Accounting for Differences in Cold Hardiness Advice

[DrTolhur]

As with anything, bonsai obviously carries with it plenty of varied advice as far as what works and what doesn't. And for things like soils, fertilizer, etc., I'm sure there's lots of things that work, and part of it depends on things like what water you use, how often you water, and whatnot. People can kind of try things and see what gives good enough results for their liking. And you get feedback within the season as far as how the tree responds.

But there's one thing that I can't understand why there's varied advice on, which is hardiness of trees to winter temperatures. Specifically: some sources say Japanese maple roots will die if they get below 15ºF. Then other people say they leave them outside, largely unprotected when it's around 0ºF for days on end. These seem unreconcilable. Any tree will die at some temperature, but what is it? Does a root temperature of 15ºF kill Japanese maples or not? If not, why do people say it does?

Here are some of my guesses as to why they differ so much, but I'd love to hear input if anyone else has better knowledge or specific experience (i.e. lost Japanese maples due to cold):
- Sources saying to keep above 15ºF just being overly conservative.
- Healthy trees can survive colder temperatures (below 15ºF) better than weaker trees.
- Larger trees are hardier.
- Being on the ground might give enough thermal boost to keep roots above 15ºF even when air temperatures are well below that for extended periods.
- Sometimes trees just die with no obvious cause, and someone guessed it was because of temperature.

 

[Eckhoffw]

Yep. Pretty much sums it up.
I’m still new at this, but I feel like giving trees a little pampering throughout the winter isn’t the worst thing. As long as they get the dormancy hours in.
I shoot for between 32 and 40° temps in a heated cold frame with all my less hardy trees. Sometimes newly potted trees in smaller pots may go here as well.
The others get like emperor penguins and I mulch them in with chips/leaves along the fence. A wind break is good.
To many variables to figure when wintering. Keep it simple.

 

[penumbra]

My response is to care of JM and not JM bonsai so allowances need to be made. As I have seen and experienced, the 15F is conservative but I was taught 10F on JM. If the tree is cared for and does not freeze dried, as I understand it, the fine feeder roots, particularly those from a current season, will die at those low temperatures. Come spring, a healthy JM makes new roots from the primary roots.
There seems to be no rhyme or reason to it all because I have half a dozen JM in large ceramic pots and have survived 14 below F. Some of the other cultivars were not so lucky. That winter (I think it was 5 winters back) it hit nighttime temps of minus 14F and the high the following day was below 20. This happened two nights in a row. While I was surprised by some of the things that lived, I admit that the next spring was a very slow start for many plants and there was really no seasonal gain for those plants that struggled.
So, It depends................. but ere on the side of caution.

 

[Scorpius]

Cold hardiness varies within varieties of JM as well. Coral bark varieties are less cold hardy for example.

 

[GGB]

The temperature of the air and the temperature inside of a bonsai pot won’t be exactly the same. People who set their trees on the ground are banking on the 30 some degree earth providing warmth to the pot. If the pot is large or if temperatures only dip for a short time there more chance that will work. But air temps aren’t root temps, at least not right off the bat

 

[0soyoung]

Nothing beats real data - see appendix 1.

https://agsci.oregonstate.edu/sites/agscid7/files/horticulture/osu-nursery-greenhouse-and-christmas-trees/onn090210.pdf

The paper is very helpful for sorting out fact from fancy/fiction.

 

[penumbra]

If I relied on fact alone most of my plants would be dead. I have read too many facts over the past 60 years that are contradictory to one another and to experience.

 

[sorce]

We, as humans, can't know the ability of any one tree to survive a temperature.

They are not all created equal.

That's just the variable we can't know, which should be the end all, but we are too curious for it to be.

Never mind the list of variables we don't know because they are never shared.

There's at least 5 variables for everything already mentioned in this thread.

I don't believe trees have the capability to die over winter without outside influences not including being in a pot.

Sorce

 

[Shibui]

Here are some of my guesses as to why they differ so much, but I'd love to hear input if anyone else has better knowledge or specific experience (i.e. lost Japanese maples due to cold):
- Sources saying to keep above 15ºF just being overly conservative.
- Healthy trees can survive colder temperatures (below 15ºF) better than weaker trees.
- Larger trees are hardier.
- Being on the ground might give enough thermal boost to keep roots above 15ºF even when air temperatures are well below that for extended periods.
- Sometimes trees just die with no obvious cause, and someone guessed it was because of temperature.

all of the above.

 

[rockm]

I don't know who is advising that leaving a JM out unprotected in 0 F is OK, but I'd ask to see their collection of JM...Fifteen F is the benchmark people use to measure potential damage to bonsai in containers. That temp is the threshold where INTRAcellular water freezes and damages cells themselves(think frozen soda can exploding--much the same thing in root cells with frozen intacellular water)...Some trees are more able to withstand/put off that kind of damage. Others not so much.

 

[Deep Sea Diver]

The reference @Osoyoung gave is pretty old, 41 years, yet its one of the only ones for basic benchmark kill temperatures…. given one’s trees are potted up in standard nursery mixes and not bonsai media with numerous larger interstitial spaces between particles which allow faster freezing of the roots in the core of a bonsai…

One might also recall nursery trees are stored on the ground and huddled together during winter

Here’s a bit of an interesting wisdom from this reference that has been (modified by bonsai hobbyists over time to an approximation of +10F for a tree in a bonsai pot as a starting point.)

"The actual killing temperature will vary with the degree of plant dormancy at the time of exposure to the low-temperature. When plants are preconditioned for low-temperature storage and the temperature remains uniformly low prior to exposure to cold-temperature, the roots will generally survive provided that the temperature does not go below the average killing temperature for the species. However, if the low-temperature follows a period of fluctuating temperatures, especially temperatures that are conducive to growth, the roots could be injured at temperatures significantly higher than the average killing temperature. Also, the roots of newly rooted cuttings have a tendency to be less tolerant of cold-temperature stress than plants with a year or more of growth. To minimize root injury, plants should be stored at a temperature at least 3-5°F above the average killing temperature." - Harold Davidson and Roy Mecklenburg. 1981. NURSERY MANAGE MENT - ADMINISTRATION AND CULTURE. 450 pages. Prentice-Hall, Inc.

Recent literature seems to be even more conservative as data shows root damage delays tree growth until the roots can be repaired (yet not necessarily budding).

Also data shows climate change has brought a marked increase in the amount of unseasonable dormancy breaking and sudden freeze thaw refreeze events.

Imho tree survival is not what a hobbyist ought to aim for as their goal. Instead for all trees to complete dormancy undamaged so each is robust and resilient, ready for the rigors of bonsai for an entire growing year.

Practically as a hobbyist the above translates to “Protect one’s trees properly or be prepared for a shortened, and likely less robust, growing year”

cheers
DSD sends

 

[dbonsaiw]

Personally, I'm going to try and keep things simple this winter. After almost 50 winters complaining how cold it is in NY, I finally realize that it's fairly mild here. Last winter I went bat shit crazy trying to prevent winter from reaching my trees with tents and the like (an impossible task, and counterproductive in any event). This year, I have mulched all my trees close together in a giant mulch heap (after 40 bags I lost count) - about 15' X 10' X 10" deep. I will use last year's greenhouse tent this year as nothing more than a wall to block one side from wind. Not sure anything more is needed in my neck of the woods. I tend towards the OCD side and as winter approaches there is less and less bonsai stuff to do - in other words, I may just build something to enclose the trees after all.

 

[Colorado]

Personally, I'm going to try and keep things simple this winter. After almost 50 winters complaining how cold it is in NY, I finally realize that it's fairly mild here. Last winter I went bat shit crazy trying to prevent winter from reaching my trees with tents and the like (an impossible task, and counterproductive in any event). This year, I have mulched all my trees close together in a giant mulch heap (after 40 bags I lost count) - about 15' X 10' X 10" deep. I will use last year's greenhouse tent this year as nothing more than a wall to block one side from wind. Not sure anything more is needed in my neck of the woods. I tend towards the OCD side and as winter approaches there is less and less bonsai stuff to do - in other words, I may just build something to enclose the trees after all.

Keeping it “simple” with 40 bags of mulch? We have different ideas about “Simple” Looks like a very effective setup though.