Extending the Season with Cold Frames

(Welcome, Brad, SIP-growing Chicago friend and guest-poster. We hope to see your updates often.)

Last summer I started growing with SIPs after reading Bruce's post. I'll share more on last year later, but it was a great success. This year I'm tripling the number of SIPs, and trying some other new projects.

The greens, tomatoes, and peppers I planted last year were seedlings purchased from friends and farmers markets. This year I want to start more from seeds. I don't have a good south window to start seeds indoors, so I'm trying cold frames.

The idea is to trap heat in the SIP, warm the soil, and get the seeds a head start on the season - essentially a mini greenhouse. Ideally, the frame will help the seeds germinate weeks earlier, boosting the harvest.

I'll still plant seedlings around May 15th (the estimated date of our last frost in Chicago), but this year I'm going to start some seeds at the beginning of April using the cold frames. If they perform well, next year I may start some earlier, and will certainly start more seeds (and purchase fewer seedlings).

The cold frames will also help at the end of the season, allowing roots and greens to produce a bit longer than they would otherwise.

The Design

There are countless ways to make these - I used this design because it fit perfectly on the frames I built around my SIPs, and I had most the parts left over from other projects. Because this is a test run, I made them easy to disassemble should I decide to change the design.

The top is a thin acrylic sheet, though anything that will let light through will work. They're held to the 2x4 frame with some L-brackets. I am considering sealing the edges of the acrylic with some silicone, though I'm not sure if that's necessary or will help significantly.

If you want to make this design more effective, add a 10+ degree slope facing south to increase the sun they capture. Integrating an automatic vent will significantly reduce the time spent monitoring the box temperature.

How it's working

I thought I would measure the effect the cold frame is having on the soil temperature. It's definitely going up during the day, which I assume will get the seeds moving along. The temperatures drop in the evenings, so I'm planning to cover the SIPs any night that will get close to or below freezing.

Around 5 pm Tuesday (a cool but sunny day),
the soil temperature in the SIP with a cold frame was 62, and

47 degrees in the SIP without a cold frame.

By the next morning, around 6 a.m. (an evening that dropped to the low 40s), the soil temperatures were: 48 & 42.