upon this post they may learn from my success/mistake.