OOPs sorry I dont have a Mikawa, I have a Sango.. sorry about that...

I'm not sure I'd go to much bother :-) Provided the pots are sufficiently large, I doubt you have much to worry about in zone 7. It takes some seriously cold weather over an extended period for the soil to freeze solidly. If something like that is predicted, you might want to consider wrapping just the pots with bubble wrap. It is an excellent insulator and the roots are the only part of the plant likely to be damaged by extreme cold.

I grow virtually all my JM's in containers and never bother with any winter protection - ditto with the nursery plants. Last winter we had some abnormally cold weather - into the single digits - and none of my maples were the slightest bit affected by it.

Pam always offers such good advice, but I want to muse a little, & maybe expand a little, too.

Blanketing pots with insulative wrappings is productive when you're capturing heat within the insulation that is generated by an extraneous source - often the earth. It can be effective to insulate your pot if it is resting on the ground or garage floor, or it's partially buried, but it's not very effective if your pot is on a deck or railing ..... because there is no extraneous heat to trap.

If you were trying to keep a marginally hardy plant on a balcony, I would tell you to build a box around the plant, but leave one side open and that side goes against the building. Then insulate the other sides. This traps and holds heat conducting/radiating through and from the wall. The same box a foot removed from the wall & insulated all around would probably be useless - no extraneous heat source.

The industry standard for the lower limit of A palmatum's resistance to cold is a root/soil temperature of 14 or 15* F. IOW, your plant is more likely to die if it sees actual root temperatures below this threshold low than it is to survive, and many cultivars will not tolerate these low temperatures.

It's very unusual for 6" soil temperatures, even in zone 4, to ever drop below 25* F, even in bare ag fields. We know that roots of varying ages and stages of lignification do not all have the same degree of cold-hardiness. The finest and more herbaceous roots, the actual workhorses, are the first to succumb to freezing temperatures. This can happen (in the finest roots) at temperatures as high as 25-28*, so even though there is no visible damage to the plant, we are still experiencing some minor degree of cold-injury while temperatures remain in the upper 20s. As actual root/soil temperatures continue to fall, progressively larger roots succumb to freezing of bound water (water inside of cells, which is the true determining factor in whether cells/tissue succumb(s) to freeze injury or remains viable).

In some cases, plants exposed to cold extremes (for the species) lose most of their rootage and begin spring with the same approximate energy levels and reactions to their environment as a large cutting, having to regenerate their lost rootage from stored energy reserves before they can supply the water needed to keep the canopy hydrated. This regeneration of lost rootage is an expensive energy outlay, because the energy that went into roots would have gone to branch extension and foliage production had it not been used to regenerate roots. From this, we can see that unnecessarily exposing your tree's roots to temperatures even in the low 20s, will have an affect on energy management, even if you don't actually see/recognize it.

In the end, chill injury of any kind to roots, or even above-ground parts, slows development and growth. Where plants are concerned, once growth potential has been lost, it can never be regained. Plants can't grow faster or better than they are genetically programmed to grow, no matter what lengths we might go to, so they can't play catch-up.

Ideally, for all deciduous trees, the safe bet for ensuring maximum vitality for the spring push is to try your best to keep root temperatures between the upper 20s to no warmer than 42* for the winter. This ensures maximum root survival and the highest energy levels possible, within the influence exerted by other cultural conditions.

There are lots of over-wintering strategies, including unheated garages or out-buildings, burying the containers, mulching against a building, pits, etc. Most of these take advantage of geothermal heat. Just putting a pot on the ground or garage floor, as opposed to leaving it on a deck or bench, can moderate root temperatures by more than 20* when it's REALLY cold, so keep that free source of heat in mind when you plan your strategy; and remember, the more roots you save, the better it will be for your plant.

Al

Al, thanks for the additional clarification/explanations. I never think about containers on a deck or porch cuz I never had one!! All of mine are on the ground. I'm also giving Staceybeth's zone the benefit of the doubt......it just isn't usually necessary to provide much winter protection for container-grown hardy plants in that climate. No doubt I've lost a few roots on my container plants over the years - I am a great believer in benign neglect, especially when the weather gets cold - but it doesn't seem to have done anyone any serious harm :-) If I think the weather is going to get really funky, I gather them all together so they create their own little microclimate.

YPP

I had thought about the fact we were each a zone removed from Stacy's, with yours a zone warmer and mine cooler, and meant to note that our practical experience is a little different. Maybe she can put us both in a bag & shake us up & come up with something she thinks will work to her plant's advantage. I think what I offered would probably be more valuable to zone 5 & 6 growers of JMs in containers, but it's still very good info to keep in the back of your mind whenever you have a special plant (other than JMs, too) you're worried might be marginally hardy in a container in your zone.

Take good care, Pam.

YPA