Hardiness of Japanese and Korean Maples

ZONE HARDINESS and CLIMATE ZONES

1. Hardiness - Japanese Maples are supposed to be hardy to Zone 5, -20 F according to J.D. Vertrees. (I question that, but I do recommend the book!) It depends on many factors including the age of the plant.

2. Variety - Some Maple varieties are hardier than others. Japonicas are supposed to be cold hardy, as are Pseudosieboldianum (Korean Maple). A. Pseudosieboldianum is rated as hardy to Zone 4 (-40), but it is hard to find and has (as I know it) only one named cultivar. It is a columnar form. I only have a few of those. (The same caveat applies, ALL young plants need winter protection.)

3. Is the plant in the ground or in a container? A container grown plant may be hardy in a Zone 6 or 7 but not in a Zone 5-4. Root Stock - Since almost all named Japanese Maples are grafted, the top (depending on variety) may be hardier but the Root Stock is almost always an Acer palmatum seedling. Root Stock plants are seed grown and so hardiness of the under stock would vary. Mulching would therefore be a good idea.

4. Site conditions - Zone microclimates play distinct havoc with stated hardiness statistics. This would include slope exposure, frost drainage, fluctuation of daily temperatures. In Denver, it might get to 10 below and climb on a sunny day to 70. The shaded side stays frozen and the south facing bark will rapidly thaw. This causes splits in the trunk and often the death of the tree. This applies to the Pacific Northwest as well, explaining the need of shade cloth or protective lath.

5. Length of your cold snap, snow cover etc. The worst combination would be very cold temperatures with no snow cover. Equally of concern would be wildly varying weather conditions. One year I had three major arctic expresses with warming periods between. I was in a Zone 6 zone and it went to 7 below with no snow cover and bright sun. All the worst combinations. The Nursery was toast. So would be most potted bonsai.

6. Suggested mitigations: Mulch, provide wind protection, pile snow up against the trunk and perhaps use anti-desiccant sprays, etc All the above factors heavily as to how much damage is done by these temps. Read snow is really GOOD! The best news is that we usually do not have a prolonged period of such ultra-cold temps. This is a major factor in the survival of the trees and how much damage they may receive.. Further wet weather may yet exacerbate the winter damaged root conditions; so protect the plants from further excessive moisture until new growth has begun.

'7. Oxygen plus' might be one mitigating commercial chemical that might help recover some root-damaged plants. Having a dried out hose available to a frost free spigot is another helpful tool. Spraying the frozen trees (lightly) will do much to mitigate damaging thawing – much as they do in the citrus fields. Bring the hose back in and allow it to drain, should you need it again.

8. Always brush snow carefully off your trees as the accumulations might worsen, freeze and accumulate more. … breaking limbs.