Just moved about 20 potted JMs outside (from garage). Temperature during the day is very warm, but at night drops to 25-30. Is it OK for potted JMs to be outside?
Why did you move them out?
Is it OK for potted JMs to be outside?
Not if they're in leaf and the temp is below freezing. They need protection. Roots won't likely freeze if the plant is resting directly on the soil or a slab, but they will freeze at 25-30 if they are elevated & not under shelter.
Frozen roots do not necessarily mean a dead tree, but it's best to keep roots from ANY freezing temps after budswell is underway.
If your JMs have been protected all winter, it's probably not a good idea to move them out now -- particularly if they are already leafing out. Generally speaking, dormant JMs in containers should be able to handle low temperatures to about 15F. I keep my containerized JMs outside all winter (in NC), but move them into my garage if low temps are forecasted to dip down into the teens. However, if your maples have been in the garage all winter, they probably are not really acclimated to real cold temperatures -- unless your garage is unheated and temperatures get below freezing there.
This winter, I moved my JMs into my garage about mid-January when lows were forecasted to reach about 15F one week. It never really got that cold, but they had been outside down to 17F on a previous night. They all made it through the winter just fine. I kept mine in the garage until some of them started to break bud in early March. We had a light freeze (30-32F) about 10 days ago, when most of my maples had already started leafing out, and they all handled the cold with no problems.
Plants I am growing in pots have been outdoors in Philly for 10 years. It's been close to 0F several times.
This year about 40 JM's overwintered in pots outdoors. The small pots had some protection but the larger pots none. No casualties so far this year.
If they are not in leaf and the soil is moist they will live to some where just below 20F, but they will or can experience root or wood damage...the larger the plant the better off it will be.
If they are breaking bud or just beginning to leaf, the leaves will generally go unharmed to around 27-25F. If there is some frost, or ice to insulate them it is often better (depending of the variety or tenderness of the leaf tissue), but they can quickly become dessicated as can the new stems. Variegates and yellow varieties are most tender.