Japanese maples in pots are not supposed to be outside in temperatures below 15 degrees or they can suffer root damage. I would let that be your guide. When there doesn't seem to be much risk for very low temperatures, it ought to be OK. However, if your maples start leaving out or breaking bud while in the garage, you may have to wait until any danger of hard freeze is past. I live in NC and we haven't had any temperatures this winter below about 24 so far, so I have left all of my container maples outside.

You would do well to bury the containers to the soil line & mulch if you have a location out of wind. Against a structure out of sun (north elevation) is best. The object is to keep root temperatures low to prevent the early leaf-out that is inevitable when over-wintering in a garage. When chill requirements are met and as soon as soil temperatures climb above 45* F. or so for several consecutive days, the plant will begin growth. At this time, cold hardiness is seriously compromised & the plant will need protection from freezing/frost. The problem is, the plant also needs light while growing. Unless you can prevent leaf-out, your best strategy is to move the plant(s) outdoors during daylight when temperatures allow and back under protection when freezing temps threaten.

Al

I'd just keep them in the garage until all danger of frost has passed. If they break dormancy early, drag them in the basement or wherever or simply keep them in the garage just so long as the temps don't drop to freezing or below. Mother's Day is pretty much the marker here. Just be careful in whatever you do. If you've taken them to the outdoors and a light freeze is expected, simply cover them with poly, fabric sheets, etc.

Dax

P.s. Burying them as suggested above is the easiest solution if you have the space. Still, if temps drop for some reason and they've awoken outdoors, you'll still need to cover them or you will lose buds/stems.

Trees overwintered in a garage (live ones) ;o) will always leaf out well before danger of frost has past. There are many good reasons I suggest moving them outdoors when temperatures allow. An important one is the growth produced in the garage will spoil the appearance of the plant. The plant will have gone into dormancy with internodes a normal length & growth that is typical of the plant in normal light conditions. The growth produced while the plant is light-deprived (in garage) will be extremely long in internodal length, weak to the point of being unable to support itself, and noticeably different from both prior and future growth produced in normal light.

Low light conditions also contribute substantially to the weakening of the tree. Energy reserves begin to decline rapidly when leaf-out begins. Good light helps the tree reach a break-even point much earlier in the growth cycle. Poor light keeps the tree in continual decline and can reduce energy reserves to dangerously low levels.

Remember, trees are most vulnerable in the period immediately following leaf-out. Allowing them to grow in low light will speed decline, lower energy reserves, prolong the recovery period and have a negative effect on development/bio-mass. Trees need all their reserve energy to fight off insects and pathogens, and to keep their systems orderly.

Al

What do you suggest if they are starting to leaf out. I have one that just has the buds swelling but I have another that is starting to unfold a few leaves. I live in Zone 2 so it is quite amazing that they made it this far and I don't want to lose them now. Thanks in advance.

Keep them cold, but not exposed to freezing temps and give them good light. Early leaf out is a big problem in containerized woody plants. It is always better to keep plants quiescent as long as possible to avoid your predicament, but even then, it's very probable the plant will begin spring growth too early anyway. I have about 75 deciduous trees in containers & I load them on a large wagon that I move in and out as temperatures allow. I would be greatly dismayed if we had a prolonged warm spell & my trees started moving this early.

Al

Hi Healyjet,

Bring them indoors (or if you're garage is heated keep them there and give them at least a lightbulb's worth of light for say 6-8 hours a day. Since they've broken, they'll need to be completely void of any "freezing temperature" and being that you're in zone 2, it's a reasonably easy thing for me to say that it will probably happen anytime and then again and most-likely sooner than later !

Once they've broken bud there isn't any going back. As I said above, you can bring them in the basement with light, (or keep them near windows actually in the house and now treat them as houseplants). In order for you to understand the answer to your question, you have to think in terms of: "When does 'my tree' normally leaf out naturally, and if something unseasoningly unusual is currently taking place, how do I mimick nature as best as I can? Also considering that now until your climate's last freeze is over, you're now in charge to maintain that 'mimick' using any other resource you may have.

To repeat myself, obviously should your garage not be capable of doing the job, it's my recommendation that you find a spot elsewhere where the plant can continue to pursue this unseasonably change in your trees bilogical clock.

Best,

Dax

P.s. Your tree has definitely already met any dormancy requirement, so taking it indoors so it can fully leaf need not be of any concern you may have.

Healey - There is a reason I suggested a cold spot with good light. The chill will slow tree systems dramatically, which will ease energy outlay to leaf production - hopefully until the plant can be shuttled in and out. You will find that Acer tolerates indoor humidity poorly. It will react by closing stomata, which effectively shuts down photosynthesis already diminished by low light conditions and speeds decline.

Also to be considered, and as mentioned, is "what to do with the growth abnormally long in internodal length? Leave it & it spoils the appearance of the plant. Prune it later & it's a horrible waste of energy & leaves the tree deeper in energy debt & sacrifices development while the tree plays catch-up.

Al

What do you mean "tolerates indoor humidity poorly"? Once I bring it into the warmth should I give it supplemental humidity?
Ward

Yes, if you can, and if you're intent on bringing it indoors. You'll notice that most houseplants that are able to tolerate indoor conditions during the heating season reasonably well are those that have leaves rich in sub-cuticular waxes and are able to maintain leaf turgidity with open stomata. In winter, the air in most homes is drier than air in the earth's driest deserts. Don't depend on misting to raise humidity. It is ineffective and can negatively affect the plants ability to make food, also affecting photosynthesis by stimulating the closing of stomata.

Al

Al, what temp do you consider a cold spot?
Ward

From 35 - 50* F.

Al