In the majority of the world, cold weather puts an end to the growing season. When temperatures start to dive, most gardeners know that it soon will be time to pack it in for the year and run from that deadly frost. That is true unless you start finding ways to extend your seasons. One of my favorite methods is using cold frames.
What is a cold frame?
A cold frame is simply an enclosure with a transparent roof used like a miniature greenhouse. The transparent top allows sunlight to reach the plants as well as warm the enclosed space. It provides protection from frost and a warm humid environment perfect for seedlings and young plants. The common cold frame consists of a hinged window with a simple wood frame. The cold frame is adaptable to available materials which can from stone walls to hay bales walls. The lids can be windows or made from plastic as long as it is transparent. A cold frame can extend the planting season by a month or more both at the beginning and the end of the year.
How do I use a cold frame?
Cold frames provide tons of ways that you can extend the growing season, which can all pay huge dividends. Not only can it provide you a place to start seeds early it can allow you to grow many plants in it well into the cold parts of the winter. The key to using a cold frame is to closely manage the inside temperatures by opening and closing the cover. You want to keep the temperature below 75° for warm season plants and below 60° for cool season plants.
Start seedlings early
The warm humid environment created in a cold frame is perfect for the development of young seedlings. You can either plant in plastic pots and flats or seed directly in prepared soil within the cold frame. Once seeds have germinated, be sure to vent the enclosure frequently to prevent the seedlings from damping off. Keep all seedlings well watered. More plants die from heat and drought in a cold frame than from frost damage.
Harden seedlings off
Many gardeners start with plants grown in a greenhouse or indoors under lights or in a window. Before being planted in the garden, the tender plants must be acclimated to the temperature and sunlight. I usually wait until temperatures do not fall below 35° or so before setting out cool season plants. For warmer season plants, wait until a few weeks before your last frost date. Transition plants grown indoors to the cold frame by placing them outside for a couple of hours a day gradually increasing over a week.
Overwinter tender plants
While cold frames do not provide anywhere near the same level of frost protection as that of a greenhouse, they can still help keep some more tender plants through the tough months. Take care to monitor the temperature fluctuations especially on sunnier days. If the temperature rises above 40°, slightly open the lid. If the temperature reaches above about 55°, fully open the lid.
Extend the season
Cold frames can be used to stretch the growing season for many cool season plants well into the winter. These plants can withstand much lower temperatures and typically just need the protection of the cold frame during night times when temperatures drop below freezing most frequently.
Where do I put a cold frame?
Because the cold frames rely on solar gain, getting direct sunlight is important. Cold frames should be sited on a south-facing spot that is well protected from wind. It can be permanent or a temporary installation.
How do I make a cold frame?
Cold frames can be made from a variety of materials. The key is to have four walls to trap heat with a transparent top. One of the simplest and most efficient models uses wood walls and framing with a recycled window for the top. That is essentially what we have built. Here are the basics on how to construct a simple cold frame:
- salvaged window
- 1×6 cedar boards (5 sides)
- 1×8 cedar boards (3 sides)
- 2×2 corner posts (deck balusters)
- 2 t-hinges
- 1-1/2” wood screws
- 1/2″ wood screw
- River rock or gravel
- Custom cut the cedar boards to the dimensions of your window. You will want to cut the top angled side boards first and match the lower side boards to the length. Then, cut the front to match the width of your window.
- Build the bottom frame. Cut the (4) 2×2 corner posts to lengths of 17 ½”. Attach the 1x 6 boards to the center of corner posts using wood screws. Leave the corner posts extended about 3” on the bottom side to extend into the ground. Attach the four sides and move to the top frame.
- Attach the top angled frame to the corner posts using wood screws. Be sure to match up sides and fronts and the top frame should fit right in if you matched your board lengths when you cut them. I cut the tip of the angled piece a little narrow, so I put a screw into the top.
- Lay out the area you where you plan to install the cold frame. Cut and remove any sod and excavate a flat area 2-3 inches deep. Dig out holes in the corners 3 inches deeper to sink the corner posts in. You can see now that I modified my cold frame to be installed against my raised gardens. I just used the side of the raised bed in place of the back boards.
- Fit the cold frame down into the sunken area. Pack the dirt in around the edges. I spread about 3” of gravel across the bottom. This helps insulated the cold frame and will hold heat better.
- Attach lid. I used a basic t-hinge to attach the window to the back boards. Here is the result:
- Next step is to add the arms. This is the basic plan for adding arms. (This is next on my to do list!)
The cold frames have definitely come in handy this year. We can’t build gardens fast enough for all of the seedlings we have and are still starting. I can’t wait to see how far I can stretch out the fall/winter.