A cold frame is the most underused piece of garden infrastructure in most backyards. Gardeners build or buy one, use it for a few weeks in spring to harden off transplants, then ignore it for the rest of the year. This is like buying a car and only driving it in April. A well-managed cold frame can be productive for ten or eleven months of the year, providing fresh greens through winter, starting transplants weeks earlier than indoor lights alone, extending fall harvest deep into December, and even serving a useful purpose during the summer months when most people assume it should be stashed in the shed.
The key to getting year-round value from a cold frame is understanding that it is not a single-purpose tool but a microclimate management system. In January, it is a miniature greenhouse that traps solar heat and protects hardy greens from wind and deep freezes. In March, it is a seed-starting station that gives transplants a head start over anything started indoors. In June, it is a curing box for garlic and onions or a propagation chamber for cuttings. In September, it is a season-extension device that lets you start a second fall crop cycle. The structure does not change — your management of it does.
This calendar is based on cold frame performance in USDA zones 5 through 7, which covers the majority of the continental United States where cold frames are most useful. If you garden in zones 3 or 4, shift the timeline about two to three weeks later in spring and two to three weeks earlier in fall. If you garden in zones 8 or 9, you may find the cold frame more useful as a summer shade frame and rain protection than for cold protection. Adjust to your local conditions, but the monthly framework applies everywhere that has a winter.
January: Harvesting Winter Greens and Forcing Early Growth
January is the coldest month in most of the country, and it is the month that proves the value of a cold frame. If you planted the cold frame properly in September and October (covered later in this calendar), you should have established plants of cold-hardy greens that are alive, green, and harvestable even when outside temperatures drop to single digits.
The crops that perform best in a January cold frame are spinach, mâche (corn salad), claytonia (miner's lettuce), and certain varieties of lettuce — particularly Winter Density, Rouge d'Hiver, and Arctic King. These varieties were selected specifically for cold tolerance and short-day performance. They will not grow much in January — the short days and weak sunlight limit photosynthesis regardless of temperature — but they will hold in good condition, and you can harvest outer leaves for fresh salads throughout the month.
Kale and tatsoi are also excellent January cold frame crops. Both are extremely cold-hardy (surviving into the single digits with cold frame protection) and both actually improve in flavor after frost exposure. The cold converts starches in the leaves to sugars, which is why frost-kissed kale tastes sweeter than summer kale. Harvest outer leaves and the plants will continue producing from the center as daylight increases in February and March.
Vent management in January is minimal. On most January days, you can leave the cold frame closed. However, on sunny days when the outside temperature rises above forty degrees, crack the lid an inch or two to prevent condensation buildup, which promotes fungal disease. If your cold frame has a thermometer (and it should — a simple min/max thermometer is one of the best five-dollar investments in the garden), open the lid when the interior reaches sixty degrees and close it when the outside temperature starts dropping in the afternoon.
This is also the time to start planning your spring cold frame succession. Order seeds for the fast-growing crops you will direct-sow in the cold frame starting in February — radishes, lettuce, spinach, and mesclun mixes. Having seeds on hand and a planting plan written out means you can act as soon as conditions allow rather than waiting for a seed order to arrive.
February: The Transition Month — First Sowings
February is when the cold frame begins its transition from harvest mode to production mode. The increasing day length — gaining about two minutes of daylight per day in early February and accelerating through the month — triggers growth in overwintered crops and makes new sowings viable for the first time since fall.
In the first half of February, continue harvesting overwintered greens. The plants will start showing new growth as the light increases. Cut-and-come-again harvesting works well — take the outer leaves and let the center grow. Spinach that has been static since December will suddenly start pushing new leaves, and the fresh growth is tender and sweet.
In the second half of February (zones 5–7), you can make your first direct sowings of the year inside the cold frame. The cold frame soil is typically ten to fifteen degrees warmer than exposed garden soil, which means seeds that need soil temperatures of forty to fifty degrees for germination can go in the cold frame while the outdoor garden is still frozen or waterlogged. Sow radishes (ready in 25–30 days), lettuce (harvest baby leaves in 30 days), spinach (45 days), and arugula (30–40 days). Scatter seeds thickly in rows or broadcast across the available space — you will thin as the seedlings grow.
Ventilation becomes more important in February as the sun gets stronger. Even on days when the outside temperature is in the thirties, a closed cold frame in direct sun can reach seventy or eighty degrees inside — too warm for the cool-season crops you are growing. Open the lid by mid-morning on any sunny day and close it in late afternoon. If you cannot be home during the day, a solar-powered automatic vent opener is invaluable — it opens the lid when the wax cylinder heats up and closes it when it cools, maintaining a comfortable temperature range without any attention from you.
March: Seed Starting Season Begins
March is the most active month in the cold frame calendar. The combination of lengthening days, strengthening sun, and cold frame protection creates ideal conditions for starting a wide range of crops from seed — many of them weeks before outdoor sowing is possible.
Continue direct sowing cool-season crops inside the cold frame: lettuce, spinach, radishes, carrots (yes, carrots germinate well in a cold frame — the consistent moisture and warmth speeds up their famously slow germination), beets, and turnips. Succession sow lettuce and radishes every ten to fourteen days for a continuous supply. The cold frame gives these crops a protected start and produces harvests four to six weeks earlier than outdoor plantings.
Start transplants for the outdoor garden. Sow broccoli, cauliflower, cabbage, and kale seeds in cell trays or small pots inside the cold frame. These brassicas need cool temperatures (sixty to seventy degrees) for germination and strong early growth, and the cold frame provides exactly that without the leggy, etiolated growth that plagues indoor seedlings under weak lights. Onion sets and leek seedlings also benefit from cold frame starting — they transplant better as stocky, well-rooted plants than as spindly indoor-grown seedlings.
In late March (zones 6–7) or early April (zone 5), you can start tomato, pepper, and eggplant seeds inside the cold frame if you can maintain nighttime temperatures above fifty degrees. In practice, this usually means starting these warm-season crops indoors under lights and moving them to the cold frame once they have their first true leaves. The cold frame then serves as a hardening-off station — the plants get real sunlight, natural temperature fluctuations, and wind exposure that toughens them for the outdoor garden.
For product options that pair with your cold frame setup, our cedar cold frame and white oak cold frame are designed to fit standard raised bed dimensions and include automatic vent openers.
April: Hardening Off and Transitioning Out
April is the bridge month between the cold frame growing season and the outdoor growing season. The cold frame's primary role shifts from growing crops to hardening off transplants that will soon move to the open garden.
Hardening off is the process of gradually exposing indoor-grown seedlings to outdoor conditions — direct sunlight, wind, temperature swings, and lower humidity. Plants that go directly from a climate-controlled indoor environment to full outdoor exposure suffer transplant shock: sunburned leaves, wind-snapped stems, and wilting from the sudden increase in transpiration demand. The cold frame is the ideal hardening-off tool because you can control the exposure incrementally.
Start by placing transplants in the cold frame with the lid mostly closed. Over the course of seven to ten days, progressively open the lid more during the day, starting with a two-inch crack and working up to fully open. Leave the lid cracked at night starting around day four or five (weather permitting). By the end of the hardening-off period, the transplants are in a fully open cold frame experiencing the same conditions as the outdoor garden, and they are ready to transplant with minimal shock.
April is also a major direct-sowing month in the cold frame. If you have not already started carrots, beets, turnips, and second successions of lettuce and spinach, sow them now. The cold frame soil in April is warm enough for reliable germination of all cool-season crops, and the protection from April rain and wind gives seedlings a gentle start. Start your first outdoor sowings alongside the cold frame plantings — you will be able to compare the growth rates and see exactly how much advantage the cold frame provides.
Begin harvesting radishes and baby lettuce from February and March sowings. These quick crops are ready in twenty-five to forty-five days, and the cold frame environment produces tender, sweet leaves that rival anything from the farmers market. Pull radishes when they reach the size of a large marble — cold frame radishes tend to be milder and more crisp than midsummer radishes because they grow in cool, even conditions without heat stress.
May and June: Summer Transition
By May, the cold frame has largely completed its spring duties. Outdoor temperatures in zones 5–7 are warm enough for direct sowing and transplanting without protection, and the cold frame can become dangerously hot if left closed on warm days. Many gardeners simply remove the lid and store it until fall, and there is nothing wrong with that approach. But a cold frame without its lid — or with the lid propped fully open — still has uses.
In May, the bottomless box of the cold frame makes an excellent slug and wind barrier for newly transplanted warm-season crops. Place tomato, pepper, or eggplant transplants inside the open cold frame for their first week or two in the garden. The walls block wind (which is a major stress on transplants) and provide a physical barrier against slugs and cutworms. Remove the plants or the cold frame walls once the transplants are established and growing vigorously.
In June, use the cold frame as a curing station for garlic and onions. Lay freshly harvested garlic bulbs or onions on a wire rack inside the cold frame with the lid propped open a few inches. The glass or polycarbonate lid keeps rain off while allowing air circulation, and the solar heat speeds the curing process. Two to three weeks of curing in a ventilated cold frame produces well-dried garlic with papery wrappers that stores for months.
You can also use the cold frame in summer as a propagation chamber for plant cuttings. Close the lid to create a humid environment, place pots of cuttings inside, and the high humidity and warmth encourage rooting. Shade the lid with a piece of burlap or shade cloth to prevent overheating — you want warmth and humidity, not a solar oven. Herb cuttings (rosemary, lavender, sage) and perennial flower cuttings root well in a summer cold frame.
July and August: Off-Season Maintenance and Planning
July and August are the cold frame's quietest months. If you are not using it for curing or propagation, remove the lid entirely and store it out of direct sunlight to preserve the glazing material. Use this time to inspect the cold frame box for needed repairs: tighten loose screws, sand rough spots that could splinter, apply a fresh coat of linseed oil or exterior sealer to wooden frames, and replace any weatherstripping around the lid seal.
This is also the time to plan your fall cold frame planting schedule. The most productive cold frame seasons are fall and winter — the months when the cold frame provides the greatest advantage over unprotected outdoor growing. Decide which crops you will plant in September (lettuce, spinach, mâche, claytonia, Asian greens), order seeds if needed, and prepare the cold frame soil. Remove any summer weeds, add an inch or two of finished compost, and lightly work it into the top few inches. If the soil has become compacted from spring use, loosen it with a garden fork.
By late August, you should be watching the weather forecasts and thinking about your first fall cold frame sowings. In zones 5–6, the target is to have seedlings established before the first frost, which typically arrives in mid to late October. Working backward from a first frost date of October 15, you need seedlings of lettuce and spinach in the ground by mid-September at the latest, which means sowing in the cold frame in early September or starting seeds in trays in late August for transplanting.
September: The Second Spring
September is the most important planting month for cold frame gardeners. What you plant now determines what you will harvest from November through March. Think of September as a second spring — the same cool-season crops that thrive in March and April grow beautifully in September and October, and the cold frame carries them through the winter months that follow.
Direct sow in the cold frame: lettuce (Winter Density, Rouge d'Hiver, Arctic King, and other cold-tolerant varieties), spinach (Bloomsdale Long Standing, Giant Winter, Space), mâche (Vit is the standard variety), claytonia (also called miner's lettuce), arugula, tatsoi, and mizuna. Sow thickly — you can thin later, and dense plantings shade out weeds. Water well after sowing and keep the cold frame lid open during the day — September temperatures are too warm for closed-frame growing.
Transplant into the cold frame: kale seedlings started in August, lettuce starts from late-August sowings, and any other cold-hardy brassicas you want to carry through winter. Set transplants slightly closer together than outdoor spacing — the cold frame's protected environment and the short-day growing conditions mean plants will stay smaller than their summer counterparts.
Leave the cold frame lid off or fully open throughout September. The crops do not need protection yet — they need the full sun and air circulation of early fall to establish strong root systems and put on as much growth as possible before the short days of November and December slow everything down. The goal is to get plants to near-harvest size by late October, because after that point, growth nearly stops and you are essentially harvesting from a living refrigerator.
October: Closing Down for Winter Protection
October is the transition from open-frame growing to closed-frame protection. Watch your local forecast closely — the first frost can arrive any time from early October in zone 5 to late October in zone 7. When nighttime temperatures consistently drop below freezing, it is time to put the lid on and begin managing the cold frame for winter.
Replace the lid when nighttime lows reach the upper twenties. Close the frame in the late afternoon to trap the day's heat, and open it on sunny mornings once the interior warms above fifty degrees. This daily open-and-close cycle is the rhythm of cold frame management from October through March — trapping heat when you can, venting when you must, and keeping the interior temperature in the range where your crops stay alive and harvestable.
Continue harvesting throughout October. The crops you planted in September should be reaching harvest size: lettuce for salads, spinach for cooking and salads, radishes from succession sowings, arugula for peppery greens, and tatsoi for stir-fries. Harvest by cutting outer leaves and letting the plants continue growing from the center. This cut-and-come-again approach provides a continuous supply rather than a single large harvest.
Make your last sowings of the year in early October: fast-growing lettuce varieties and radishes can still produce a harvest before the short days slow growth. Mâche and claytonia are also good last-chance sowings — they are extremely cold-hardy and will germinate in cool soil, establishing just enough before winter to provide harvests in January and February.
Add a layer of mulch — straw or dry leaves — around the base of the cold frame to insulate the soil from the sides. The soil inside the cold frame is your thermal battery, and insulating it helps retain heat through cold nights. If you expect temperatures in the single digits or below, throw an old blanket or heavyweight row cover over the closed cold frame at night for extra insulation. Remove it in the morning to allow light in.
November and December: Winter Harvest Season
November and December are pure harvest months in the cold frame. No new sowings, no transplanting — just daily or weekly harvests of the crops you established in September and October. The plants are alive and green, but growth has effectively stopped due to the short days (less than ten hours of daylight) and low light angles. You are harvesting stored production, not new growth, which is why it was critical to get plants to near-mature size before November.
Lettuce, spinach, mâche, and claytonia are the four best winter cold frame crops. All tolerate repeated freezing and thawing without damage, all taste excellent when grown in cold conditions (cold-grown greens are noticeably sweeter and more tender than summer-grown equivalents), and all hold in harvestable condition for weeks without bolting or deteriorating. A single planting of mâche made in September will provide handfuls of sweet, nutty-flavored rosettes from November through March with no attention beyond opening and closing the lid.
Kale and tatsoi add variety to the winter cold frame harvest. Both are extremely cold-hardy and both benefit from the flavor improvement that cold exposure brings. Harvest kale leaves for smoothies, soups, and sautés. Tatsoi's flat rosette shape makes it particularly well-suited to cold frame growing because it stays low and does not crowd the lid.
Vent management in November and December is straightforward: open the lid on any sunny day when the interior temperature exceeds fifty-five degrees, and close it by mid-afternoon. On overcast days or days when the temperature stays below freezing all day, leave the lid closed. Moisture management matters more than temperature management in winter — condensation dripping from the lid onto leaves can cause crown rot and fungal problems. Cracking the lid an inch on dry, calm days helps reduce condensation even when temperatures are marginal for full venting.
By the time you are harvesting fresh greens from your cold frame while the outdoor garden is frozen solid under snow, you will understand why cold frame enthusiasts consider these simple boxes the highest-value tool in the garden. The total cost of a cold frame plus seeds is less than you would spend on a few weeks of winter salad greens from the grocery store, and the quality of cold frame lettuce — crisp, sweet, and harvested minutes before eating — is incomparably better than anything shipped from California or Arizona.
Frequently Asked Questions
Can I grow root vegetables in a cold frame through winter?
Yes, with a modification to your approach. Root vegetables like carrots, beets, and turnips need to be sown in mid to late summer so they reach mature size before growth stops in November. Once mature, the cold frame acts as a refrigerator, keeping the roots in perfect condition in the ground for months. Dig as needed through winter. Carrots stored this way in a cold frame are sweeter and crisper than anything you will find in a store — the cold converts starches to sugars, and the soil keeps them perfectly hydrated.
How cold is too cold for a cold frame?
A well-built cold frame with cold-hardy crops inside can handle nighttime lows in the single digits Fahrenheit without losing plants. Below zero, you risk killing even the hardiest greens. Adding thermal mass inside the cold frame — a few gallon jugs of water painted black — helps moderate temperature extremes by absorbing heat during the day and releasing it at night. Throwing an insulating blanket over the cold frame on the coldest nights adds another five to ten degrees of protection. In zones 3 and 4, a cold frame alone may not be sufficient for winter growing, but it will still extend the season by several weeks on each end.
Do I need to water plants in a cold frame during winter?
Usually very little. The enclosed environment reduces evaporation dramatically, and cold-season crops have low water demands because their growth rate is minimal. Check soil moisture weekly by sticking your finger an inch into the soil. If it feels dry, water lightly — just enough to moisten the top couple of inches. Overwatering is a bigger risk than underwatering in a winter cold frame because excess moisture promotes the fungal diseases that are the primary threat to winter greens. Water on a sunny morning so any moisture on the leaves can dry during the day.
What is the minimum sun exposure needed for a cold frame?
A cold frame needs at least four to six hours of direct sun during the shortest days of winter to maintain reasonable growing conditions. South-facing exposure is ideal. If your only available location gets less than four hours of winter sun, the cold frame will still work for fall extension and spring starting, but winter production will be limited to the hardiest, most shade-tolerant crops — mâche and claytonia will survive on surprisingly little light, while lettuce and spinach need more to remain productive. Avoid placing a cold frame under deciduous trees that drop leaves onto the lid, blocking light and adding weight.
Can I stack row covers inside a cold frame for extra protection?
Absolutely, and this is one of the most effective cold-protection strategies available. A heavyweight row cover draped directly over plants inside a closed cold frame creates a double layer of protection — the cold frame handles wind, rain, and the first tier of insulation, while the row cover adds another six to ten degrees of frost protection. This combination can keep plants alive and harvestable through temperatures well below zero in many cases. The row cover also reduces condensation drip from the cold frame lid, which helps prevent the fungal problems that are the main winter threat to cold frame crops.