OVERWINTERING: IN FROM THE COLD
Looking down into the warm “Long House” at Logee’s Greenhouses in Danielson, Connecticut.
The sliding scale of effort involved in keeping tender plants alive over the winter ranges from really easy to not difficult at all. What you’ll be capable of depends on how much you can lift without injury, how much control you have over the indoor climate, and how much space is available. After that, all you need to do is monitor your plants’ needs, mostly for water.
During the winter, you can store and enjoy plants in three kinds of spaces. Some plants prefer to keep growing, and even blooming, indoors on warm windowsills. More will be happy to slow down to a standstill in a space that is bright and chilly but above freezing, such as a greenhouse, cold frame, or enclosed porch. And others will willingly go dormant in cool, dark storage. Each space and the plants overwintering in it will require different timing (for coming inside and going back out again in spring) and different levels of attention through the winter.
Overwintering on the warm windowsill
The same temperature range that keeps exiled gardeners comfortable and the pipes from freezing (high 50s to 60s F at night into the 60s and 70s F during the day) is also ideal for growing cuttings and for overwintering tropical plants, otherwise known, at least while they’re indoors, as houseplants.
Welcoming plants into living spaces, particularly over the winter when we miss the garden madly, offers a variety of benefits. Walk into a plant-filled room and your heart rate actually lowers: plants reduce stress. Houseplants have also been proven to filter gaseous toxins known as Volatile Organic Compounds (VOCs) out of the air we breathe. As plants purify our air they take in carbon dioxide and emit oxygen, which reduces the likelihood of headaches and improves our sleep. And according to studies (and personal experience, as I stare deeply at a begonia for just the right words), plants inspire creativity. Indoor plants do require watering and regular attention but that is part of their charm. Taking care of a living thing can alleviate symptoms of depression, the blues, and cabin fever.
Plants also raise indoor humidity levels, staving off cold symptoms such as sniffly noses and coughing. Most houses, particularly when the heat is on, are very dry, usually below 30 percent humidity. Plants and people both prefer more humid conditions: 40 percent for cacti and succulents, 50 to 60 percent for the rest of us. The more plants that are transpiring water vapor from their leaves, the higher the moisture level in the air. Placing indoor plants on humidity trays will further increase air moisture. These trays, or saucers, should be 1 or 2 inches deep, filled with water and enough pebbles to keep the pots from sitting directly in the water. As the water evaporates, the humidity level will rise around your plants.
Locating light
Even in houses with covetable natural light, daylight is elusive during the winter. The sun, low on the horizon, weakens in intensity, and glass lets in less light than you might think. Add to that, most of us who desire privacy have also covered our windows either with foundation shrubs or curtains.
Start by taking an inventory of shadows. If you or the cat lying in a sunbeam cast a sharp shadow for a few hours a day, you’ve got decent light. The farther you move away, even from your brightest window, the darker it gets, the more diffuse the shadows. You might still be able to read the fine print on a sunny day but a sun-loving plant will have to stretch toward the window in order to photosynthesize the sun’s energy. Use your findings to evaluate available windowsill real estate in your house and place your plants wherever their specific light requirements are most likely to be met. Be sure to position all sun-loving plants as close to the glass as possible.
• South-facing windows provide the brightest overall exposure through the winter (in the Northern hemisphere) so reserve those windowsills for sun-loving succulents, geraniums, and citrus.
• West-facing windows offer a warm afternoon glow. This is a fine second choice for tropical plants like hibiscus and glory bush.
• East-facing windows, with their morning sun, give a gentle boost to cuttings and seedlings.
• North-facing windows deliver diffuse light that will be fine for most ferns and understory plants like aspidistra and farfugium. But when the sun is distant and weak even shade-loving plants like these would do well in an east-facing window, or away from the windows in a bright room with southern or western exposure.
A window full of begonias, a fig, hoya, and orchids raise the humidity level in my studio.
A Meyer lemon, scented geranium, and begonia share a sunny west-facing corner.
Time it right: coming in for winter
The timing of the move inside can mean the difference between life and death. In fall, scan the weather forecasts for frost warnings and play it safe. Start bringing your tropical plants inside as soon as night temperatures are forecast to dip into the mid-50s F.
If temperatures plunge quickly in your area, plan ahead and give the plants destined for the living room a chance to acclimate by bringing them inside two weeks or so before you close windows and turn on the heat. Give potted plants their last dose of fertilizer in late summer. (Continue to feed winter bloomers every two weeks to a month until blooms fade.) Even though natural predators will generally help keep insect infestations to a minimum over summer outside, check your plants for hitchhikers, and wash them off if necessary before bringing them inside.
A tillandsia and a few rabbit’s foot ferns thrive in the
light and humidity of an east-facing windowsill above the
kitchen sink.
Winter watering
Judging the amount of water to give plants is probably the trickiest variable for most indoor gardeners. When getting to know your plants, think about where they grow wild. Plants from a rainforest floor will need to be kept evenly moist, which means the soil should never be allowed to dry out. But that doesn’t mean they should sit constantly in a full dish of water; only plants from a bog habitat will tolerate that. Plants from climates with limited rainfall do better if their soil dries to the touch between watering. Their leaves may even be allowed to wilt, but then we have to drench them. And we have to pay attention to any seasonal watering requirements too. Many plants’ dormant season is a dry one.
Plants send out distress signals when they need water but they will become stressed and unhealthy if wilting is the only cue you use. Instead, make a habit of checking their soil a few times a week. You’ll see indicators by looking at the potting soil (it lightens in color as it dries) and sticking your finger in it (the soil will either feel cool and moist, or dry and dusty, to the touch). But the best way to test for soil dryness beyond the surface is to test its weight. If the pot and plant feel heavy, the soil is moist; if it feels light, it’s dry.
Water plants in the morning so they can take advantage of the day’s light to pull water up into their leaves through photosynthesis. Always water thoroughly enough that it runs out the drainage hole at the bottom of the pot. (This will encourage the roots to grow down rather than staying near the surface.) Allow your plants to sit in puddled saucers for a few hours to give the soil and roots a chance to absorb what’s needed, then empty the excess. The warmer the temperature, outside or in, and the brighter the light, the more often plants will need to be watered. And vice versa: the cooler the temperature, the lower the light, the slower the growth, the less water is required.
Many plants have phototropic hormones in their stems that cause them to lean toward the sun, so rotate them a quarter turn every time you water to keep them balanced and standing straight.
Whenever you pot up plants, be sure to leave a watering well of at least half an inch between the soil and the pot’s rim to prevent overflows and washouts.
Insect pests indoors
Even if we encourage insects in our garden we’re unlikely to welcome them inside the house because they’re messy eaters and they gross out the other members of our household. The most common houseplant pests are aphids, whitefly, scale, mealybug, and spider mite.
Aphids on an African blue basil (Ocimum ‘African Blue’) cutting.
To control spider mite, which thrive in low humidity, and are evident when
leaves become stippled and pale, hose plants off and place them in a humidity tray.
When plants are thriving and unstressed, adequately watered and groomed, devastating infestations are unlikely. So aim for health and keep your insecticidal arsenal as friendly as possible. If you can’t dislodge insects with your fingers or a blast of water, try spraying with insecticidal soap or diluted dish soap (one small squeeze to a spray bottle full of water). Along with killing most insect pests on contact, this is also useful for washing dust and mold off of the leaves.
Other nontoxic insecticides include horticultural oil (a petroleum-based oil suspended in water), neem oil (which is pressed from the seeds of an Indian tree), and alcohol. Even nontoxic insecticides can damage some plants so always test your chosen remedy on a couple of leaves before dousing the whole plant. If the leaves shed, or look scorched or bruised, try something else. Keep your plants out of direct sun for a day or two after treatment and rinse them off to remove dead bugs and residual insecticide.
Time it right: spring fever
In late winter or early spring, plants begin to respond to the sun’s climb in the sky, just as we do. The brighter light and warmer temperatures that entice us out into the garden also spur our houseplants to grow and increase their rate of transpiration. Do not abandon them. They’ll suddenly need to be watered more often. Early spring is also the time to start fertilizing again. Any plant destined to spend the summer in a container, especially those that flower or fruit heavily, will need to be fertilized regularly through the growing season to make up for the lack of nutrients available in potting soil and to replace what gets used up and washed out.
Gather a grooming kit—watering can, shears, a spray bottle of soapy water, a damp rag, and a dustpan and whiskbroom—so you can tidy as you go. Prune and sweep dead leaves and spent flowers into the tub. Squirt insect pests and use the rag to clean off their honeydew excreta. In the winter, I call this “gardening” and find it pretty gratifying.
Give your plants a very dilute (about half a teaspoon to a gallon) dose of water-soluble fertilizer formulated for houseplants such as a standard NPK (nitrogen, phosphorus, potassium) ratio of 20-20-20, once every two weeks to a month, watered into soil that is already moist. Plants in sunny windows or full-sun outside will need to be fed more often than those growing in lower light.
Salt deposits on the soil and pot indicate that I over-fertilized this fern.
Some children and pets will eat anything, and unfortunately, unless we’re talking about Brussels sprouts, dietary adventurism isn’t necessarily a good thing. Plants, like sharp tacks, have ingenious ways of protecting themselves from predators.
A lot of beautiful plants are toxic to eat and most of us have had poisonous plants in our garden forever and never had to rush anyone to the hospital. For a list of plants toxic to humans, visit the National Capital Poison Center (www.poison.org/prevent/plants.asp). Children are quick learners. Encourage them to garden with you and then watch them like a hawk. Call 1-800-222-1222 if they ever swallow anything they shouldn’t.
Before trying to memorize the entire list of poisonous plants compiled by the ASPCA (www.aspca.org/pet-care/poison-control/plants/), and restricting your houseplant choices, observe your pets’ habits and try behavior modification. Most cats, for instance, like to chew on grassy blades. Cabbage palm is on the do-not-eat list but they can chew papyrus to nubs and New Zealand flax to ribbons without risk of depression or internal bleeding. Even so, it’s better to give cats their very-own dish of cat grass, which is usually a sweet mix of rye, oat, barley, and wheat grasses that germinates from seed within a week. If your cats are anything like mine, they’ll go for that and ignore the rest. (Sow a fresh crop every two to three weeks.)
Dogs often chew anything and everything out of boredom and frustration but also will go for grass-like plants when their stomachs are upset. If they need grass to settle their stomach, ask the cats to share. If you suspect your pet has been poisoned by ingesting a plant, call their vet immediately.
Pigeon strikes a pose mid-snack.
You might think that if a little is good, more is better: not so with fertilizer. Salts leached on the outside of terracotta pots or the surface of the soil is a sign of overdoing it. Scrape salty crusts off the soil, replace that soil if any roots have become exposed, and give your over-fertilized plant a good soaking to run the excess down the drain. And make your solution even more dilute next time. Keep track of your feedings in a calendar or garden notebook.
Also remember to pay extra-close attention to your plants after you have fertilized them and they are putting on succulent new growth. That is temptation enough for any herbivorous insect that might be able to find its way in, but spring’s changes in temperature and humidity add to their stress making them easy prey. Don’t let them be eaten alive.
Plants that spend the whole year in containers will indicate that they need to be repotted when they dry out daily, fail to thrive, or bust through their pot. Spring and early summer, before or as you move them outside is a fine time to tackle that task.
When outdoor temperatures rise into the mid-50s at night, begin to move your plants back outside. Even if they have been in sunny windows, let them acclimate to higher light levels by hardening them off in the shade for a week or two. Check for scorch marks after moving them into the sun and give them more time in the shade if necessary.
Overwintering in a bright and chilly space
It’s Murphy’s Law that the majority of tender plants we might want to winter over will thrive in the kind of environment most of us have very little of: a brightly sunlit space that warms gently during the day and is chilly (35 to 60 degrees F) at night. What gardener wouldn’t give up chocolate for a cool greenhouse in which to protect temperate zone evergreens like cuphea, felicia, rosemary, succulents, phormium. and farfugium?
A variety of keepers including phormium, colocasia, abutilon, geranium, and echeveria in my plantry.
In lieu of that dream greenhouse, which would likely be cost prohibitive to erect and maintain, consider using an enclosed porch (I call mine “the plantry”) or attaching a cold frame to the south side of the house. A cold frame is simply a glass or polycarbonate topped (and sometimes sided) box that sits on the ground or over a pit for extra insulation. Make it yourself from old window sashes—slope the top to shed snow and hinge it for easy venting—or buy one premade from any garden or greenhouse supply company. Whether your ersatz greenhouse is an enclosed porch like mine or a cold frame, use a portable heater plugged into a timer to keep your plants from freezing on cold nights.
A frosted morning in Blithewold’s Idea Garden.
Another option, if your house’s thermostat has settings for more than one zone, is to populate your brightest room with plants and turn the heat down low, to 55 degrees F or lower, at night. Supplement
any daylight that falls across that room with grow-lights. A combination of one cool-white and one full-spectrum fluorescent tube in a standard double-channeled fixture fitted with a reflector, will sufficiently cover the wavelengths necessary for plants’ health. Hang it above a wide shelf fitted with a humidity tray. Plug the fixture into a timer that will turn it off only after your plants have received twelve to sixteen hours of light and adjust the lamp to keep it 8 to 12 inches above the tips of your plants. With a bank of grow-lights, you can turn even a corner of the basement or garage into a cool winter “greenhouse.”
Time it right: coming in for winter
Plants destined to spend the winter in bright, chilly storage can stand to stay outside until just before a killing frost. Keep your eye on the forecast and don’t let them get nipped. If your area tends to get premature light frosts interspersed with summery weather that would otherwise extend the garden season, cover frost-tender plants with a sheet on cold nights to enjoy them outside as long as possible. As with plants that come into the house’s living spaces, stop fertilizing potted plants in late summer. Make it easier on yourself by digging and potting plants up in stages rather than all at once the day before a forecast frost warning. Cut large plants back by at least a third to help prevent transplant shock and wilting, and so they’ll take up less space, and be easier to move.
Winter care
Plants that are kept cold and barely growing won’t need quite as much attention as those growing in the warmth of the house. As a rule, water temperate-zone perennials and shrubs just before their soil dries completely. Allow succulents and desert plants to go dry; depending on how cold the space is, they may only need to be drenched once every two weeks to a month. Groom dead leaves and give plants a quarter turn every week or two to keep them standing straight.
If mold begins to grow, cut affected parts back to healthy stems, give plants more space, and make sure saucers are empty. Turn a small fan on low to help circulate the air.
Keep an eye out for insect pests even though cold temperatures tend to slow infestations. Control insects as you would with plants growing in the house.
Geraniums (Pelargonium) growing in a sunny barn “orangerie.”
Time it right: spring fever
In late winter or early spring, cut herbaceous perennials and shrubs back again by half or to the lowest set of buds. New growth will form that, in four to eight weeks’ time, will be perfect for taking cuttings. Begin to fertilize again in early spring as new growth emerges.
Just as inside the house, plants will respond vigorously to the changes in temperature and day length. Your “greenhouse” will heat up during the day and may need to be opened and cooled to keep plants from becoming stressed by heat and dryness, and susceptible to predation. Water daily to weekly in spring and remember to close the windows, doors, or vents again on cold nights.
If you don’t have enough room inside for all of your favorite tender plants, or if you’ve taken all the cuttings you want, you have nothing to lose by leaving some plants in the ground. Occasionally we are dealt an unseasonably warm winter or an insulating blanket of snow. Or, if we take advantage of garden microclimates such as south-facing stone walls and foundations that hold and radiate extra warmth during the winter, we might just get lucky. Every gardener has at least one story about a surprising survivor and can tell you that nothing is more thrilling than seeing a salvia or dahlia return from the dead. Take your chances with plants at the edge of your own hardiness zone.
To give marginally hardy plants a fighting chance, leave their stems standing to protect their crown and to avoid stimulating new growth too tender to survive a cold snap. Cover their root zone with shredded leaves, bark mulch, or leftover holiday greens. In spring, pull any matted mulch and heavy greens away from the crown of the plant. Shredded leaves, being lighter in texture, may be left longer as protection. Cut stems back in late spring and do a little probing to check for signs of life but don’t expect to see any new shoots until the soil warms.
Placing shredded leaf protection around the crown of Agastache ‘Heatwave’.
As outside temperatures warm above freezing at night, move the hardiest plants back outside. Harden them off in the shade for a week or two before moving them gradually into more sun. Week-by-week, as weather permits, continue moving the plants outside until all have found their places in the garden.
Overwintering in a dark and cool space
Closets, storage crates, basements, crawl spaces, garages, or any place that can be kept dark and above freezing, are ideal for storing tuberous rooted and herbaceous perennials, and deciduous shrubs and trees, which lose their leaves in their native climate and expect to get a real rest between growing seasons. The temperature should hover between 35 and 60 degrees F; the light level should be dim to pitch black, day and night.
Every gardener’s dark storage space is different: a storage method that results in moldy rot for me, might foster plump, healthy root tissue for you. Use a humidity meter to chart moisture levels. In dry spaces, potted plants will need to be watered more often, whereas in damp spaces, you’ll have to watch for signs of mold and rot. The humidity level will also affect storage. When dry, try storing plants in open plastic bags or in boxes barely covered with dampened potting mix or peat moss; when damp, pack bareroot tubers and corms loosely in open bins, wrapped in newspaper, and in paper bags rather than plastic. Dusting with fungicide is optional. Experiment.
Dahlia ‘Baronesse’ touched by frost.
Time it right: coming in for winter
Plants destined for dark storage should be hit by frost before coming inside—a cold zap is the best way to shut down bloom production and trigger dormancy. Stop fertilizing potted plants in late summer or early fall and water them less frequently, allowing growth to slow as the temperatures dip. Also make sure you have labeled your plants before frost blackens their stems and blooms.
Unless an arctic freeze is forecast, wait a couple of days to two weeks after the first frost before digging plants out of the garden. This will give their roots a chance to get the message and gather resources for the winter. (Dahlia tubers swell with carbohydrates and water as temperatures plunge.)
Plants with tuberous roots or corms (such as dahlia, canna, four o’clocks, and gladiolus) may be stored barerooted. Use a digging fork to gently lift them out of the ground. Cut stems down to 3 to 6 inches, remove any soil from around the roots and dry them off in the sun upside-down for a day or two before storing them. Plants with fibrous roots (such as salvia, lemon verbena, fuchsia, and palm grass) should be stored potted up. Use a spade to dig them out of the ground, keeping plenty of soil around the roots. Cut their stems down as well, by at least a third to make moving them inside easier. Water them in well after potting up.
Dahlia tubers brought up from storage already beginning
to sprout.
Winter care
Out of sight out of mind? Plants overwintering in
dark storage need very little attention but try not to forget about them completely: check them at least once a month.
If bareooted plants begin to shrivel, mist tubers or dampen packing material. Air them out if you see signs of mold, and promptly remove any mushy or rotten bits.
Potted plants should be kept “just moist,” meaning they should be watered only sparingly before the soil becomes bone-dry. The darker and damper the space, the less frequently they will require watering and vice versa. Some plants, like tuberous begonia and pineapple lily won’t need any water at all. Tip those pots on their side as a reminder.
Potting up dahlia tubers.
Time it right: spring fever
As light levels increase in spring, so do ambient temperatures and even plants that have been dormant in the dark will notice. Begin watering potted plants if they are noticeably dry, or as soon as they show signs of growing. If it has been a wet spring, tubers might need to be aired out to keep them plump and firm. If the spring is dry, mist packing material to keep roots from desiccating. But don’t be overly worried if the tubers shrivel this late in the game. Sometimes they will still sprout when you plant them and you’ve got nothing to lose by trying.
If you have room in your makeshift greenhouse (you should, after moving the hardiest plants out), bring your dormant plants into the light to give them a head start of about a month before planting them outside. It is a good idea to pot up tubers at this point too. This will help them bloom sooner, and visibly growing plants will be easier to work around after planting them in the garden. If you decide to plant tubers straight into the ground, wait until after your average last frost date, when the soil is warm, and mark the sites well. It usually takes two or three weeks for their shoots to emerge.
Fuchsia, brugmansia, four o’clocks, palm grass, begonias, and dahlias spending the winter in my cellar.
Flowering maple (variegated Abutilon pictum ‘Gold Dust’ in the foreground with
A. ×hybridum ‘Red Monarch’) and other tender-perennial temptations at Avant Gardens.
