To what temperature are the roots of Japanese maples and Stewartia hardy.? I live in northern Virginia and I have been putting them in the ground. It occurred to me that this might not be necessary. Any thoughts?
Michael Hagedorn listed 15 degrees F (-9.4 C) for both A. palmatum and S. psuedocamilia in chapter 13 of Bonsai Heresy. He stated that the root hardiness list is based upon a combination of Oregon State and Iseli Nursery data/experience. He also found that for short term cold exposure (i.e. overnight) merely putting the pots on the ground made a several degree difference so I imagine that burying the pots would make a similar difference that would probably last longer. However, since the roots are contained and not spread very far in the ground I doubt that it would result in the getting the full top growth hardiness of -20F (-28.9C) that is listed for both species.
Thank you, Marty. I was hoping to get out some work…oh well
Also this blog by Michael Hagedorn explains his technique to overwinter in Oregon.
Seasonal Care for Cold Weather… | Michael Hagedorn (crataegus.com)
I have taken to surround the trees with stone or landscape blocks, and cover the pots with cedar wood shavings (purchased at farm outlet used for animal bedding) or particles of granite used for chicken grit (also from farm store) and protect from desiccating wind. I try to allow morning sun and ensure air flow to reduce fungus and bacteria from having a calm environment to grow. If I put them in the ground the soil is contaminated with very heavy loam that restricts drainage. Good luck.
We Bobs need to stick together!
When the snow flies, I lightly cover the enclosed trees to add another insulating blanket.
I am outside Spokane, WA which is USDA zone 6a with expected lows every few years of -5 to -10F (-20 to -23C). Most years we don’t drop below 0F, but we have gotten lower than -10F once or twice. I built a poly tunnel to protect my trees over 20 years ago that has slowly become a cold greenhouse. The current version is build from custom wood frames that are 39" x 48" (1 m x 1.2m). this year I had to recover then and I used UV resistant polyethylene with a layer of bubble wrap inside the poly for insulation. Last year I had the bubble wrap on about 2/3 of the frames stapled from the inside and it made it easy to keep the greenhouse right around freezing using a relatively inexpensive in-line digital temperature controller (the one I got can also turn on a ventilation fan to cool it) and a little 1500W box heater. I have a propane heater if it decides to get really cold. Perhaps overkill, but it is nice to be able to bring trees in to work on during the winter and take them back out without fear of damage from the work.
Thank you both. Your posts have been ext helpful and encouraging