Cold frame is a transparent-roofed enclosure that is built into the ground and is used to protect plants from cold weather. The transparent top of the structure allows sunlight to penetrate and stops heat from escaping the enclosure through convection. A cold frame is like a miniature greenhouse that can extend the plant\’s season.
Cold frames create microclimates that provide a difference between air and soil temperature and insulates the plants from harsh weather like the cold. This allows the home gardener to plant earlier in the spring, and to survive longer into the cold weather. The temperature difference between the outside and inside the cold frame normally is not more than 5 to 10 degrees difference. Cold frame are also used to start seedlings which are then transplanted into the garden, and also act as a growing area for cold-hardy vegetables that are grown for harvest in autumn and winter.
Location
You should place cold frames in an area with southern exposure so that it receives the maximum amount of sunlight daily. As per always, ensure that the site has good natural drainage that will not collect water. Use drainage tile or a thick layer of coarse gravel to correct areas with poor drainage. Building the cold frame above ground level may give proper drainage but there will be a lost of heat. Ensure that you build the cold frame within water supply, and if the structure is built with heating, make sure there are outlets close by.
Tip: To reduce the cost of heating, utilize the north or northwest windbreak by positioning the cold frame as required next to a building, a board fence or hedge. Bales of hay or straw can be used as temporary windbreak.
How to Build a Cold Frame
A cold frame is a rectangular box with its back higher than the front, and is covered with a transparent roof. You can also opt for an old window sash instead of glass for the transparent roof to reduce costs.
Tools and Materials:
- treated plywood cut to size and shape desired
- 2-8\’ lengths of 1×3 inch lumber
- 6 pieces of 2½” hinges with removable pins
- 4 pieces of 5″ flat angle irons
- 1 roll of 6 mil clear plastic
- 36 ½” wood screws
- 24 ¾” screws
- Staple gun
- Drill
- Cut the plywood into panels that will make up the front, back, and sides of the cold frame. Hold the walls together with four hinges (on each corner) that is attached to the panels with ½” wood screws. Then secure the walls together again with more ½” wood screws (see Figure 1).
Figure 1 Building the cold frame panels
Figure 2 Joining the cold frame top
- Once the panel is done you will now construct the top of the cold frame. Construct the top of the cold frame according to size with 4 pieces of 1×3 inch lumber. Attach these lumbers together to form a cover with flat angle irons and ¾” screws. (see Figure 2).
- Roll out the plastic that will function as the transparent cover of the top and place the newly constructed frame in the middle. Leave about 5 inch of overlapping plastic on the edges around the top frame and cut to size. Fold the 5 inch overlap over the frame and secure it on the frame by stapling it down.
- Place the top of the frame onto the bottom panel and secure it to the back of the panel with the last two hinges.
Figure 3 Completed cold frame
Watering
Water the plants in a cold frame early during the day and not in the evening so that the foliage will have enough time to dry before nightfall. When watering during temperatures below freezing, the cold frame should not be opened too widely.
Generally there is no need for constant watering over cool season. The frame should be open longer and wider as the plant becomes larger and the temperature rises over the season. It is then that the frequency of watering must also be increased. When the soil is obviously dry, water the plants. Always allow the soil surface to dry between watering, but make sure that it does not dry to a point that the plant wilts.
Soil pasteurization
The cold frame should have about 4 to 6 inches of good garden soil when you start you seedlings. To ensure that your soil is free from weed seeds, insects and diseases during germination, bake the soil in a 140 degree F oven for 30 minutes before placing into the beds. Alternatively you can pour hot water on the bed to pasteurize it. This must be done in advance prior to planting your seeds so that the soil can dry adequately.
If you are germinating your seeds in flats or pots, and not using your cold frame to germinate your seeds, then you do not need to do the above. Planting your seedlings onto the bed will be sufficient enough.
Fertilization and Pest Control
Do a soil test and make the necessary amendments to your soil according to the type of plants you will be placing in the cold frame. Once the seeds germinate and develop leaves, add a little liquid fertilizer every two weeks while the plants are in the frame. Keep an eye on your cold frame bed for pests and apply the necessary pest controls before plants are injured.
More reading:
Cold Frame: http://en.wikipedia.org/wiki/Cold_frame
Growing Vegetables in a Cold Frame or Hot Bed:
http://www.humeseeds.com/cldfrm.htm
Building and Using Hotbeds and Coldframes:
http://extension.missouri.edu/publications/DisplayPub.aspx?P=G6965
