Sciency Answers: Overwintering tender plants

I've gotten a couple questions from people about how to over winter tender plants indoors -- Cathy asked about some species of tender salvia, and Joel asked about echium, so I thought I'd do a little (or not so little) Sciency answer on the basic principles of overwintering stuff.

First, I've got to tell a story. I was a student at Ohio State, and my friend Beth was teaching me to drive (Yes, I didn't get around to learning to drive until I was 22... I actually took my driver's test the same week I took the GRE for grad school. Aced the GRE. Technically failed the drivers test, but the testing lady was nice and let me get my license anyway. No, I'm not really one for practical life skills... which is why I get on so well in grad school.) So we were driving slowly around neighborhoods in Columbus when I suddenly slammed on the breaks.
"Um... there is no stop sign." Beth said.
"Those are BANANAS!" I yelled as I jumped out of the car. They were bananas -- over a dozen 10 foot tall banana plants with fruit on them. In Columbus Ohio. Zone 5. I ran up to them and admired their beauty, then knocked on the door to ask them how they managed to pull it off.
Turns out it was simple. Every fall they dig them up, knock most of the soil off, cut off all the leaves, and throw them in the basement. Come spring, they shove them in the ground, and the bananas take up where they left off. That is the beauty of bringing in tender plants -- you can grow all kinds of cool stuff that you wouldn't be able to otherwise, because while it may be a zone 5 winter outside, in the house it is a solid zone 10. So every fall, I bring in loads of plants. Sometimes I plan ahead for this, growing things in containers that can easily be moved, or taking cuttings of large plants in the late summer. But more often I see frost on the forecast, look at some cool plant I was growing as an annual, and decide to dig it up, cut it back, and see if it will make it. Usually they do. I've actually had very few failures, provided I follow these rules:

My rosemary standard has been wintering happily indoors for several years now

1. Don't expect them to look pretty. My goal for most things is merely that they survive. Some plants will look marvelous indoors all winter, but most others will limp along, loosing a leaf here, getting leggy and ragged looking -- but they survive, and come spring I cut off all the raggedy growth, and pretty soon they're looking gorgeous again.

Various small, tender things in my sunniest window.

2. As much light as possible. This is a no brainer. Shove everything in the sunniest windows you have. Florescent lights are wonderful for plants, so if you really have a lot of plants to bring through, invest in a few shop lights. Oh, and don't bother with "grow light" bulbs. They don't make plants grow any better, they're just designed to make plants look prettier. So unless you are displaying them in your living room, any old florescent bulb is fine.

My elephant ears always look sad over the winter, but perk up as soon as they go back outside.

3. As cool as possible. This may seem counter-intuitive, but in my experience it is the most important thing of all. You don't want plants to freeze, of course, or really drop much below about 50 F (10 C), but especially if you don't have as much light as they might want, cool temperatures are the best. At 50 or 60 (10-15 C) the plants don't really grow, but they don't die either. They just sit in suspended animation until spring, which is exactly what I want them to do. For me, this is easy, because I'm cheap and keep my house chilly. When I'm home and awake, I keep my thermostat at 63. At night and while I'm at work, it is at 50. Perfect for everything... except guests. But serious, plants always come before guests! If you like to be warmer, a cool basement or garage might be a better bet. If they are cool enough, many plants will stay dormant and you won't even need lights, or you can rig up some florescent bulbs to keep them happier.

Kept cool and dry, succulents like this agave are effortless. I basically ignore them, and they're fine.

4. Keep them dry. Don't dehydrate them, but be careful not to over water. These plants are going to be stressed and barely growing, and too much water will make them rot. Also, lots of plants are adapted to going dormant through seasonal droughts, and keeping them dry will signal them to shut down and wait -- which is exaclty what we want them to do.

That is my standard protocol, and it works for just about everything I've tried.

Some plants will come through fine with even more extreme treatments without any light at all. Just about anything which forms a bulb or tuber can be dug up, branches chopped off, wrapped in dry newspaper, shoved in a plastic bag, and left somewhere cool and dry like a basement. I do this regularly with tuberous begonias, Salvia guaranitica 'Black and Blue', sweet potato vine, dahlias, callas, cannas, and gladiolus. When I try new plants, I often go out in the fall with a garden fork and pop them out of the ground to see if they have a tuber I can save -- turns out that four-o'clocks (Mirabilis jalapa) and hyacinth bean (Dolichos lablab) both form easily overwintered tubers. I've heard of people doing the same with the thick, fleshy roots of nicotiana (ornamental tobacco) and scarlet runner beans as well. Some plants that don't have obvious bulbs or fleshy roots will take the same treatment, as with the bananas I talked about at the beginning. Similarly, the shrubby Hibiscus rosa-sinensis can be over wintered in the dark, fully dormant. Just put them somewhere cool (like a basement) and let them dry down. They leaves will drop off, and they'll sit patiently until warmth and water returns in the spring. I've heard Pelargonium will do the same, and are probably oodles of other plants that can be overwintered this way as well. The only way to find out is to try. It doesn't take long to pop something out of the ground and throw it in the basement. If it dies, oh well, it would have died anyway had you left it outside. If it lives, well! Then you've got a lovely plant for next year, and a trick to show other gardeners!

To sum it all up: Keep them cool, light, and dry, and experiment with everything. If you have success with something unexpected, be sure to brag, and please let me know, so I can share your methods with other gardeners.