Ampersand’s cold frame is usually the favorite stop on our site tours. If you have ever grown in one, you know that a cold frame is a season extender. It’s a way to create a milder climate so that we can grow outside all year round.
Our cold frame is made out of an old shower door, it is sunken into the ground on the north and west sides and it is insulated along the walls. The north wall holds a stack of bottles filled with water, adding thermal mass to our design in order to further stabilize the temperatures. If they were painted dark, they would collect more heat, but we have not needed that with this design.
All that makes a great place for frost tolerant greens to overwinter or make use of the spring and fall seasons. But the funnest thing about our cold frame tender is the opening mechanism.
It’s important to open the cold frame in the morning and close it at night on sunny days. We don’t want to turn our cold frame into a solar oven. We built a Passive Solar Cold Frame Tender which allows the sun to do this for us.
Here’s how it works: When the sun rises over the hill in the morning, it heats up this 5 gallon blue water jug, which has about a gallon of water in it. It pressurizes, just like a closed water bottle that is left in your car in the summer will. The pressure builds. There is only one place to release this pressure, and that is by sending water up the clear vinyl tube into the old vinegar jug that hangs on the outrigger, an extension of the lid of the cold frame.
Water flows up into the jug and adds just enough counterweight to open the cold frame lid. In the evening the blue jug cools down, depressurizes, and sucks the water back in. Voila! No need to worry about frying our plants.
We saw this design in our friend Lu Yoder’s back yard many years ago. He and Steve Baer designed this system. Lu and collaborator Dawn created a great zine on how to make it yourself. Now it’s posted online so you can make your own!