Throughout this blog and at other gardening sites, we often refer to a plant’s “hardiness zone”. Find your location on the map to identify your zone.
Since moving to northern North Carolina in 2021, I’m now in Zone 7b. Here, the average annual extreme minimum winter temperature is 5-10° F. My location, though, is not far from the higher elevations of zone 7a (0-5° F). According to the older 2012 map, where I live was clearly in zone 7a.
At this USDA map website, enter your zip code to see if your area increased or decreased in the average extreme minimum temperature. Where I live, that temperature is 2° higher than it was in 2012. This does not mean that overall temperatures have increased an average of 2° F, just the lowest extreme minimum temperature.
Gardeners in other countries have similar systems of categorizing climate ratings.
Since I moved to North Carolina in 2013, we’ve had a few winters when the lowest temperatures fell almost to 0°F. We’ve also had a couple of seasons in the past few years when I rarely needed a winter jacket.
The 2023 map above replaces the 2012 map. Most zones of the country have been shifted upward a bit to reflect the slightly higher average extreme winter temperatures. But that won’t stop Mother Nature from testing the limits. These are averages, after all.
Microclimates
A microclimate that raises the temperature just a few degrees, compared to out in the open, can make a huge difference. You can find a microclimate simply by placing the winter garden on the south side (in the northern hemisphere) of the house, a solid fence, or the shed. There’s a little wiggle room in your USDA plant hardiness zone if you can take advantage of warmer microclimates.
In late February of 2024, I placed flats of the first cool season greens of the year on the south/southeast side of the enclosed porch. Here, they can grow where the temperatures are 1-3 degrees warmer at night than 15′ from the house and they’re protected from the wind. Now that they’re well rooted, the brassicas and lettuces will tolerate freezing temperatures before they’re sold or set into the garden.
Potted spinach ‘Monstrueux de Viroflay’, a French heirloom.
The USDA Plant Hardiness Zone On the Label
Most vegetables, greens, annual flowers, and herbs are rated “cool season” or “warm or main season” plants. They are not generally assigned USDA plant hardiness zone numbers on their labels. Check with your local agricultural extension office for a planting timetable.
Parsley and cilantro, pansies and violas, kale and green onions are considered cool tolerant. And some will tolerate warm temperatures as well. But basil, zinnias, and bell peppers are warm season plants and cannot tolerate frost or prolonged exposure to temperatures below 50°F. Depending on your climate zone, you’ll be able to grow tender sweet basil outdoors beginning somewhere between February and June.
Each tree, shrub, and perennial is given a zone number, an indication of how much cold it can tolerate in winter. A plant’s zone rating signifies the likelihood of its survival after an average winter. Some plants also are assigned heat zone ratings, which is helpful in hot climates.
Strawberries and raspberries, for example, are hardy in zones 3 or 4 to 7 or 8, depending on variety. Blueberries, as a group, have a wide range of temperature tolerance, growing in zones 3 to 10. Certain varieties, such as highbush and lowbush blueberries, grow toward the cooler end of the range, while rabbiteye blueberries are suited to the warmer southeastern U.S.
A Little More Protection
Cold frame in Maryland.
With simple materials, there are a number of ways to protect cool season greens in winter. From wind barriers to low tunnels to a cool greenhouse, some of these tips will work for you.
On very cold nights, cover with clear plastic sheeting to retain the heat gained during the daytime. Adding a string of miniature incandescent lights will raise the temperature even higher in a cold frame. Don’t forget to open the cover in the morning. Temperatures rise quickly inside a closed frame.
During bitterly cold or icy weather, the clear plastic sheets stay in place in the daytime, without over-heating the plants beneath the cover. Loosely drape the plastic over the crops, allowing excess heat to escape when the sun shines. All of our winter crops bounce back after a night of below-freezing temperatures. I loosely cover them at 25° F to prevent an interruption in their progress and damage to the foliage.
In late afternoon, I’ll secure the plastic if the nighttime temperatures are expected to drop below the low 20’s. Many of the cool season greens can survive with no winter protection in warmer zones. Each variety of cabbage, broccoli, or cauliflower, for example, has a more defined range of tolerance, so choose cultivars carefully.
Another option is to plant a dense evergreen hedge on the north or northwest side of the garden, or wherever your prevailing winter winds come from. Evergreens help diffuse and divert frigid winter winds.
The Sun’s Path
The sun’s zenith rises in the sky as summer approaches and begins to fall again after the first day of summer. Although the shortest day and the first day of winter is December 21, our lowest winter temperatures occur around the third week of January. Similarly, the hottest part of summer occurs a few weeks after the first day of summer.
As you plan the location of your vegetable garden, remember that a winter garden receives fewer hours of light. Try to orient it toward due south for the best exposure.
If there are deciduous trees south of the garden, you might actually get more sun in the winter, when the trees are leafless, than in the summer.
The sun’s path across the sky, in southern North Carolina.
As an avid vegetable gardener, I will find ways to grow a few rounds of greens and vegetables all through the year. I’ll never move to a colder zone if I can’t grow my favorite greens in winter. The average extreme minimum temperature creeping a bit higher on the thermometer makes it just slightly easier to succeed.
In addition to the plant’s hardiness zone, other factors contribute to the tree’s or the perennial’s success or failure in your garden. Time of planting, general health of the plant, ongoing care, and soil condition all have an impact. Begin with the plant’s hardiness zone, though, because that’s the one factor we can’t control.
In a few days, we finally will see some almostnormal temperatures in this part of the southeast. It’s time to think about spring-flowering bulbs that will be planted over the next 2 or 3 months. Get comfortable; this is a long essay on the big topic of spring-flowering bulbs.
It often happens: when the daffodils, tulips, and wonderfully fragrant hyacinths bloom in February, March, and April, that’s when many customers ask for the bulbs. But most bulbs are planted in the falland into early winter. Their roots develop as the soil cools in late summer and autumn, but growth above ground is delayed until months later.
It is no longer uncommon for garden centers to sell potted bulbs in spring. So, if you missed out on the fall planting season, or if you want to see the flower colors before planting, inquire with the growers. Vendors at spring home and garden shows and farmers’ markets also sell pots of budded or blooming bulbs.
Daffodils are reliable, easy to grow, and not eaten by animals.
Many thousands of varieties of bulbs are available to gardeners. Each region of the country has its favorites, based on ease of culture, resistance to hungry animals, and familiarity with what people “usually” grow there.
If you want something more unusual but your local sources don’t carry it, you can probably find it online or through mail order catalogs. Find out what the bulbs require before investing in 2,000 pink tulips that can’t take the heat in your back yard.
On the Subject of Tulips
Tulips are not deer-proof.
Large-flowering hybrid tulips grow in the cooler northern half of the United States, down to Zone 7. A few will perennialize in slightly warmer climates. The smaller species tulips will grow a bit farther into the South.
Tulip bulbs planted among the roots of trees and shrubs might work slightly better than those in open beds. The soil there is somewhat cooler and dryer. They’re not fond of warm temperatures and high moisture levels.
Many gardeners here in the piedmont of North Carolina plant tulips with the expectation of only one glorious show, and then tear them out when flowers fade. Beautiful as they are, tulips in zone 8 or warmer usually do not come back for an encore performance the next year. Even zone 7 is a challenge for them.
And that’s okay with a lot of people, including designers of public parks and municipal common spaces. But, wow, what a show...if you can keep the deer and rabbits away from them! They are especially fond of tulips, and I highly recommended using deer repellents. When using a solution in a sprayer, set the nozzle to a fine mist. A coarse droplet will simply bead off the waxy foliage and flowers without sticking.
Voles, too, eat tulip bulbs. Planting tulips with PermaTill (small, expanded gravel used for drainage) around them usually deters the voles. Stainless steel mesh planting baskets will exclude burrowing animals from the root zone. And squirrels have been known to do a little transplanting of their own. Products are available to help prevent these problems.
How Bulbs Work
Striped ‘Pickwick’ and purple ‘Remembrance’ crocus, in the Maryland garden.
A bulb is a shortened stem, with roots that emerge from the basal plate when the temperature cools in the fall. The basal plate, clearly visible on hyacinths, is a rounded disc of tissue at the bottom of the bulb. Tightly folded undeveloped leaves surrounding the flower shoot contain food for the plant during dormancy. A bulb has all the parts necessary to complete its life cycle: root initials in the basal plate, a growing tip, leaves, stems, and flowers.
The rooting bulbs remain safely tucked underground until the soil begins to warm up in spring. That signals the plants to burst from the ground with their beautiful, cheerful flowers. We bid another winter adieu! Hungry bees and other pollinators emerging on warm spring days feed on pollen and nectar from spring-flowering bulbs, when little else is in bloom.
The foliage must be given enough time to photosynthesize and to store carbohydrates in the bulbs. This food will sustain the plant during dormancy, and ensure that more buds will develop for next year’s flowers. So, after the flowers finish up, let the plants wither naturally. Foliage can be removed after at least half of it has yellowed.
First, a few words on summer- and fall-blooming bulbs
Garden centers stock most of the summer-blooming bulbs in spring to early summer. You can find bulbs, corms, tubers, and rhizomes of lilies, gladiolus, canna, caladium, iris, and calla at that time of the year. Nurseries also offer potted specimens as they come into growth.
Elephant Ears
For dramatic foliage plants, look for elephant ears (Alocasia and Colocasia spp.), which grow only a foot tall to over 6′ tall. These are tropical plants and will die with frost, unless the tubers are dug and stored over winter. Planted in the ground in regions where frost doesn’t penetrate the soil, they usually survive the winter.
Caladium
Caladium, in late spring.
The colorful caladiums are tropical and need warmer soil temperatures in winter in order to survive, even when dormant. Some gardeners dig up and save the tubers every year, but most purchase new tubers or potted plants in spring to mid summer.
Foliage colors include green, white, chartreuse, pink, peach, and red, and can be mottled, edged in a contrasting color, streaked, or spotted. Their spathe-like flowers are not showy and usually removed.
For the past four years, I’ve kept a large Italian terra cotta pot outdoors in the summer. It contains a white caladium, a few kinds of brake (Pteris) ferns, and trailing clumps of Pilea ‘Aquamarine’, with its reddish stems and incredible pewter-blue, rounded leaves. It’s also home to whatever else dropped in. This year: some bulbs of tender Ledebouria socialis (leopard lily), a miniature African violet, and Dendrobium kingianum, a small orchid that has lived in a 3″ clay pot for over 4 decades.
Dormant Caladium
When the weather cools, the whole pot comes indoors to the sunny kitchen, where the caladium goes dormant. One by one, the leaves turn yellow, and the caladium sleeps through the winter. At average indoor temperatures, it remains dormant.
Instead of digging out the tubers, I leave them in the soil, caring for the rest of the plants as needed. Every couple of years, some potting soil is added under the plants, and the pot goes outdoors to light shade. And each year, the caladium comes back after about a month of warm weather. It’s a heavy feeder, so fertilize caladium every 2-3 weeks while it’s in leaf.
Dormant caladium tubers that remain surrounded by soil over the winter are more likely to return the next year than dry, loose tubers kept in a bag. Keep them dry if by themselves in a pot; damp is okay, if in company with other plants.
This variety is always the first amaryllis to bloom.
Amaryllis
Amaryllis (photo, above) is a beautiful late fall to spring flowering plant. Dormant bulbs can be found at garden centers at this time of year, alongside the spring-flowering bulbs. Potted in the fall, Amaryllis bulbs begin blooming indoors around the holidays.
Colchicum Autumnale
Colchicum ‘Waterlily’, nice with….
…purple beautyberry, Callicarpa americana.
This is one of my favorites partly because the plant is animal-proof and partly because it blooms in thefall. Sometimes slugs can be a problem. Scattering Sluggo granules near the Colchicum will take care of that.
The bulbs will appear in some, but not many, garden centers, along with the daffodils and other spring-flowering bulbs. Left on a windowsill, the bulb produces flowers with no encouragement necessary! But I prefer to plant these hardy bulbs in the garden. Colchicum is the source of a potentially toxic pharmaceutical component, so grow with care.
Only the flowers make an appearance in autumn; colchicum foliage arrives in spring and lasts a short period of time.
Placed near a shrub called Purple Beautyberry (Callicarpa dichotoma or C.americana), the Colchicum ‘Waterlily’ flowers perfectly echo the color of the shrub’s berries (photo, above).
Once planted, they soon begin blooming in white (‘Alba’) and shades of lavender-pink. ‘Waterlily’, available from Wayside Gardens, has many petals, pale in color until exposed to light. It has the appearance of a water lily, but it is not an aquatic plant. After 2 years in the ground, one bulb will produce dozens of flowers. This plant is sometimes called “fall crocus”, but it is not to be confused with…
…Fall Crocus
Species crocus bulbs, with saffron on the left.
A few hardy crocuses bloom in the fall. One is saffron crocus, Crocus sativus, which gives us our most expensive spice, harvested from the female flower part, the red style and stigma. Only 3 threads of saffron are laboriously harvested from each flower, explaining its very high cost. Incidentally, saffron has been used as a spice for 3500 years. And for more trivia–saffron crocus is a sterile triploid whose ancestry is open to speculation.
There are other bluish (C. speciosus ‘Cartwrightianus’) and white (C. kotschyanus) fall crocuses available. The lavender-blue color positively glows in the setting sun. This color is rare in the garden at this time of the year, and a welcome surprise when the flowers emerge through the Liriope and other short groundcovers.
Crocus, technically, grows from a corm, but, for simplicity, is often called a bulb.
Page 1: Time For Spring Flowering Bulbs, On the Subject of Tulips, How Bulbs Work, and First, a few words on summer- and fall-blooming bulbs (Elephant Ears, Caladium, Amaryllis, Colchicum Autumnale, Fall Crocus)
Page 2: Color Effects, Formal and Informal, Naturalizing in Lawns, Naturalizing in Meadows, If You’re New To Spring-Flowering Bulbs, When Planting Drifts of Bulbs (Microclimates and Timing, Laying Out the Beds), and Aftercare
Page 4: Hyacinthoides, Tulipa (Tulips In Zones 7 and South, Species Tulips), Hyacinthus, Narcissus (Buying Daffodil Bulbs, Linnaeus, the RHS, and the ADS, Planting Daffodils, Fragrant Daffodils), and Allium
Page 5: Maintenance, Tricks to Hide Maturing Foliage, Stinzenplanten, Forcing Spring-Flowering Bulbs, Rock Gardens, and Don’t Overlook the Little Ones
Yes! You can use strings of miniature lights to prevent frost from damaging tender transplants in the garden.
This article originally was written for springtime applications. But read through, and you’ll see how the same principles work for cold weather gardening in many regions. These lights will keep greens in better condition, sometimes all the way through winter. Your cool season greens can produce for weeks or months beyond the early freezes!
Unpredictable
The weather has been so weird this spring. Some days it feels like June, and others skim the bird bath in ice. Lots of us already planted tender (easily damaged or killed by frost) vegetables and flowers. Without some protection, they will be injured by the cold nights that are still in our forecast.
If the nighttime temperatures are predicted to fall below 50°F, I’ll cover tomato and pepper transplants in the garden. A night or two in the 40’s probably won’t hurt most transplants if they’ve been properly hardened off. But consistently chilly weather will slowly rob the plants of their vigor. Peppers and basil are particularly sensitive to cold soil, and can be killed by it, even if temperatures aren’t close to freezing.
It’s amazing to see snow still flying in some of our northern states. It is beautiful…to watch on TV.
Early winter in the cold frame. Light strings snake around the bottoms of the pots.
Miniature Lights
Incandescent lights, as opposed to LED’s, give off gentle heat. Those 100-bulb strings of miniature incandescent Christmas lights can increase the temperature under cold frames. They also protect tender plants in garden beds when covered with plastic sheets to hold the warmth.
Potted plants under cover, warmed by miniature incandescent lights.
This is especially beneficial in those locations where an upside-down bushel basket will not sufficiently ward off the cold. And I’m not suggesting that you attempt to defy good garden sense by planting peppers in February in Minneapolis. There are limits!
When I calculated the wattages, square foot coverage, and costs, the miniature lights were the most economical option for my purposes. Each 100-bulb string provides warmth equivalent to a 40-watt incandescent bulb. I also considered heat mats and heat cables. Have a look around hardware stores and see what garden centers have available. Look for ideas online or with mail order companies selling farm, garden, horticultural, or greenhouse supplies.
Use “indoor/outdoor” products that are safe to use in humid environments. Space heaters designed for indoors are not good candidates outdoors. If you’re planning a project outside your comfort zone, check with licensed contractors, especially for electrical work.
(***Update***: Above average temperatures in the Carolinas carried into mid-fall, so the amaryllis bulbs refused to die down. Recent nighttime readings in the low to mid 20’s would have damaged the potted bulbs, so they’ve been enjoying temperatures in the mid 40’s, thanks to miniature lights and a plastic cover. Not forcing early dormancy on the plants gives them extra time to photosynthesize and, perhaps, an additional flower bud. Finally, the leaves are beginning to yellow, and soon they’ll come indoors to rest and to begin the bloom cycle. December, 2018)
And More Bright Ideas
Include snow loads and wind events (they will happen) in your plans. Slope the top of a cold frame away from the house to allow run-off. During the occasional heavy snowstorm in Maryland, I got up in the middle of the night to sweep off snow that had accumulated 8-12″. Toby, our second Rhodesian ridgeback, thought this was lots of fun. Although polycarbonate is very durable and will bend with weight, too much distortion will allow frigid air to enter and might collapse the frame.
There are more permanent alternatives to consider.Do-it-yourself greenhouse kits could be used specifically for fall and winter crops, heated or unheated. Familiarize yourself with local weather patterns to decide what’s feasible…or necessary. You might be able to convert a shed or construct a lean-to by adding a few materials readily available from the hardware store. Contact your local agricultural extension agent for practical suggestions or check out the numerous online videos.
Cold Frames With Miniature Lights
Setting up cold frame with new strings of miniature lights.
Cold frames are simple glazed structures that give extra protection to plants when the weather is unsuitable for planting out. Young seedlings or transplants can be hardened off here until they’re ready for the garden. Also, plants that normally go dormant or die in winter can be kept green in a frame. And edible cool season vegetables can grow all winter in a cold frame where it’s too cold out in the open.
Try to orient the frame’s exposure toward the south (in the northern hemisphere), which will give the greatest benefit in terms of heat gain. Bricks on cinder blocks (photo, right) elevate the north edge of the glazing for rain run-off and a more favorable aspect to the sun.
Each hour of direct sunlight shining on the frame keeps the temperatures elevated and stores warmth in the ground. If possible, locate the cold frame out of the wind and in a warm microclimate on the south side of the house or shed.
Materials
Some gardeners use lengths of sturdy wire, arched and stuck in the ground, covered with clear plastic secured by fasteners. Or use long pieces of PVC tubing anchored over rebar sunk in the ground. Repurpose old storm windows or doors (without lead paint) propped on a support.
“Low tunnels” or “caterpillar tunnels” are available at garden centers or online, complete with supports and coverings.
For the large frame in Maryland (photos), I used rigid twinwall polycarbonate panels, covered with clear plastic film to close the sides. On very cold nights, I added more sheets of plastic, separated by “netted” plant flats that trapped air between the layers. This created dead air space and better insulation.
Cold frame in MD, vented in the daytime.
Houseplants
Oxalis, succulents, stellar geraniums, a variegated fig tree, and other houseplants (photo, top) survived the winter, although I did not expect them to grow. When cyclamen plants went out of bloom in the house, I moved them to the cold frame, where they happily set buds once again.
Plants that do not tolerate the slightest chill, such as African violet, Alocasia, pothos, philodendron, and basil, would not survive very long at temperatures below 60°F. They stayed indoors for the winter.
“Frame to Table”
Without using miniature lights in Maryland’s colder winters, I would not have been able to harvest this assortment of greens. I grew lettuces, leeks, arugula, hon tsai tai, mustard spinach, dinosaur and ‘Red Russian’ kales, and spinach. And Swiss chard, tatsoi, pac choi, mini broccoli ‘Happy Rich’ in wide, shallow pots, about 6″ deep (such as those in photo). ‘Nabechan’ bunching onions and some greens grew in the ground on one end of the cold frame, while potted crops sat on the brick surface.
Greens growing in wide bowls.
Dense materials, such as brick and stone, absorb heat during the daytime and release it at night. I threaded strings of miniature lights between pots, on top of the brick patio. Broken light bulbs cause part or all of the string to fail, so be careful with them. Pots of greens and herbs placed close to the edge of the frame allowed easy access.
Herbs grew all winter in this cool but frost-free environment. Parsley, oregano, sweet marjoram, rosemary, thyme, chives, sage, French tarragon, and bronze fennel are a few of the herbs I grew in the frame. Cold frames are great for half-hardy species and those that normally stay dormant in winter. Italian oregano, tarragon, and chives, for example, retained their foliage instead of going fully dormant.
I added an adapter to the porch light socket near the patio door, and the lights’ extension cords plugged into that. With a simple flip of the switch from the cozy kitchen, the lights turned on or off. That’s one advantage of building your frames close to the house.
Remember to vent the frame on sunny days, turning lights on or off as necessary. Close it up by mid- to late afternoon.
Water as necessary, keeping the soil damp, but not wet. During mild periods of the season, add a dilute dose of fertilizer to those plants (mostly the greens) that are growing.
Power Outages
Have a contingency plan during power failures if you’re going to keep tender plants in your cold frame. Use old blankets (keep them dry), more layers of plastic, and gallon jugs filled with hot water. When this happened a few times in MD, I heated water on the wood stove for those hot water bottles and brought the most tender plants indoors.
Snow or straw piled up around the sides and a layer on top holds in a substantial amount of heat. Clear the top in the morning to allow sun to reach the plants. But if the weather is especially harsh and overcast, the plants won’t mind staying covered for a day or two.
The Maryland Greenhouse
The small greenhouse in Maryland.
I built this greenhouse from a kit purchased from a horticultural supply company. It was fairly easy to put together, although some of the pre-drilled holes didn’t quite match up and the instructions weren’t great. But the twinwall polycarbonate and sturdy framing were able to withstand considerable weight from ice and snow. The structure was attached to a foundation, which was optional but recommended.
This is where I kept more flats of tender succulents. Placed on 2 x 2″ x 8′ posts propped up on cinder blocks near the ends, the flats were heated from below with strings of miniature lights. They had the additional protection of 1 or 2 layers of clear plastic secured on the sides and suspended a few inches over the plants. When nighttime temperatures plummeted to single digits, I added a couple of old bedsheets and another layer of plastic, and then removed them once the sun hit the greenhouse. None of the plants froze.
A second heavy duty indoor/outdoor extension cord reached the porch light adapter near the cold frame, so I could turn all the lights (in the cold frame and in the greenhouse) on or off from indoors.
Double Bubble
This extra “bubble” of lightly warmed air made it possible for frost intolerant plants to survive our zone 6b winter. A bubble inside a tightly sealed unheated greenhouse (or other structure) feels up to 2 or 3 zones (up to 20-30°F) warmer at night than outside air, and even warmer with miniature lights. Keep in mind, though, that each layer reduces the amount of light reaching the plants.
Two or three rows of tender plants—all under a single bubble—occupied the center of the greenhouse. Near the perimeter sat frost tolerant cool season greens in large black nursery pots. Dark colors absorb the sun’s energy, keeping soil unfrozen for longer periods of time. Large pots work better than small pots because they store more warmth. Sometimes I stored bales of potting soil, wrapped in black plastic, on the north edge, where the winter sun would not be blocked.
States in the middle and northern parts of the U.S., in USDA zones 4-6, benefit greatly from an extra barrier of protection from frigid temperatures. Cold hardy plants are more likely to survive than tender species in spaces with no supplemental heat.
Here in North Carolina
Since moving to Charlotte, North Carolina (USDA zone 7b/8a) in 2013, I’ve used clear plastic and miniature lights to ripen up the last of the summer vegetables. And they keep cool season greens and vegetables growing through the worst of our winter weather. To avoid any surprises, check the weather forecast daily.
Succulents that moved with me from Maryland spend the winters in lighted cold frames after filling the sunny windows indoors (photo, right). Delicate species stay inside.
One year, an HVAC contractor came to check the heat pump, located near the lower patio. There were several flats of plants in a second cold frame down there, with miniature lights. That frame was not easily visible from the house. A couple of days later I discovered that the contractor had inadvertently disconnected the extension cord feeding power to those lights…and life to the plants! You know the rest of that story.
(***Update***: I just finished enclosing the sunny front porch at my new home in northern North Carolina, where I moved in October, 2021. This is where the succulents and other plants will spend the winter, aided by miniature Christmas lights. December, 2022)
Cautions
Cold frame covered with clear plastic and weighed down.
Make sure the miniature lights are rated for indoor/outdoor use, and that extension cords are also safe to use outdoors. I’ve been warming up the cold frames and garden plants like this for decades without mishap, except as noted above.
The tender succulents and bowls of mixed greens and herbs covered during the winter survived without damage. On nights that fell below 15°F, the temperature in the cold frame stayed in the high 30’s or 40’s, depending on the number of lights used.
Plants survived single-digit temperatures, helped by the old quilts and extra plastic placed over the frame at night. When needed, I added another string of lights. Placing the frame cover fairly close to the tops of the plants limited the volume of air those lights needed to heat. Not too close, though, or the foliage will feel the cold.
This works like a charm, but only if there’s a power source nearby. Be careful; don’t use extension cords that can’t carry the demand for power. Don’t hook up more than 2 or 3 light strings together.
There are few products on the market that demonstrate the phenomenon of planned obsolescence as clearly as cheap imported miniature lights. Stock up on them when they’re plentiful. They don’t last more than a couple of years with frequent use, and they’re becoming increasingly scarce in favor of LED’s. Tread carefully!
Lastly, instructions on the packaging recommend washing hands thoroughly after handling strings of lights. There might be some lead in the wire covering.
Miniature Lights Preferred
Mini lights warm up 2 large pots of young tomatoes.
There are other types of incandescents available but watch the wattage! If bulbs are hot to the touch, the plants will think so, too, so don’t place them close to the plants. The preferred miniature lights are barely warm, so they distribute the heat evenly and gently.
Make sure the bulbs are not in contact with the plastic covering or the leaves. Avoid letting the plastic cover touch the leaves; cold transfers through the plastic and will damage foliage.
It’s best to lay the strings on the ground, around—but not touching—the stem. The warmth will rise. For potted plants, wind some of the lights around the pots. Tall plants might require lights up into their branches.
If you’re not really trying to push the envelope, the plastic bag or old sheets probably will do the trick. It’s when that sudden blast of uncommonly cold weather comes around that prompts us to take extra measures.
(***Update***: In the photograph above, mini lights kept the young potted tomatoes [in 20″ pots] and a few other plants 15° warmer on an unusually cold spring night. I placed the miniature lights on the soil surface and wound them around the pots to keep the roots a bit warmer. Plastic was closed and secured with clips. April, 2019)
Out of Reach
Where lights are impractical, there is a solution. Use large plastic mulch or trash bags, and tuck in a hot water bottle. Seriously! A repurposed gallon milk jug filled with very hot water slowly gives off heat through the night. A little experimentation will tell you if the heat is given up too quickly (wrap the jug in an old towel to slow heat loss) or insufficiently (use 2 milk jugs or larger containers). Don’t place the hot water close to the foliage. Keep this in mind when the power goes out.
Those “water wall” products you see at garden centers work on the same principle. Water that heats up in the sunlight slowly gives up that heat to the plant at night.
Plastic bags that are nested, or layered, give better protection than a single layer. Spread the opening over a wide footprint (soil’s square footage) to capture heat rising from the ground. Or simply secure a large plastic sheet to a tall stake in the ground. Weigh down the covering with rocks or bricks to keep it in place and to seal out the cold. Remove the cover in the morning.
* * * * * * * * *
I hope this information gets your garden successfully through the “shoulder” seasons, when temperatures can fluctuate wildly. Just a few readily available and inexpensive materials can prevent a total loss.
Later in the year, when autumn temperatures tumble, having materials ready for duty will extend the season. Rolls of 4-mil clear plastic—actually somewhat translucent—can be found in the hardware store’s paint department. This product (4- or 6-mil plastic) is less expensive than greenhouse film. If, however, you opt for permanent structures, the longer lasting film will be worth the investment.
If you’ve never grown cool season vegetables, these methods will open up a new world of growing opportunities. You can extend the season before and after the summer crops or protect tender ornamentals through the winter. No need to retire the trowel with the first fall frost!
Young broccoli ‘Arcadia’ in 20″ pot, where a tomato grew in the summer. Sharp-tipped pinecones discourage squirrels.
Just when you thought it was safe to plant tomatoes!
Microclimate
C and F.
Call it impulse or ignorance, the natural inclination for most of us gardeners is to push the season. I’ll admit it; I planted tomatoes and a sweet pepper here in North Carolina about 2 weeks ago, knowing very well that I might have to protect them at night.
But I’m ready, with plastic bags and bamboo stakes, and large sheets of clear plastic, if needed. Plus, the garden is located close to the warmer south side of the house. The sun’s warmth collects in the soil and in the wall, releasing it slowly over the nighttime hours. Peppers are more sensitive to chilly temperatures, so they get covered first.
Even though this cozy little microclimate gives a few degrees of protection, I’m not about to cause any discomfort to my new plants, nor to check their growth. So, I covered them when the nighttime temperature was predicted to fall below 50°F. That’s not even close to freezing, but that’s what I mean. I don’t want them to get close to freezing. And neither do they!
Incidentally, research indicates that tomatoes and peppers might yield more fruit if they have been exposed, when young, to temperatures in the high 40’s to low 50’s. But those plants will need to have been fully hardened off in order to withstand the less hospitable environment.
Microclimate: Where To Find It
Maybe your property has a warmer microclimate that will protect newly planted flowers or vegetables. Close to the south side of the house, a shed or garage could work. Or the sunny side of a solid fence. Perhaps your vegetable and cutting gardens are surrounded by massive stone walls…perfect. A dense hedge will cut wind speed and offer some protection.
A sunny porch, carport, or the protected corner of a patio might keep those young plants happier than if they were planted out in the garden just now. Who isn’t tempted by those first plants offered at garden centers, even though we know it’s way too early?
Direct sunlight warms the soil and surrounding structures. The area needs to be open to the southern sky (if you live in the northern hemisphere). A garden on the south side of a brick wall is great, but not if it’s shaded by a white pine tree. Dark colors absorb more of the sun’s energy, so they’ll release more warmth at night.
You can create a microclimate simply by covering sensitive plants with old bedsheets or sheets of plastic before the sun sets. Prop them up so the material is not touching the tender foliage. During the shoulder seasons, temperatures can fluctuate between seasonal and cruel. There are limits to the degree of protection afforded by microclimates.
Another option is to construct a temporary cold frame, with some sort of a support (bales of hay, cinderblocks, wooden boards) and a clear covering (lead-free old windows, patio door, clear rigid plastic, or plastic film). The cold frame can stay in place for as long as it serves your purpose, but you’ll need to open the covering to let cool, fresh air inside. Heat builds up quickly under clear glazing.
Always Check the Weather Forecast
In this Zone 7b location, April 15th is considered “safe” for most tender, main season edibles and flowers, but it is always advised to check the weather forecast daily. If the forecast had called for a long stretch of cold rainy weather, I would have delayed planting. No one wants to be planted in cold, wet soil.
More sensitive species, such as cucumber, zucchini, and basil, will not be planted until the soil has warmed up considerably.
Diascia ‘Flirtation’ is one of my favorites for the shoulder seasons, when it could be warm or frosty.
Among flowers, caladium, scaevola, streptocarpella, zinnia, and vinca are a few that prefer warm soil before planting out. Their foliage will wilt or show damage even if the temperature hasn’t reached the freezing point.
The alyssum, snapdragon, and diascia, however, did just fine without protection, and even prefer cooler temperatures.
Frost occurs first out in the open—in the middle of the lawn or on the rooftops. Plants that can tolerate cool temperatures will be unharmed in a protective microclimate. They might be damaged, however, if they’re far from the house and subjected to a freeze. Just 5 or 10′ can make a difference!
Frost Last Night
This morning I was surprised to see frost on the rooftops and ice in the puddles on top of the cold frame. It wasn’t “supposed” to get below 35°, but the temperature dropped well below freezing. The plants in the cold frame and in the garden are fine. Late yesterday afternoon, I covered the tomatoes and the pepper with large, heavy plastic bags (with more plastic bags layered inside for insulating dead air space) supported by a couple of sticks in the ground.
The larger the “footprint” of the covering, the better; the larger area of covered soil allows more heat stored in the ground to rise. And it was breezy yesterday, so a couple of bricks held the plastic in place.
Don’t forget to uncover the plants in the morning, as soon as sunlight reaches the area. It doesn’t take long for the inside air temperature to rise high enough to stress the plants.
There are some tricks you can use to fool…or assist…Mother Nature, and I’ll write about those in the future. You might want to adopt some common sense methods of protecting plants to stretch the season. In addition to taking advantage of microclimates, one of those tricks involves strings of miniature incandescent lights, like the ones you strung on the Christmas tree. So don’t pack them away in the attic just yet!
Since moving to Charlotte, North Carolina (USDA zone 7b/8a) in 2013, I’ve used clear plastic and miniature lights to ripen up the last of the summer vegetables. And they keep cool season greens and vegetables growing through the worst of our winter weather. To avoid any surprises, check the weather forecast daily.
There are few products on the market that demonstrate the phenomenon of planned obsolescence as clearly as cheap imported miniature lights. Stock up on them when they’re plentiful. They don’t last more than a couple of years with frequent use, and they’re becoming increasingly scarce in favor of LED’s. Tread carefully!
