A THREAT OF FROST always sends me scurrying to haul in the last houseplants, something that if I were a better person, I’d have done a week or two before. Even if a particular weather warning proves a false alarm, it’s signal that it’s time: time to make plans for houseplants and 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. How to overwinter some favorite tender plants:

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–try forcing dormancy or semi-dormancy versus forcing it to limp along, suffering. If you have non-hardy plants you’ve tried keeping as “houseplants” in your heated home, 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 possible.

Extra heroics: Adding a growlight hood for 12 hours a day in, say, a cool basement could 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 right, alongside a bromeliad). But first, as with all my “houseplants,” they get a physical: a checkup in the wheelbarrow (below) 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, a transition instead of a shock to the system. These guys are tough, but resent drafty, cold spots; I try to find enough bright but protected places. Here’s how begonia expert Tovah Martin cares for her collection.

BROMELIADS: Bromeliads (including the Vriesea, below, and the one above left) are great in the shady garden all summer, and in the house all winter. I have had some of mine 10 years, and have only occasionally had to repot. I 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. In fact it needs at least 40 days below 50F (but above 35 degrees). I simply stop watering both my yellow and orange ones for a month and a half or two, and deliberately grow them in my bright but less-heated mudroom, then let them warm up a bit to like 60 and resume watering. How to grow clivia like a pro.

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 bulblike 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. This is the hardest environment for me to simulate in my Zone 5B winter and heated home. Perhaps an enclosed porch, mudroom, or windowed basement (or bright garage if yours stays in the mid-40s or a bit warmer) can provide this where you live. 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, and are easy to overwinter here. Place pots in the basement or another cool spot; no light or water required.

AMARYLLIS: In mid-August or early September, I begin withholding water as I will for eight weeks or thereabouts, leaving the pots in a closet in the dark. 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 even 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 very 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.

TUBEROUS BEGONIAS and CALLA LILIES are easy to store, after a light frost (not a hard freeze!) signals to them it’s time to begin going dormant. The former are best stored right in their pots, cool, dark and dry; the latter don’t care so much about the cool part of the equation. Tuberous begonia and calla how-to with expert Dugald Cameron.

CANNA: These are the easiest of the easy, I think. 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 to cure. Store tubers layered in boxes or crates (but not in plastic) in sand, peat, wood shavings 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; you can at least cut them in half and wait another day or two for cuts to heal before stashing. Kathy Tracey of Avant Gardens Nursery stores them in her New England location this way.

ELEPHANT EARS is the name people give to both Colocasia (known as taro; the “elephant-ears” with typically matte leaves like ‘Mojito,’ above) 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 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.

“The entire group of new elephant ears tend in that direction,” says Tony Avent of Plant Delights Nursery, “hence they will never be sold as dormant tubers by most bulb firms. These are best if kept potted through the winter under one of two regimes…actively growing at over 60 degrees F and in good light, or semi-dormant at 45-50 degrees and kept fairly dry and as such, light is not as critical.” How I try to overwinter the matte-leaved Colocasia types.

Larger tubers of some more traditional shiny-leaved kinds, the Alocasia, 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 Colocasia that I like to use in water gardens (above) in a big plastic tub of shallow water in the cellar one winter. They looked like hell, but grew back into good plants anyhow this summer.

EUCOMIS, or pineapple lilies, are dead-easy. From tiny ones to towering giants, I stash them all, right in their pots, dry and dark and cool all winter, then take them out come spring and water again. My growing Eucomis collection.

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 go too far and 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!). 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; 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 annual “geraniums” like ‘Vancouver Centennial,’ above, and scented geraniums, or even zonals): I interviewed the co-owner of the longtime leader in these plants, Shady Hill Nursery’s Joe Heidgen, about how to care for pelargoniums year-round, including various overwintering tips. If you didn’t take cuttings to root in August, the best way to have the freshest plants for next year, you can also 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. If I were a zone or so warmer, a bright garage would probably work, but not here. 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. I have several (including one above) 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. No light needed. 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.

FIG TREES IN POTS: Lee Reich has grown more figs up North than anyone else I know. He has tried every method from tipped over in the ground and mulched, to dragged into the cellar in pots, dark and cold (or even without their pots), to in the ground inside a barely heated plastic greenhouse, just above freezing. Overwintering potted figs, with Lee Reich.

CITRUS IN POTS: Since they are evergreen and also ripening fruit over the winter, these are really tricky for me, but cool and very bright is what’s wanted.        Overwintering potted citrus, with leading source Four Winds Growers.

ROSEMARY: Here’s yet another “cool and bright” subject, my biggest challenge in a cold Zone. How longtime herb specialist Rose Marie Nichols McGee of Nichols Garden Nursery advises we overwinter rosemary, indoors or out–depending where we live.

more information:

• “Fine Gardening” magazine on the subject