Temperatures in the mid-20s likely will zap the new growth on your hydrangea, especially if it's a macrophylla-type hydrangea. If it is a macrophylla-type that's been "forced", I would dig it up and store it in the garage overnight. In fact, I'd put it back in a pot for easy transport to and from the garage until you've passed the last threat-of-frost date for your area to give yourself the best chances for getting blooms this year.

Thank you for responding, Al! I was just checking out your Trees in Containers thread. Sorry; my zone info is 6, NY and the containers will be placed against a wall protecting them from any South-West wind while still receiving sunlight from the South-East. I have baby Royal Star Magnolia, Ann Magnolia, chocolate mimosa, regular Albizia Julibrissin, Furman's Red Salvia, and black and blue Salvia. The trees are in 22" wooden planters. The salvias are in smaller insulated--2" thick plastic pots. Would encircling a layer of cardboard around all the containers pressed together be enough? Maybe a blue tarp or black plastic garbage bags wrapped around with the tops uncovered for ventilation too? By the way, is more sun a threat or benefit in winter? I'm also wondering, with the eventual need for plant lifting/root pruning anyway, perhaps wintering in the ground is better? The only issue then is a drainage problem in the minimal available ground I have. Sorry for the deluge of questions.. I should have prepared for this ahead of time. With winter approaching, I'm really starting to become concerned. Any additional advice from you is most welcome. Thanks again.

Artisticcheese, my plants are in zone 9 coastal, just off the bay. Mulch will keep the ground from freezing. I learned this from a master gardener in San Antonio named Manuel Flores in 1994, when I asked for help on how to keep my bouganvilla from freezing in San Antonio, where it does freeze on a regular basis. My tropical, are in Rockport, it has only gone below 32 degrees twice in the past 19 years there. I lost all my tropical plants in 2010, so I do not want to take a chance of loosing them again, so I protect the most expensive ones. Torch lilies, exotic hibiscus, plumeria, hong kong orchids, and desert roses. I do not protect datura, brugmansia, gingers, iochroma, ti plants, variegated rubber trees, bird of paradise, bouganvilla, and a lot of other tropical plants and vines. In the spring I give the rooted cuttings to my neighbors, family, and friends. Barbra

I do overwinter mine inside, but use lights on a timer. I have a shelf in my basement where I keep my pots, using the lights I keep for seed starting. I mix some bonsai or succulent soil in with the potting mix to help with the drainage, and then am careful with the watering. I use a probe moisture meter to make sure I know how dry the pot is, or don't water until I see the new growth tips droop slightly. We have dry air here, and I mist the plants when I think about it to help with some humidity. I also have to be careful to watch for powdery mildew on my indoor herbs. You will have to be careful moving them outside for short periods since they won't be used to the sun/wind. i would worry about the bucket not allowing for any light or air circulation. Plus, it won't control the moisture level of the soil. I did manage to overwinter an Arp here with lots of straw, but it didn't grow much for me the next summer, so I quit trying.