The Cold Frame Corner

By: Susan Perry, Colorado Master Gardener in Larimer County

Two years ago, at the end of the growing season, I had several hundred un-harvested carrots.  So I decided to try an experiment & cover them with grass clippings & leaves and see if I could harvest them all winter.  Well, I could – sort of.  They were easy to harvest & tasty until the ground froze but much harder to harvest (still tasty, however) after the ground froze.  Harvesting from frozen ground literally required a chisel & a lot of determination.  Unfortunately, things changed when we had a thaw & re-freeze, rendering the remaining carrots inedible & useful only for compost.

Last winter at the last minute, my husband Tom & I cobbled together two cold frame-like boxes, hinged on the corners for easy storage, from stackable boxes sold by Kerrie the handywoman/salvager on the main drag of Timnath.  The drawback was that while the hinges made storage easy because the boxes folded, the hinged corners weren’t “tight” (there were gaps where air could get in).  I solved that by buying foil-wrapped rigid Styrofoam insulation at Home Depot & cut to the correct height so that the entire box was lined.  The boxes were topped with two single pane glass storm windows that cost $3 each from Uncle Benny’s in Loveland.  From a temperature perspective, the inside of the boxes worked just like a cold frame – warming up enough to need venting on the morning of fall, spring, & winter days.  But I’d shot myself in the foot by using the foil-wrapped insulation because the foil foiled me, so to speak.  I had a remote wireless thermometer in the box but the foil prevented the signal from reaching the monitor inside so I could never tell the temperature inside the box.  So I just had to use an old fashioned thermometer & hope for the best on really cold nights, never really knowing.  But the goal of last year’s experiment was to use the boxes around the carrots to prevent the ground from freezing & enable an easier harvest all winter.  That goal was accomplished & we harvested our last carrots in mid-January.  But obviously, there were improvements that could be made:  the ability to know the temperature inside the box before going to bed at night on super-cold nights without having to go outside; something better than single pane glass storm windows, which ended up being nerve-wracking to handle because you didn’t want them to break (one did break when I attempted to tilt some snow off – duh); the boxes weren’t sloped to take full advantage of solar gain; and the ability to expand the crops we had all winter which also meant the ability to do more than just prevent the ground from freezing – possibly add some heat in the boxes if needed.

Figure 1 - Sloping & Notched Side of Cold Frame

Well, winter is a great time for gardeners to take stock & see what they could do differently, do additional research, etc.  One offshoot was that I learned that besides extending the fall harvest through the winter, I could also use the cold frames to get an earlier start in the spring.  So we set one carrot box out in the spring to warm the soil where we were going to plant carrots this year.  I was able to sow carrot seeds on 4-17-14 that germinated by 4-28-14.  Once this happened, my motivation really ratcheted up so I prowled the local Home Depot stores for culled wood for new, better cold frames.  Why culled wood?  Well, I view all this as an experiment and cannot bring myself to spend a fortune on something that might be a dud.  Culled wood is 70% off the normal price so I was able to get enough wood for 6 cold frames for under $10 a box.  (This took about  4 – 6 weeks of looking in the spring).  Not bad but the down side, which I knew, is that culled wood is discounted because it’s imperfect so I was buying warped or curved wood.  I just tried to pick the least warped wood possible. 

Figure 2a - Dowels in Upper Corner

Figure 2b - Corner Fits Together (Phew!)

During the 4 – 6 weeks of hunting for wood, I was able to get enough for one box and to find handy Ted, a handyman, carpenter, & otherwise talented fellow, who could cut the sides of the cold frames.  The sides have to slope down, with the “top” of the box (the part that will be on the north end) being taller than the “bottom” of the box that is located on the south end.  (FYI if you Google “cold frame diagrams”, you’ll see various diagrams that will give you the general idea).  Anyway, back to the sloping sides …. I was fortunate to find Ted the handyman because you have to be pretty comfortable with a circular saw to do the cuts on the wood that will become the sides of the box.  Ted also notched the top of each side so I could put a stabilizing bar across the box.  The box is 3’ wide by 6’ long.  Many resources on cold frames suggest making them widest from east to west and shorter from north to south.  But my beds run long north to south and I certainly didn’t plan to re-do the entire garden for this.  So just do what’s going to work in your garden.  

The prototype cold frame was where Tom & I planned to work out all the kinks – at least that’s what we told ourselves.  One thing we liked about the boxes from Kerrie was that they folded together for easy storage that didn’t take up much space.  So we wanted to figure out how we could make boxes that would be easily stored.  What follows is not for the faint of heart, especially when using culled wood … I decided we’d try to make the cold frames “knock down” which meant we’d dowel the corners so that we could knock the sides apart in the spring & stack everything in our basement.  No screws, hinges, or nails but anyone who works with wood will tell you dowels can be tough to work with.  There’s not a lot of room for error because you’re drilling holes in two pieces of wood – the holes have to match up & you stick a little piece of wood (a dowel) in them to match it all up & fit it together. 

We also decided that the wood would last longer if it wasn’t directly on the ground, so I found some 2” x 2” redwood on Craigslist for next to nothing.

Figure 3 - Redwood "Under-Frame" on Ground

We don’t have to go into all the details because if you want cold frames, you need to make it work for your situation.  Maybe you don’t know handy Ted but you have a handy neighbor or wood-working husband.  Maybe you don’t want to use culled wood because you’re just going to do one cold frame not six (or because you know that with culled wood, you might not get the corners to fit well …. I’ll get to the solution I have for that later).

So, prototype done, all wood bought & cut, but honestly then everything sat in the basement till early September – we got busy with the garden & other stuff.   

Figure 4 - The Prototype Minus Cover

To my dismay, Uncle Benny’s in Loveland did not have any storm windows so for the cover of each cold frame, we had to go with Plan B, much more expensive … twin-wall polycarbonate.  It’s what they use in greenhouses but holy cow, it ain’t cheap.  But it was cut to size so we only have to construct the remaining five boxes.  We’ve been doing that off and on for the past few weeks.  Tom has primed & painted 4 of the six boxes (bright colors because I know they will cheer up our HOA board).  Five boxes are constructed so we have one left to put together.  

Since we’re looking at some cool (30 degrees) nighttime temps in the next few days, we put up 2 boxes around the lettuce and spinach.  Of course, it was so quick to type that sentence but getting them set up took a few hours.  We have to level the ground out so the redwood would sit flush on the dirt, then set the cold frames on top of the redwood.  And that’s where we left it.  We’ll get them covered soon, either with the polycarbonate or just floating row covers, which will be fine for 30 degrees.

Figure 5 - First Two Boxes for Lettuce & Spinach