I grow all my Japanese maples in containers and they stay out all winter. Most are in pretty large containers but a few newer ones are still small and they are all fine. If it is predicted to be very cold - for us that means down into the low 20's or teens (and that happens rarely) - I group them all closely together. That provides enough of a microclimate to see them through safely. Root damage in containers can occur if they are exposed to extended periods below around 25F. Even wrapping the outside of the pots with bubble wrap will help. The top growth can withstand much colder temps so covering them is not a concern.

If you do feel a need to bring them inside, they need to be somewhere where the temperatures do not exceed 40F. And colder is better. Light is not a concern while the trees are dormant, so a dark unheated garage is fine. Soil should remain barely moist to the touch - never too wet in winter and never allowed to dry out completely.

I can't tell you anything about the jacaranda :-) Never grown one.

knowing that potted plants are less cold-tolerant?

==>>> not really ...

you MIGHT have it backwards ...

the key is to GET THEM DORMANT.. AND KEEP THEM DORMANT...

the usual problem with pots.. is since the roots are not in mother earth.. the pot can warm in winter.. and the roots can come out of dormancy.. due to sun heating the pot .... and then it gets cold again ...

even worse if you store them in dark .. and then bring them out.. to bask in the sun ... and the pot warms ....

many plants .. do not appreciate going in and out of dormancy thru winter ...

if you can .... you might be better off.. sinking pot and all.. in mother earth.. so she will temper temp swings ...

also keep in mind.. dormant plants do NOT need a lot of water ... or they might root rot ... just enough water.. to keep the media damp. is enough .. and let it nearly dry ... ice cubes can be used to add little amounts of water .... rather than swamping them with a watering can ...

finally ... my attached garage is at least 2 zones warmer ... up here in MI ... and the problem is that stuff can come out of dormancy months early in spring .. if not in mid winter .. so it can create more problems.. than it solves ....

im surely no expert in your zone ... but putting you pots .... in full shade.. on the north side of the house.. might be the easiest way to go ..but never forget your goal.....

GET THEM DORMANT.. AND KEEP THEM DORMANT... as best you can ...

ken

btw... i would suggest planting a 4 foot tree .. and get rid of the pot .. of course.. this is where you add.. that you live in an apartment.. and have no land to plant in ... you could always repot in spring ...

Here in StLouis where it gets below freezing I will 2nd or 3rd the idea a potted plant is less cols hardy than one in tbe ground.

Gardengal's statement about the root mass being surrounded by cold air is 100% on.

Throw in the heating effects of sun on the pot and the faster freezing of rain in the pot and ya get the picture. Black pots are the worst for getting hot lol.

FWIW, I have found our hot summers to be equally challenging.

How much that applies to your zone? I dunno. I can keep Japanese maples in pots here without extraordinary measures.

Ken, I actually could plant the Jacaranda in my yard, but I know she's a tropical tree, and probably won't ever be cold-hardy enough to plant in the ground in zone 8b. I just need to be able to move her inside when it's cold outside so she doesn't almost die like last time.

Gardengal, so that means I should bring my Japanese maples inside my garage? Or will the fact that they're in 21-inch pots where their roots haven't spread out to the sides yet mean that I can keep them outside? I don't know if my (unheated) garage is cold enough to maintain dormancy (it definitely gets above 40 degrees), but the lows here rarely go below 30, so if that won't harm the maples even in pots then I'll keep them outside. Seeing as how you've been at it 30 years I trust your advice is the most accurate. Thanks.

Toronado, if you can keep them in pots in zone 6 then it shouldn't be a problem in zone 8b, no?

Grouping potted plants together for cold conditions only protects the ones in the center of the grouping. But as stated potted Japanese maples should be hardy outside in Zone 8 unless the pots are quite small or a cold winter comes along. The jacaranda on the other hand needs to be grown as a house plant in climates where sharp frosts occur.

Yeah, I'm pretty sure that the Jacaranda can never go outside permanently. However, I have read that the bark of mature Jacarandas can take temperatures down to 26 degrees, so maybe somewhere along the line she can go in the ground, where plants are happier anyways.

Alright, I trust that the 21-inch pots for what are young trade-gallon Japanese maples should suffice to insulate them from the cold.

A further consideration is not the cold resistance of the trees but of the containers. Terracotta can be damaged unless you've bought frost proof. What are your pots made of?

They're metal. I'm guessing that's not good?

Depends on how you define 'good' :-) Metal containers by definition are frost proof so no worry about the cold damaging the container. But they also have minimal insulation value so the cold from the ambient air in winter penetrates faster and easier.

I do use some terra cotta outside all winter. High-fired or Italian terra cotta is much more cold tolerant than Mexican terra cotta. The firing temperature dictates how porous the clay is - the hotter/higher the firing, the denser the clay and the less able it is to absorb a lot of moisture. It is the moisture in the clay that causes the problems with terra cotta - it freezes and expands, cracking or spalling the container. I do try to keep any outdoor TC out of the worst of the rains but it seldom gets cold enough in a z8b climate to cause a lot of problems with these. I have a few large TC pots from Italy that are going on 20 years old!

Yeah, I was figuring that they certainly wouldn't have problems with cracking, but they would quickly reach the ambient air temperature as well due to being made of metal. So the final word here is that I should leave those maples outside since they are in very large pots for their size?

As I said, I have always left ALL my maples in containers outside all winter. Even those in very small 1G pots. They will be fine. (I would only express this assurance in a zone 8 climate).

Alright, got it. Thanks again.

Testing of potted Japanese maples showed a tendency for root damage to start at about 20 degrees F. Container plants of any kind can look "fine" after cold snaps if all you are going by is how the tops look afterward, and haven't looked at the roots - the new small roots on the outside of the root masses are the first thing to go when potted plants are damaged by cold. Next is the older roots farther back inside, and if you ever lose those then you have lost the entire specimen.

If you don't see well developed, long established Jacaranda in public and private plantings around you there then it is not an outdoor tree for you.