By: Teo Spengler
Cold frames protect your crops from the cold weather and frost of autumn. You can extend the growing season several months with cold frames and enjoy fresh veggies long after your outdoor garden crops are gone. Read on for more information on fall gardening in a cold frame, as well as tips on constructing cold frames for fall.
Autumn cold frames work like greenhouses, sheltering and insulating tender plants from chilly weather, breezes and frost. But, unlike greenhouses, cold frames for fall are easy to construct yourself.
A cold frame is a simple structure. It isn’t “walk-in” like a greenhouse, and its sides are solid. This makes it easier to build. Like a greenhouse, it uses the energy of the sun to create a warm microclimate in a chilly garden, a place where crops can thrive as the weather turns cold.
When you extend the growing season with cold frames, you can grow fresh greens or bright flowers well past frost. And autumn is the perfect time to allow cold frames and frost to coexist. But keep in mind that some plants grow better in cold frames than others. Those that work the best are low-growing, cool-season plants like lettuce, radishes and scallions.
Expect a cold frame to extend your growing season up to three months.
The attraction of fall gardening in a cold frame starts with a longer growing season, but that isn’t all. If you install cold frames for fall, you can overwinter tender plants that won’t make it on their own through winter.
And the same autumn cold frames can serve in late winter to start seeds before the last frost. You can also harden off young seedlings in a cold frame.
When you decide to extend the growing season with cold frames, you first must purchase or build a frame or two. You’ll find innumerable varieties available in commerce, but it’s cheaper and more ecological to make your own from materials around your house.
Think of these garden-helpers as bottomless containers with removable glass lids. You can use leftover lumber to construct the four walls of a large container, then build a “lid” from old windows.
The glass on the top lets sunshine enter and heat up the space. On very hot days, you’ll need to prop it open so your crops don’t cook. On cold days, keep it closed and let solar power keep your autumn crops happy and healthy.
This article was last updated on
Read more about Greenhouses
The typical Minnesota gardening season ranges from May to September. Minnesota gardeners wait impatiently for the last spring freeze date in their area. The first frost in the fall is also important to know.
The Midwestern Regional Climate Center has produced an up-to-date interactive map of first fall and last spring freeze dates. Although the growing season is longer in the Twin Cities metro area and the far southeast corner of the state than other regions, most gardeners in Minnesota agree: the season is too short!
There are a number of ways to get an early start in the garden and stretch the harvest longer into the fall.
The Minnesota Department of Agriculture (MDA) keeps track of current soil temperatures throughout much of Minnesota. Even if you do not use any techniques to start early, you will still want to know when the soil is warm enough to plant heat-loving crops such as sweet corn, cucumbers, melons and squash. If MDA’s map does not cover your area, use a thermometer to measure the temperature two to four inches below the surface.
To warm the soil for heat-loving plants, put plastic mulch in place one or two weeks before planting. You can also lay the plastic and plant the same day. With this method, the effect of the plastic will come later.
The surface of the soil needs to be smooth and level for black, green/olive, blue and red plastic mulch to warm the soil. The plastic absorbs the sun’s energy and conducts it to the soil touching the plastic. If there are air pockets under the mulch, the plastic will not transfer the heat.
Clear plastic mulch does not need to be in complete contact with the soil, although it is best to apply it smoothly. It warms the soil more than black plastic because a film of water forms on the underside of the mulch. Heat cannot escape from the soil because the water blocks energy that is trying to escape. In this way, the soil absorbs more solar energy under clear plastic than under black plastic.
Depending on your garden soil type, and how much rain your garden gets, rain that falls on soil not covered by plastic may move laterally and supply your plants with water. Plastic mulch helps soil retain water. The soil under the plastic can also become very dry. You can design and install a system of drip irrigation under the mulch, or plan to water your plants at the holes you cut for planting.
Black plastic and clear plastic
Covering garden soil with black plastic film has been a standard soil-warming practice for many years. Black plastic can increase the soil temperature about 5°F, and prevents weeds.
Clear plastic mulch is even more effective at soil warming, increasing soil temperature by as much as 14°F. Unfortunately, weeds can grow and flourish under clear plastic in mild weather. If the weather is hot and sunny enough to kill the weeds, in a process called “soil solarization,” it is also hot enough to harm the roots of vegetable plants.
Green or olive wavelength-sensitive plastic
Red plastic mulch has improved yields of some crops, including tomato and eggplant, by changing the quality of the light reflected up into the plants. Blue mulch improved yields in cucumber, melon and squash.
These colored mulches warm soil as much as black plastic, but do not control weeds as well. You must stake or trellis crops grown on these plastic mulches for vertical growth, so that the mulch surface stays exposed for maximum reflection.
For home gardeners, it may not be worth using them instead of black plastic. A change in reflected wavelengths from red or blue plastic will probably not contribute much to the success of heat-loving crops in the short Minnesota growing season. In Minnesota, yields of heat-loving plants will improve by:
Problems with plastic mulch
Biodegradable and photodegradable alternatives to plastic mulches
Landscape fabric or permeable fabric mulches
Winter sowing in milk jugs
Using empty milk jugs as tiny greenhouses, you can start seeds for transplants outdoors instead of inside. Winter sowing is a good idea for gardeners who do not have space to set up trays, plant pots, lights and heat mats.
A cold frame consists of a shallow, unheated box with a transparent cover. Cold frames use the warmth of the sun to heat up the growing environment during the day, and protect plants from the cold at night.
You can repurpose scrap lumber and old window sashes as cold frame construction materials. The lid should close snugly to keep warm air from leaking out, but must be on a hinge so that you can vent the cold frame during the day.
You can also use cold frames with an electric heating mat as a bottom heat source, or "hot bed", to extend the growing season.
Cool-season crops such as lettuce, kale, cabbage, cauliflower or broccoli will perform well when grown in cold frames during cool months. You can also use cold frames to harden-off heat-loving seedlings such as tomatoes, peppers or eggplants, acclimating them to outdoor conditions.
Building a simple cold frame takes a few hours and requires only simple tools: a hammer, a drill, a saw, and nails or wood screws.
The finished product will be an open-ended box: four feet wide, four feet deep and two feet high. Paint it inside and out with a white, oil-based paint to prevent deterioration. The white paint reflects light inside the coldframe.
Place the cold frame in a site that receives direct sun at least half the day. The best position is facing due south, but a southeastern or southwestern exposure could work. To let the sunlight in, lower the front of the cold frame by digging a trench six inches deep along the front and tapering back along the sides. Secure the cold frame by packing the soil back along the sides and front.
Cover with old storm windows, attached with hinges to the back wall. The more snugly the windows fit together and to the cold frame body, the better.
Eight standard 21"x 10.5" nursery trays of seedlings will fit in this cold frame. To keep the undersides of the trays clean, put a piece of landscape fabric in the bottom of the cold frame.
Cloche is the French word for “bell,” and describes a bell-shaped glass dome placed over a garden plant. Any cover for an individual plant that acts like a mini-greenhouse by trapping heat from the sun is a hot cap. Hot caps can raise daytime temperatures and maintain warmer nighttime temperatures, as well as provide protection from wind. Sometimes removal or venting during the day is necessary because plants may overheat.
Garden centers usually carry relatively inexpensive waxed-paper hot caps in spring. Some have a flap at the top that opens to vent excess heat during the day, and then closes to protect the plant at night. If such a flap is not already present, cut slots in the top. Anchor the hot cap with stakes, soil staples or soil pushed over the edge. When the top of the plant reaches the top of the hot cap, remove and discard it.
You can also use plastic milk jugs or soda bottles with the bottoms cut off as cloches. In this case, remove the plastic lid for ventilation, and then replace it at night. Anchor milk jugs with stakes or soil staples so they do not blow away. Plants will quickly outgrow soda bottles and milk jugs, but since spring can turn quickly to summer in Minnesota, these no-cost cloches may be all that you need.
More expensive, but more durable, are glass and clear plastic cloches. Glass cloches are heavy enough that they will not blow away in strong wind. Anchor plastic cloches by staking or burying the rim. The cloches should have vented tops.
Water-filled tomato protectors
Tomato water walls such as the “Frost Jacket,” ‘Kozy-Coat®” and “Wall o’ Water™” are for warmth-loving transplants such as tomato, pepper and eggplant. The water-filled cells accumulate heat during the day and release it slowly during the night. When using these protectors, you can transplant tomatoes, peppers and eggplant out to the garden three weeks earlier than normal.
Remove the water walls once the plant begins to emerge from the top. Empty out the water, wash and allow drying before storage. With good care, these plant protectors can last many years. Even if one or two cells leak, they will still provide protection.
A new type of individual plant protector based on camping tents and beach sun-shelters is available. Equipped with zippers and pop-up mechanisms, these cylindrical jackets and rectangular shelters accumulate heat, protect against wind and maintain warmth overnight. Set them up after planting and installing cages or stakes.
A simpler jacket available for gardeners to try is a tube of red, micro-perforated plastic film. It is similar to the plastic film that protects clothing after dry-cleaning. This plastic jacket is red, which may promote higher yields. The plastic will protect against wind and disease spores carried by soil splash. It will provide limited insulation against cold nights.
Cut the plastic in sections, and slip it over the tomato plant and cage. Fasten it to the base of the cage or anchor it to the soil. Tie the top closed, and then open for ventilation on warm days.
Floating row covers are lightweight synthetic fabrics laid directly on top of a row of plants to protect against cold and insects. They are usually synthetic fabrics, either spun-bonded, knit or woven, with varying degrees of light transmission and insulation against cold temperatures. From fleece to fine mesh, there are many different weights of row cover material. They all allow rain to flow through them.
Lay down row cover fabrics over the row after planting. Allow some slack in the fabric so the plants have room to grow. If the fabric comes loose in the wind, it can harm young plants. Anchor it on all sides with soil pins and/or soil to keep the edges firmly in place.
Most gardeners use row covers for a month or less, then remove them. For plants that require insect pollination, including cucumbers, squash, melons and pumpkins, remove the cover once the plants start to bloom, so that bees can reach the flowers.
Row cover fabric can last more than one season with care and luck.
Low tunnels and supported row covers
You can use the same fabrics from floating row covers with semi-circular supports to form a low tunnel over the row of plants. Many gardeners prefer to use supported row covers, because even a well-secured floating row cover can whip and flap on a windy day, harming tender plants.
You can also cover low tunnels with plastic film, but a plastic-covered tunnel requires more attention. Plastic transmits more light and retains more heat and humidity during the day, compared to row cover fabric. Even on a cool day, if the sun is bright, the air inside the low tunnel may become too hot, and kill the plants. It is important to open the plastic and allow ventilation.
At night, the air temperature within a plastic-covered tunnel will decrease, becoming the same as the temperature outside the tunnel. Plastic does not allow rainwater to penetrate, so you may need to water plants in a plastic-covered tunnel.
New clear plastic materials have slits pre-cut along the sides. Once the plastic warms up to a certain temperature, the slits expand and open. As it cools, they tighten back up and close. This film is more expensive than plain clear plastic, but it may be a better solution for low tunnels.
A high tunnel is a large, semi-permanent structure, tall enough for an adult to stand up inside. A high tunnel is ideal for very large gardens or commercial vegetable farms. High tunnels provide a growing environment protected from most of the elements, as well as some disease and insect pests. Unlike a greenhouse, a high tunnel has no heating or cooling system, and usually does not have benches or tables for plants.
While wire supports form low tunnels, high tunnels use metal tubing. They may also have wooden sills or low walls, framed ends with hinged doors, and mechanisms for venting the tunnel along the sides or through slats at the ends.
Item number: XHT1158
Cold frames and hot beds, hoop houses, cloches, and floating row covers allow gardeners to grow plants earlier in spring and later in fall. Although these structures are used primarily for growing vegetables, they may be used for growing ornamentals, including flowering plants, as well.
Cold frames: Cold frames are by far the most useful and versatile of the structures that extend the gardening season. They are simple, easy-to-make structures that can be used year-round to provide warmth from the sun, and prevent water loss and damage from the wind. Many gardeners use cold frames to harden off transplants in the spring. In addition, cold frames are great places to grow salad crops such as lettuce, radishes, and spinach before and after their regular outdoor planting season. In winter, cold frames can be used for forcing bulbs (see University of Wisconsin Garden Facts XHT1144 – “Forcing Bulbs”), storing root vegetables (e.g., carrots or parsnips), or propagating trees and shrubs using hardwood cuttings.
Permanent cold frames should be sturdy enough to withstand years of sun and adverse weather. Most are constructed of wood and have a hinged cover. Glass windows make great covers, but are heavy and breakable. Alternative covers made of plexiglass or double-layered clear plastic weigh less and can be more durable. Commercially available cold frames are often built of lighter-weight materials and are often portable, allowing for movement of the frame to different sun exposures as seasons and plants change. Portable frames can also be set on concrete blocks or bricks to add height for taller plants.
Cold frame lids should be hinged so that they can be easily propped open to allow cool air to enter the frame. This can be important on sunny days when temperatures inside cold frames can get extremely hot, causing plants to wilt. Some mail order catalogs offer temperature controlled cold frame hinges that automatically open and close to vent the frame.
Hot beds: Cold frames become hot beds when heating of some kind is added to the ground under a cold frame. The modern, high-tech version of a hot bed involves burying a waterproof, thermostatically-controlled heating cable in a layer of sand two inches beneath where plants will grow. The bottom heat of hot beds encourages root growth in the plants. An older, low-tech method of creating hot beds has been to place fresh manure in the bottom of a cold frame, with decomposition of the manure providing the heat. This technique is not recommended due to potential safety issues surrounding the use of fresh manure, particularly for food crops (see University of Wisconsin Garden Facts XHT1143 – “Safely Using Manure in the Garden”).
A hot bed is ideal for starting vegetable and flower seeds in sunlight rather than under artificial light. Many seedlings require constant warm soil temperatures to germinate, exactly the sort of environment that a hot bed can provide.
Hoop houses: Hoop houses are similar to cold frames, only larger. Metal or plastic pipes are bent into a series of hoops that are stuck into the ground or attached to raised beds. The hoops are covered with four to six millimeter polyethylene that is buried in the soil at the base of the hoops. Hoop houses can extend the growing season by an additional six to eight weeks. Like cold frames, hoop houses must be ventilated on warm days. To ventilate a hoop house, simply lift the plastic on the ends of the house, or make openings in the plastic at the top or sides of the house.
Cloches and hot caps: Frequently used for tomatoes and peppers, cloches and hot caps add three to four weeks to the spring growing season. There are many variations on cloches, but generally they are any transparent (not opaque) structure that covers a single plant (e.g., a water wall or a clear juice jug with the bottom cut out). Hot caps tend to be conical in shape. Like cold frames and hoop houses, cloches and hot caps should be ventilated to allow hot air to escape on sunny days.
Floating row covers: Floating row covers are made of spun polyester or polypropylene and look like fabric. They are laid over plants and are permeable to light, water, and air. Floating row covers have many uses in the garden. Row covers keep covered plants five to ten degrees warmer than uncovered plants and provide frost protection to temperatures as low as 28°F. They also protect tender plants from wind and rain damage. In addition, row covers are excellent insect barriers.
For more information on extending the gardening season: Contact your county Extension agent.
Do you plan to grow your own food this summer? Would you like to extend that growing season a bit more in the spring and fall? If so, there are several options you might consider, including cold frames, hot beds, hoop houses, cloches, and floating row covers.
I remember my dad using a cold frame that he made of old windows to harden off plants in the spring before planting them in the garden. A cold frame is simply a miniature greenhouse that provides warmth from the sun and blocks the wind. The sun's rays enter through a transparent cover and create a greenhouse effect that heats the interior of the cold frame.
Cold frames expand the growing season one to three months. Many gardeners use cold frames to harden off transplants, but another good use is raising a few salad vegetables. Lettuce, radishes, and scallions will grow to full size in a cold frame before their regular outdoor planting season. Or in fall these same crops may be grown in the cold frame through November. Cold frames are also used to store root vegetables over the winter.
There are many different cold frame designs. The most common one is a wooden box with a clear lid that is hinged for easy opening. On a sunny day, the air in cold frames can get too hot for plants. Therefore the lid should be propped open so cold air may enter the structure.
Other designs are much simpler. Be creative. A south or east facing window-well with a plastic or glass covering might work well.
Cold frames can be converted into a hot bed by adding heat. Heat cables and pads are available for this purpose. This gives the added bonus of using the bottom heat to encourage better root growth in plants. Many seedlings require constant warm soil temperatures to germinate, so a hotbed gets them off to a better start.
Another method increasing in popularity is hoop houses. A hoop house is similar to a cold frame, only larger. Metal or plastic pipes are bent into a series of hoops that are stuck into the ground or attached to a raised bed. The hoops are covered with plastic. Hoop houses add six to eight weeks to the spring and fall growing period.
Finally, don't rule out the old-fashioned cloche method. Frequently used for tomatoes or peppers, cloches add three to four weeks to the spring growing season. These transparent "houses" covers a single plant. Examples include empty milk jugs, soda bottles, plastic covered tomato cages, or glass cloches.
This factsheet from Cornell Cooperative Extension gives more information on cold frames.
Build your cold frame today, and you'll be eating fresh greens in a few short weeks. Enjoy!
- b. sprout
- s. squash
- w. squash