What kind of Maples do you have that are so sensitive to cold? I have 5 Japanese Maples in the ground, but also 3 bonsai in shallow pots, including one laceleaf. They stay out all year, and have survived down as low as 10F. In 3-5 gallon pots they should be fine at those temperatures. If you are worried about it, just bring them in when it gets below 20.

I'd recommend adding a thick layer of leaves over the top of the root ball on each container plant. Try to make it at least 6-12 inches high. Since the wind can blow some leaves off what I do is water the leaves with a hose. After they are wet I press the leaves down with my hand so they stick to the container. Bring the Maples into the garage when there's extreme cold.

A leaf mulch on top of the container is going offer very little cold protection to the root mass. It is the sides and bottom of the container that will be most vulnerable to the cold. I wouldn't wait for "extreme cold" but move the plants into a protected area one the night time temperatures dip repeatedly into the upper 20's.

The alternative is to heel the plants - in their containers- into the ground anywhere there is a space and sufficient soil depth to accomplish - a raised bed, a disused veggie garden or just an open space in the garden. Just dig a hole, place the container and pile soil over it to the top of the rootball.

Thank you Gardengal, Stuart and Hemlock140 for your help! In that case, I will start to bring them inside the unheated garage, which typically stays above 25F. But I will take precaution as you advised.

Most of the trees are palmatum, a couple are shirasawanum, one Linearilobum (Beni (Y)Ubi Gohan), and one japonicum (Fairy Lights, small/compact tree but with the most amazing fall color ).

Sometimes the garage temperature could be in the 50's or upper 40's for several days in a row, would that cause the trees to wake up too early? I can always open the garage door and let the cold air in at night? Or am I worry too much? I have winterized successfully potted hostas but Maples in containers are new to me.

I have a few in the ground (~ 7 yr old Red Dragon, ~ 10 yr old Autumn Moon, and ~ 2-3 yr old Fire Flow, Shaina and Shirazz). I will winter protect the younger ones by pile some leaves and mulch around them.

Thanks again for your help!

Vivian

I would have brought them in before it got to the 20s.

I wouldn't be overly worried about the occasional higher temperatures. This happens naturally in my area with trees left outside in their containers and it is not enough to cause them to break dormancy. As long as it not more than a few days to a week or so - if that's the case, I would open the doors to bring the temps down :-)