Cold frames are generally found in home gardens and vegetable farms. Their primary utility is to shelter plants and seeds from wind and adverse climates by creating an insulated environment. This microclimate provides several degrees of air and soil insulation. It helps to allow plants to be started earlier in spring and these survive longer into the fall and winter when placed in cold frames.
These frames are initially used for growing seedlings which can later be transplanted into the open ground. Cold frames are extensively utilised for cold-hardy vegetables grown in autumn and for winter harvest.
A cold frame is a type of a mini greenhouse used to extend the growing season around the year and to protect plants from excessive cold and moisture. It has a transparent top enclosure that admits sunlight but stops heat from escaping by convection that would typically happen, especially at night.
Traditionally, cold frames are built in addition to a greenhouse. As the name exemplifies, these are two distinct entities, a warm greenhouse, and an unheated cold frame. This two combined helps seeds to be germinated in the greenhouse before being moved to the cold frame to be hardened before final planting outside.
A thermometer is routinely hung in the inside of the cold frame to monitor temperatures. While heat and humidity are crucial factors for germinating seeds, excessive heat in the range of 90°F and above can damage delicate seedlings. Hence the temperature inside cold frames should be closely monitored.
There are three types of cold frames. The first is a sunken cold frame with cinderblock walls and a durable plastic cover that is optimally used for overwintering tender plants and hardening off seedlings. The next is a plastic hoop tunnel that warms the soil and protects frost-sensitive plants in spring and fall.
Finally, a portable wood frame with plastic stretched tightly over the top extends the harvest season of cool-season plants and vegetables and allows seeds to be sown directly earlier in the spring. In all these cases, the temperature, moisture, sunlight and wind exposure have to be closely monitored.
You should keep certain factors in mind when using a cold frame. Some plants like the low-growing cool-season ones are more suited for cold frame growth than others. Second, the type of cold frame that should be used is determined by how much protection you want to offer to plants.
Here are a few common uses of cold frames.
Overwintering Dormant Plants – In a sunken cold frame, the ground acts as insulation, and the sun spreads warmth, but for active growth of plants, you should eliminate pockets of cold air before using it. If you want to have tropical or tender plants in the garden but feel at a loss on how to maintain them in the fall, a cold frame is an ideal solution for you. Unlike a greenhouse, you will not be able to keep your plants growing lushly in winter but you can create right conditions for gentle dormancy until they can resume growth in spring.
After storing your tender plants in a cold frame, ensure that the weather conditions are right to sustain complete dormancy. The plants should be kept moist but not wet as they might rot. There should not be much sunlight either as that will encourage active growth.
Hardening Off Young Seedlings – How are seedlings customarily hardened? They are generally carried from a warm and sheltered location like the interiors of a house or a greenhouse to the garden so that they get slowly acclimatised to extreme fluctuations of temperature and moisture. This is further achieved by gradually extending the time that the seedlings are kept outside.
All this is made easy with cold frame gardening. The same effect can be had merely by opening and closing the lid of a cold frame over a five to seven day period. It is advisable to wait until seedlings have formed multiple sets of leaves and have taken roots firmly before moving them into cold frames.
The foliage will thicken and the colours will get darker once the seedlings have been exposed increasingly to cooler temperatures, wind exposure and direct sunlight. That is a good sign and shows that the seedlings are finally ready for the permanent transition to the garden. However, if you have any doubts about their growth, especially if the seedlings are of the rare variety, you should contact professional arborists and expert tree care and tree and tree stump removal agencies in your area for advice in this regard.
Giving Seedlings A Head Start – Seedlings that are grown indoors can be settled outside weeks earlier if a cold frame is used. When you are starting seeds in a flat or putting them directly into the soil, a cold frame allows the plants to get going earlier than normal. They also become used to the prevailing temperatures and hence there is no transplantation shock as they are used to the surroundings. If seeding is for cool-season plants, the time window for seeding should be early spring or fall as they require lower threshold temperatures for early germination.
If you are putting seeds directly into the portable cold frame, you should have the set-up in place at least two weeks before seeding so that the soil will be warm and ready for germination. Ensure that the seedbed is evenly moist too. Once the seeds have germinated, the cold frame should be kept open at increasingly frequent intervals to increase air circulation. If you have started seeds in a greenhouse or indoors, you can plant them almost six weeks earlier if you put them in a cold frame in your garden.
Extend The Season Past Frost – At times, it might not be possible to bring plants to a cold frame. In that case, you can bring a cold frame to the plants. Make a plastic hoop tunnel with rebar and PVC for the frame. However, while the plastic over the tunnel will keep the plants warm at night, it should be lifted during the day. With a cold frame over your plants, you do not have to dread the oncoming of the first fall frost, it will keep the frost at bay and you can extend your season. If the external temperatures are not below freezing, plants which have been hardened by the onset of fall need not be kept covered during the day inside a cold frame or hoop tunnel with a PVC cover.
Cold frames can be bought from the local garden store or constructed from timber and plastic. However, you can also use blocks made of concrete or bricks. You can even make a bottomless wooden box and dig it into good soil in a sunny location.
Here are some guidelines on making a cold frame for your garden.
Most people use wood to make the frame as it is readily available and can be cut to size with simple tools without much of a problem. Try to get scraps of hardwood to build your cold frame as it lasts longer than softwood.
If you are using old wood, avoid ones that have been treated with non-earth-friendly products such as creosote, more so if you are placing the frame directly into the soil. Paint the wood with a non-toxic paint if you want to make your garden look colourful and to hide the scruffiness of plain wood.
You can use old and discarded materials to construct cold frames. The box can be topped with glass from an old storm window or a frame that is covered with clear plastic. The thicker the material, the higher will be the insulating properties.
Make use of discarded windows and shower doors for this project. However, ensure that the cover is hinged so that the lid can be opened for ventilation on warm days.
If you want to avoid anything permanent in your garden, even cold frames, try something temporary. Have frames or “cloches” made of old storm windows and lean them tent-style over the plants along the full length of the garden row.
If you are not the DIY types and have limited skills, cut out the bottom of plastic milk jugs and place them over individual plants. These can be kept in place by mounding soil around the jugs. For proper ventilation, the caps can be removed on sunny days.
There is another side to cold frame and that is a hotbed. A hotbed is a heated cold frame. The traditional method is using horse manure or compost but modern techniques of using electrical tapes or cables are equally effective too.
How to make a non-electric hotbed of compost or manure? Excavate about 18 to 24 inches below the frame and add compost or fresh manure.
Then, turn and moisten this material every couple of days for a week until it settles down. Add a 6 inch cover of soil over it. As the manure or compost decomposes, enough heat will be generated to protect seedlings and plants from the effects of early or late frosts.
Here are some tips you can follow to be a success in cold frame gardening.
Choosing The Right Site – Choosing the right site is very critical if you want to get the best out of your cold frame. The face of the frame should face south and the site should provide shelter from windy conditions. There should be plenty of sunshine falling on the frame too. You can make the frame stand free in the garden or lean it against the house, deck, greenhouse, shed or garage. For added winter insulation, stack bales of straw or bags against the north side.
Selecting The Right Materials – The cold frame box can be made of any material, the most preferred ones being bricks, polycarbonate, wood and straw bales. Selection of materials depends significantly on your location. If you buy cold frames with polycarbonate sides and tops, there will not be enough insulation to take your salad greens through cold and severe winters. Wood frames topped with polycarbonate are best in these circumstances. You can also use straw bales to build a cold frame quickly. Put them around herbs, tall leeks, kale or other greens and cover with a sheet of polycarbonate.
It is therefore recommended that you contact local arborist agencies to know more on the materials suited for your region. For example, if you are a resident of the State of Victoria, Australia, you can get in touch with Daryl’s Tree and Stump Removal for expert advice in this regard.
Ventilation – This is the most crucial factor between successful cold frame gardening and failure. Proper ventilation in a cold frame has to be ensured, especially in autumn or spring when temperatures during the day can fluctuate greatly. Keep cold frames open during the day if you know that temperatures are going to touch 4 C.
There are several issues connected to poor ventilation. The biggest problem is that the plants will be fried in heat if you do not keep the frame open. Without proper ventilation, the plants will be growing in an environment that is consistently warm and this leads to soft growth which can be damaged in cold weather. Plants that are more acclimatised to cooler conditions can deal better with frigid temperatures of fall and late winter. Use a digital thermometer to monitor the inside of the cold frame.
Keeping The Top Clear – If your cold frame is placed under tall deciduous trees, falling leaves in mid-autumn will cover the top of the frames. It is essential that you keep the top cleared at all times. Leaves on top of the frames will block sunlight and as a result, the plants will suffer. In winter, the top might be covered with falling snow which has to be brushed off as a matter of routine.
Follow these simple tips for proper cold frame maintenance and successful gardening.
Now that you have all the information on cold frames, what are the plants that best fit this form of gardening?
Italian Flat-Leaved Parsley – This garden plant can withstand freezing conditions and snow. Use cold frames to enjoy fresh Tabbouleh even in January.
Romaine Lettuce – This plant too can withstand freezing conditions and snow. The flavor of cold frame Romaine lettuce may not be as tasty as that grown in summer but it is nice to have some greens in winter.
If you are planting early in summer and want to harvest the crop early or late winter go for arugula, spinach, mache, radishes and leaf lettuces.
If you can plant certain crops as late as 15th August and want to harvest them by late winter, opt for carrots, scallops, kale, parsley and Swiss chard.
There is nothing more useful than a cold frame if you are an avid gardener and want to be involved with plants around the year, come spring, summer, fall or winter. The frames create a microclimate that is at the very minimum, a zone and a half warmer than your garden, enabling growth of plants throughout the year.
Annabel is a naturalist dedicated to the study of plant life. She lives in Melbourne, Australia and is closely connected with various institutions and universities, teaching the many aspects of plant life and horticulture. Annabel has a Certificate in Horticulture, and an Advanced Certificate in Arboriculture. She is also a member of the prestigious International Society of Arboriculture. She is the author of various books on plant life and spends a good part of her life touring the world and delivering lectures in seminars and workshops in different countries.
Annabel is also an avid sportswoman, having represented Australia in tennis and swimming in international events.