Root kill temps

[andrewiles]

In Bonsai Heresy Michael Hagedorn points out that many trees have roots that are less hardy than their tops, which in bonsai pots becomes an actual problem. He mentions the original list from OSU is not published but I see https://agsci.oregonstate.edu/sites...-greenhouse-and-christmas-trees/onn120509.pdf online, which is pretty old but seems to cover a similar set of trees.

I thought folks who don't have the book (recommended!) might be interested. Some of the kill temps are surprisingly high.

Most of my starter trees aren't in the list though, and since we're heading for a deep freeze here in Seattle I thought I'd check if anyone has additional thoughts on them. Yeah, 20 F is a deep freeze for us.

Curious about root kill temps for: bald cypress, dawn redwood, Chinese elm and zelkova to name a few. Maybe a temp list could go into the resources section?

 

[Cadillactaste]

Right...a tree in a pot doesn't get the insulation heat from the earths core.

I've heard many say Florida Bald Cypress aren't as cold hard as those from Louisiana. Mine came from Florida.

We have negative teens coming in next week here. I'm so relieved my trees winter in a controlled cold greenhouse environment where I just need a secondary heater to keep them in a temp I choose. I have a neagari satsuki less cold hardy. So nothing goes under 32F.

 

[Zach Smith]

Good list. I don't know the root kill temps for BC or Chinese elm but can relate that both have survived 15F on my bench (a not intentional experiment). I just posted a blog on winter protection at my website, considering what may be headed our way. Couple of species also not shown on that list: American beech didn't survive 15F, surprisingly (I just noticed the list has F. sylvatica at 15F!); Crape myrtle did. I lost a privet (L. sinense), though vulgare can evidently survive to 5F.

 

[GGB]

I can say the same about chinese elm. They have survived similar or lower temps for me on the ground in a bonsai pot. It's hard to make assumptions because zone hardiness in a typical situation doesn't always relate to the roots in a pot. It's funny how that works.

 

[Vali]

Chinese elms can take lower temperatures than -15C/5F. This is a small one from my collection pushing roots in winter

 

[Leo in N E Illinois]

I'm in zone 5b, and I am a little skeptical regarding the list from Hagedorn. I have trees in the back yard that have survived colder than the limits listed by Hagedorn. BUT, I also have lost species that Hagedorn's list do suggest were not as root hardy as I had thought. For example, I found that Ginkgo, in pots, can not take a 5b (Chicago area) winter. Healed into the ground, they are fine, growing in the ground they are fine, but on the ground, in a pot, the roots just don't seem to make it. They start to leaf out in spring, then just die, before the leaves fully expand. Sign of root death.

 

[Forsoothe!]

The ground changes in temperature very slowly whereas the air temp can turn on a dime at sunset. It takes days at 20°F for the ground to get there, 4 inches deep, and under a 6 inch mulch it could take weeks because mulch has a lot of air spaces that inhibit heat transfer at the same time that heat is still radiating upward from below and daytime sunlight still coveys some heat into the surface. There is a general rule of thumb that the fleshier roots are, the higher the damaging temp point is and conversely, the more fibrous roots contain less moisture and therefore have a lower temp damage point. I expect there are exceptions to everything.

 

[HorseloverFat]

Also we tend to forget to take into consideration the amount of temperature protection by ANY amount of “Up against the wall” or “under tables” or “grouped together in a windblocked corner”..as these rates of protection raise ,somewhat substantially, your temperatures...ESPECIALLY, surprisingly, when external air temps dip below freezing.. nice lil’ “buffers”. One could go around with a thermometer (meat thermometer is even better) to get a feel for the temperatures that YOUR winter plant area tends to “keep” the substrate around.