THE PROBLEM
The growing season where I am in Minnesota (USDA Zone 4b) is short. The average time between the last frost in the spring and the first frost in the fall is somewhere around 150 days (generally early May to late September). Sometimes considerably less.
THE SOLUTION
One of the best ways to extend a growing season is with a cold frame. A cold frame is really any unheated structure that acts as a greenhouse to help shelter plants from outdoor conditions. Depending on conditions and construction, a cold frame can maintain temperatures around 10 degrees warmer than its surroundings. And when seasons are changing those 10 degrees can be the difference between a plant thriving and dying.
Using a cold frame in the spring means young plants that have been started inside have an opportunity to acclimate and "harden off" before being planted into the garden, and in the fall cool weather plants can continue to grow while being sheltered from the increasingly harsh conditions as winter lays siege.
THE OBJECTIVES
- Extend the upcoming growing season by building a cold frame
- Use at least some existing materials
- Design it to last
- Make it aesthetically pleasing
THE STEPS
There are many ways to make a cold frame. Here's how I made mine:
1. Acquired a Window
When we moved into our home we had two double-hung windows removed from the home office. Rather than have them trashed I held onto them for just this purpose. I used 1 window for this project. (Any salvaged window or door will work. So would a piece of acrylic. But glass will do the best job trapping and retaining heat.)
2. Made a Plan
I took the measurements of my window and adapted them against this plan from Fine Gardening Magazine. I decided that I wanted to place my cold frame outside under a window in our walk-out basement. Since I didn't want the hinged lid to obstruct the view out the window, the height of my cold frame would be limited by the height off the ground where the window began. With those measurements in mind, I opted to use 2x10s for the primary box. I used 1x4s to frame the window which was dictated by the dimensions of the window itself.
3. Procured the Lumber & Hardware
I find the contrast of cedar and black hardware beautiful. Cedar also has the advantage of being naturally insect and rot resistant, won't warp due to moisture and can take a beating. It is realistic to think that I will be able to use this cold frame for 10+ years making the short-term splurge a long-term value.
With my plans and cut-list in tow, I moseyed down to my local lumber yard. Not the big box store but an actual, old school lumber yard. Not only am I a fan of shopping local and shopping small whenever I can, but I felt more comfortable being vulnerable about what I did and did not know about lumber with folks who "do" lumber day in and day out. It was a great experience and for a small fee my modest order was delivered to my driveway 48hrs later.
When it came to hardware I knew I wanted it to do the job sufficiently. But I also wanted it to be small yet substantial and I wanted it to be black. I also knew that my budget was spent on buying cedar and I was not about to pay more for speciality hardware. So after picking up some 3/8" x 2 1/2" lag screws and 3/8" flat washers (all made from Zinc and/or galvanized in order to withstand outdoor conditions), I headed for the spray paint aisle. There I found an outdoor-rated Rust-oleum 2X UltraCover black gloss paint that would adhere "to wood, metal, plastic and more". Once home, I would find a scrap piece of cardboard, cut some Xs in it with a utility knife and push my screws through the cardboard so they would stand upright. A couple of coats of spray paint later and I was in business!
Next, I needed hinges and handles for the window that would become the lid for the cold frame. My window was heavy -- nearly 40lbs -- so I opted for heavy duty gate hinges. I would also need a way to lift the lid on my own, but a pair of standard black metal handles would do the trick just fine.
4. Made my Cuts, Sanded, & Prepped
I freely admit that I can be ... well, I think the polite term would be "thorough". (Read: detail-oriented to an absurd degree.) So it should be noted that beyond cutting the boards to length none of the other steps are necessary strictly speaking. I chose to do them as a matter of personal choice, preference for how the finished product looks and feels, and as insurance on my investment.
Using a mitre saw I cut all my boards to length, save for the two triangular pieces which allow the lid to lean at an angle to collect as much sunlight as possible. Those two pieces were created from one piece cut in half along the diagonal with a 4.5" circular saw. (The Fine Gardening piece does a nice job explaining how to go about that.)
With all of my boards cut and pilot holes drilled, I sanded all six sides of every board with 120 grit sandpaper on a handheld orbital sander. I then sprayed the boards down with distilled water (I had it on hand) and allowed them to dry. This is called "raising the grain" and would definitely be considered overkill. That said, I have a weird aversion to the texture of weathered wood. I must have gotten too many splinters as a child, but I digress. I wanted these boards, and especially the surfaces I would be touching as I reach in and out of the cold frame, to be as splinter-free as possible. So with the grain raised (and the surface no longer smooth) I sanded all 6 sides of every board again, this time with 220 grit sandpaper. Pro Tip: wear a mask or you'll be coughing for days. Don't ask how I know.
With the sanding done (and it took a while), I could finally move on to applying a protective coating on the outside of my boards. Although cedar can be allowed to age naturally and take on a silver/grey-ish hue, I prefer to preserve the warm tones of the wood I see today. That means I needed to apply at least 2 coats of polyurethane with a foam brush in order to seal the wood. I opted for a hard-to-find outdoor water-based polyurethane in the name of limiting the number of chemicals my food plants would be coming into contact with.
I should also note that the previous two paragraphs make the task sound simple. And it was. But it was also time consuming. To sand all boards and apply two coats of poly took an entire day mostly because of the required drying time in between coats. You've been warned.
5. Assembled
Once my second coat of poly was dry, all that was left to assemble. Using a socket on the end of my impact driver, I quickly screwed together the pieces and that was all there was to it. My husband helped me attach the lid to the primary box using the hinges, I attached the handles with their included hardware. Snapped a few pictures and it was complete!
FINAL THOUGHTS
All that remains is for me to put the cold frame in its place and decide how I will go about keeping the lid open during days when ventilation is required. A simple 2x4 to keep the lid propped up will work but I'm exploring solar vent openers which are basically a piston whose action is triggered because the enclosed wax melts once it gets warm enough. Ain't science crazy?!
Until next time,
Jill
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