Cold frames are great to extend the growing season and not much more work if you’re already planning to use raised beds. You can start your seedlings earlier in the spring and keep plants alive longer in the fall.
Building a Cold Frame
A cold frame is just a wooden raised bed with the top at an angle facing South. Be sure to make the North side higher. An easy and cheap way to make the lid is to make the bed the same size as a door frame and get a cheap or used screen door from somewhere like Habitat for Humanity. It’s best to use greenhouse plastic or fiberglass rather than normal glass because hail or accidents could cause the glass to break and shatter into your plants and soil, ouch!
If you will be leaving the lid attached year round create a sturdy way to prop it open. You can also just remove the lid completely and store it in the summer. Fill the bed with compost-rich top soil. Leave at least three inches from the top of the short side empty for the plants to grow into and make the soil level. Be sure to mulch!
Planting in a Cold Frame
You can plant your seeds in the cold frame about two weeks earlier in the spring than is normally recommended. If you put the lid on a week or so ahead of time the soil will be warmed up and easy to plant in. Follow the instructions on the package and think about how big your plants are going to get so you don’t plant them too close together. Water and close the lid.
Growing in a Cold Frame
Check the moisture of the soil and gently water every day or two while your seeds are sprouting. As soon as the sprouts are about 2-3 inches tall you can start lightly mulching around them with straw or shredded leaves. Keep adding mulch as the plants get bigger. You want 3 or more inches of mulch around all of your adult plants to effectively protect the microbes in the soil, reduce the need for water and fertilizers, and deter weeds.
When the weather gets over 70°F, crack the lid open with a piece of wood so it doesn’t overheat and close it again when it cools down. You want to leave the lid on until there is no danger of frost. Here in Oklahoma that’s early April, we almost always get a late frost which can kill seedlings. Cold frames can be a life safer!
Either prop the lid up high or take it off completely when the weather gets hot. You don’t want the lid to accidentally come down when it’s 100°F outside and burn your plants! Put the lid back on in the fall and close it when there is a chance of freeze. You may be able to overwinter many crops in the cold frame. They need to be short enough to fit under the lid though. Think of the possibilities; year-round herbs, lettuce, and green onions, or try growing warmer climate crops like ginger!
Links to More Information
Here are more ideas to build a cheap cold frame https://misfitgardening.com/build-a-cheap-cold-frame-that-will-save-you-money-this-year/
Here’s a video that compares cold frames to tunnels and hoop houses https://youtu.be/dJxMDVLZH48
