For over ten years I have grown my own garden transplants by carrying the seed trays out into the sun on warm days and bringing them in every evening. In the last 3 years, I was juggling over 20 trays in and out of the house. This method uses natural light to produce strong, non-leggy transplants — but if you’re growing lots of seedlings it is excessively laborious!
It had to stop! However, my husband and I do not have impressive construction skills. When a friend from Pecandale Farmstead gave us some old windows, she also gave us an idea. Straw bales + old windows = instant cold frame.
The only materials you need are straw bales and some sort of windows. Craiglist or Freecycle are great options for recycled windows. If you happen to get plexiglass or acrylic ones, they are much less likely to shatter.
The only tools needed are hands (but gloves are recommended).
Choose a site that isn’t very shaded, although it is okay for it to get part sun in the south. The more sun your site gets, the more vigilant you will need to be about ventilating your cold frame on warm, sunny days. On sunny days ranging 35°F to 40°F, open it a crack. Once daytime temps reach 50°F you may need to open it entirely. Cloudy days don’t require as much ventilation.
Unheated cold frames allow you to overwinter plants that are from a zone (or two) warmer than yours. So, someone in zone 6 could overwinter plants that usually live in zone 7 or 8. In zone 8, you’d be growing plants from zones 9 & 10. It’s also a great way to get your seedlings started and ready to plant out by your frost-free date.
If you’re placing your cold frame on bare soil, weeds will be just as happy about the cozy environment as your transplants and pots. The easiest way to keep from weeding in the wintertime is to put down a barrier to block the germination of weed seeds and wandering roots. A layer of mulch will insulate plants from the cold ground but beware of pests like slugs hiding there.
You can stack your bales in several ways to make your frame tall or short. Ideally, the plants underneath shouldn’t be so deeply set they can’t reach the sun. If you only plan to do seed trays, make the frames lower or put the trays on top of something to elevate them closer to the light. If you are overwintering taller container plants, make the frames higher.
If there are gaps between the bales, stick extra hay in the cracks to reduce drafts.
Minutes later and you’re done! Nothing else is necessary to make your cold frame ready for transplants and pots.
If you have very cold weather consider adding extra heating elements such as black water containers (they absorb sun during the day and release it slowly at night) or bubble wrap on the glass for additional insulation.
Ventilation is simple: just place a brick under one end of a window to prop it up, slide it partially to the side, or remove it entirely. Be sure the window is secure to avoid crushed transplants or broken glass.
I can’t tell you how excited I am not to do the seed tray parade every morning and evening for 3 months.