brrrr! overwintering tips for tender plants

A THREAT OF FROST LAST WEEKEND sent me scurrying to haul in the houseplants, and though it was a false alarm, it’s time: time to make plans for them and for other tender things like cannas and bananas, cordyline and a favorite pelargonium or two in hopes that what I call these “investment plants” (not perennial on their own, but carried over year to year with extra effort by me) are still around come spring. With frost warnings posted here again tonight, what better day to offer tips for how to overwinter some favorites?

First, my general thinking: No two gardeners’ potential places to stash such treasures will match in temperature or humidity, so when I say the basement works well here, your cellar might not. I have identified my best spots by experimenting, and by killing many things in the process. But every year I score another victory or two because I don’t let failure stop me. (Isn’t all gardening like that?)

And this: If I don’t have the right spot for a plant–often a combination of high light but cool, 50ish-degree conditions–I prefer to try forcing dormancy or semi-dormancy to forcing it to limp along. If you have non-hardy plants you’ve tried keeping as “houseplants” in your heated home with you, only to see them go wretched and leggy, think about letting them rest, or close to it, next time–water very sparingly and keep them as cool as you can.

What I’m doing next: adding a growlight or fluorescent hood for 14 or 16 hours a day in my cool basement to make a so-so-storage space a really good one for many more things. Again: experiment.

HOUSEPLANTS

FANCY-LEAF BEGONIAS: After a summer in the high shade of trees near the house, in they came (including ‘Marmaduke,’ above, and yes, that’s my childhood teddy, Iggy; and yes, I ate his nose before I was vegetarian). But first, as with all my “houseplants,” they get a physical: a checkup in the wheelbarrow or on a tarp, one at a time, that includes a trim of any battered leaves, a gentle removal of endless spiders and the occasional tree frog trying to hitch a ride, and a wipe-down or rinsing off of the pots. Best to get all the houseplants in before the heat is on indoors to make the transition less abrupt. These guys are tough, but most of them resent drafty, cold spots; I try to find enough bright but protected places.

BROMELIADS: Bromeliads (including the Vriesea, below) are great in the shady garden all summer, and great in the house all winter. I have had some of mine close to 10 years, have only occasionally repotted and simply keep their cups filled with water all year. Talk about an investment plant.

CLIVIA: This Zone 9 South African relative of Amaryllis asks what many plants from that area do: Let it go dry and cooler in late fall to trigger the late-winter bloom cycle. I simply stop watering the yellow Clivia and also the orange ones for two and a half months, and deliberately grow them in the mudroom, where the temperature shifts noticeably with the season; 50-60 is perfect.

WITH ALL HOUSEPLANTS: Give them a physical (as my Vriesea is having, above, or as explained in the begonia paragraph); even then, you’ll import some bugs, but no big deal. Take care not to overwater (certain death) and do not feed in lower-light months, except with orchids that are out of bloom, which I feed alternating weeks. Don’t put anyone near heaters or too close to what will be ice-cold or drafty window glass in cold zones. Pebble-filled waterproof trays can add humidity to an area, as can clustering many plants or running a humidifier.

BULBS AND BULB-LIKE PLANTS

AGAPANTHUS: Traditional evergreen varieties (usually hardy to about Zone 7b) want that tricky combination of conditions that a cool greenhouse provides better than a heated home: bright light, and mid-40s or so, or as close as you can get. Perhaps an enclosed porch, mudroom, or windowed basement (or bright garage if yours stays in the mid-40s or a big warmer) can provide this. They also want just enough moisture to keep them from desiccating, but not ever to be wet. Go easy. Those that are non-evergreen, with less-thick leaves, will go dormant; place pots in the basement or another cool spot; no water required.

AMARYLLIS: I’m already withholding water the last few weeks as I will for eight weeks or thereabouts, leaving the pots in a closet in the dark, after which I’ll take them out in mid-October or so, top up the soil if needed, and water once, then place the plant in a bright spot until it wants to grow. No trying to coax a sleepy amaryllis with repeat waterings, which can rot the bulb. Wait a few weeks or a month before trying again, and once a shoot of some kind appears, begin to water regularly.

BEGONIA ‘BONFIRE’: The B. boliviensis selection ‘Bonfire,’ above, like its cousin ‘Bellfire,’ is a tuber, and wants to dry off and rest and unless you have ideal conditions (like the nearby greenhouse I have seen them prosper in all winter). For most of us, it’s just easier to let it sleep. I put mine in my 45- to 50-degree basement in the dark, pot and all, remove the withering foliage, and just let it sit, no water. Watch in late winter or earliest spring for hints of awakening: tiny sprouts at ground-level. Bring it into the light then and water carefully until fully awake. Rot from overwatering (sometimes made worse by too-deep planting) is the easiest way to kill one of these in any season, but especially when just emerging and when it wants to take a nap.

CANNA: In Zone 6 or colder, cut back frosted foliage to about 6 or 8 inches (or do this in late fall if no frost happens), then dig the rhizomes. Shake off the excess soil or rinse if you prefer. Divide into clumps of three to five eyes if really large, and place in the basement (or somewhere 45 to 55 degrees) in plastic bags left slightly open or perforated for air. Adding peat to the bags may help, but frankly I always dig up so many that losing a few is no cause for worry. And then I wake them up like this.

DAHLIA: Wait two weeks after a hard frost before cutting stems to 6 inches above the ground and harvesting tubers carefully. Note: The wait is essential for these succulent tubers to be ready to store; if there is no killing frost, I dig in November anyhow. Wash clumps, then dry in an airy, protected spot for a day or two. Store tubers layered in boxes or crates (but not in plastic) in sand, peat or vermiculite at 40-50 degrees, checking for any signs of decay once or twice during storage. Though dividing can be done before storage or in spring, big clumps may be hard to store, so at least cut them in half and wait another day for cuts to heal before stashing. Swan Island Dahlias has expert photos of how to divide.

ELEPHANT EARS is the name people give to both Colocasia (known as taro; the “elephant-ears” with typically matte leaves) or Alocasia (generally, these “elephant ears” have leaves that are shiny). Confusing, as storing them can sometimes be, since varieties on the market vary in hardiness from tough 7B-hardy types to true tropicals, meaning they have very different tolerances. It’s also particularly tricky if you live in a cold zone where the tubers often don’t get enough summer heat to really size up. Some new varieties don’t produce big tubers at all. Small tubers don’t have enough reserves to survive, dug, all winter long, so are best potted up (or left potted up) and stashed in a cool basement. Larger tubers of some kinds can be dug, the leaves cut back to the bulb, and allowed to dry until dry to the touch before placing them in the 45-50-degree spot; some people put them in peat or vermiculite containers first. Experimentation required, and I mean experiment: I even tried setting the nursery pots of some that I like to use in water gardens (above) in a big plastic tub of shallow water in the cellar last winter. They looked like hell, but grew back into good plants anyhow this summer. UPDATE 2011: I posted an article on storing Colocasia. This year I will try overwintering some indoors in 60+ degrees and bright light, as “houseplants” and some in the cellar semi-dormant for comparison.

GLADIOLUS: After the foliage dies, harvest the corms and store dry in mesh bags with good air circulation at about 40 degrees, such as hanging in a basement. Check a couple of times over the winter for any signs of decay.

OTHER TROPICALS

BANANAS: Container-grown bananas can be cut back to about 6 inches just after light frost and stored, pretty dry, in a spot that’s about 45-50 degrees, such as a basement.  I always check things I’m keeping dry to make sure they don’t desiccate in storage; sometimes by February, I need to give a little occasional water. Bananas that were grown in the ground must be dug up before frost, which disturbs the root system, so do not cut back the topgrowth too. Instead, wrap the dug rootball in a plastic bag and bring the whole plant into the same kind of cool, dark spot as above, allowing it to dry off at its own pace, cutting it back before watering and growth begin in spring.

BRUGMANSIA: In areas where Brugmansia (top photo) isn’t hardy, the best tactic is to let the Angel’s Trumpet go dry and dormant in a 45ish-degree spot, like the cellar. It can get to be quite a giant in the ground or even in very large pots, though, and big plants are hard to store (or even get into the house or basement!), so eventually you face the reality of having to cut it back partway to even get it into storage. Mine’s going in the cellar lying down, I think; Mary, a Zone 6 reader shared in comments recently that she stores hers right in her house this way; cut in half, and resting, in an out-of-the-way corner, leafless like mine in the cooler basement will be.

HIBISCUS: Many people in cold zones know to bring these tree-like plants indoors in winter, but then try to keep them growing. A more effective tactic if you have a cool spot, would be to encourage them to drop their leaves by letting the plant go dry (like the Brugmansia, above, then store the dormant plant in a 40- to 45-degree location. Check every other week to see if a little water is needed to prevent desiccation.

PELARGONIUMS (fancy-leaf like ‘Vancouver Centennial,’ above, and scented geraniums, or even zonals): If you didn’t take cuttings to root in August, the best way to have the freshest plants for next year, you can use this tactic from the Royal Horticultural Society (very much like the way my grandmother kept hers in her cellar, all cut back to stumps and barely alive, though she hung hers from the clothesline in paper bags). Pelargoniums with any hint of weakness or disease should be destroyed.

CORDYLINE and PHORMIUMS: These are really pricey plants, and get so much better when big, so I’ve been trying storage tactics for years with mixed results. Bright and cool is the key here (like high 40s but lots of light; little or no water), hence my adding a growlight to one area of the basement this winter to accommodate my ever-larger plants better than the two tiny windows can now. Keep a careful eye out for pests, particularly in late winter, and be prepared to trim off dried leaves come spring, when they can go out early.

JUST-TENDER WOODY PLANTS

JAPANESE MAPLES or HYDRANGEAS or other small trees and shrubs that are just slightly more tender than your zone allows can be grown as pot subjects, the way I do my Japanese maples (two big pots above, seen just out of the barn in late April). I have three Japanese maples here that are technically hardy, but hate the ice and wind, so I wheel them pots and all into the unheated garage on a hand cart for the winter, once the frost takes their leaves and they are sleeping. They stay there until sometime from mid-April to early May, and require water once or twice in late winter so they don’t dry out once the potting soil thaws. Every third year they must be potted up or root-pruned. I have two friends with amazing old hydrangea collections grown just this way year after year.

More Stories and Fact Sheets:

• A great article from “Fine Gardening” on the subject.

• From North Carolina State, storage needs of tender bulbs.

• Monrovia Nursery’s tips on stashing some recent popular tropicals.

• The RHS on storing Pelargoniums in winter.