Cold Frames Revisited

In case you missed them, find our previous posts about cold frames here and here.

Swiss chard growing in January

The straw bale cold frame has performed remarkably well.  With no supplemental heating, it has sustained the cold tolerant Swiss chard and the cold hardy Scotch kale quite well through overnight temperatures down to 5 degrees.  The cold intolerant sweet basil did not make it when temperatures got into the teens.  The thick bales provide a remarkable level of insulation.  The key is to grow cold tolerant crops and when the temperature is above 30-32 degrees, remove or prop open the cover to allow more sunlight in and more importantly, to not overheat the plants inside.  When temperatures are going to be lower than 30-32 degrees, cover them over for protection.  Simple.

Shadow cast by the southern wall of the cold frame

During the short days of winter, the sun sits very low in the southern horizon.  This creates a significant shadow effect from the southern straw bales as seen in the picture above.  The crops inside need as much solar exposure as they can get this time of year if they are going to not only survive, but produce a harvest of greens for you.  So this shadow was a real problem.  I thought and thought about what to do, thinking about translucent materials for the south wall such as a greenhouse panel or glass blocks.  Both of these were too expensive.

Straw bales on the southern side rolled down and away

So, I am standing there looking at the cold frame and had a Eureka! moment.  Why not just roll the two southern bales away on sunny days (as seen above) and replace them when it gets cold.  This allows full sunshine into the cold frame with no additional cost and minimal effort.  It has really worked out well.

Lettuce with freeze damage when covered by a 1 gallon glass jar

Lettuce with freeze damage when covered by a 10 gallon aquarium

The upturned glass aquarium and jars worked within their performance envelope, but didn't have nearly the insulating properties of the straw bale cold frame.  They seemed to protect the lettuce down to 25 degrees or so, but even then any lettuce that physically touched the glass at this temperature was ruined as seen above.

Have you been using a cold frame?  I'd love to hear your experiences and suggestions in the comment section below.