How warm is a cold frame? and First spinach of the season

A few months ago, I wrote a blog post detailing how to build a cold frame made from recycled materials. In that post, I mentioned that I would test to see how "warm" my cold frame really was, using two temperature probes.  I placed one probe inside the frame, and the other one nearby in the garden.  I put radiation shielding on each probe (aka pvc pipe) to try and measure shade air temperature.  By comparing the diurnal temperature profiles for each prope, I should be able to tell how effective the cold frame is at raising nighttime temperatures.  

Anecdotally, the box is working wonderfully! I just had my first spinach salad from the box, only 24 hours after it snowed 2", and in a neighborhood known for deer and deep morning shade.

After building the frame, sealing up air gaps with soil and leaves, and letting the cold frame thaw the soil, I shallowly cultivated the soil, threw a few handfulls of spinach seed down, and walked away.  I left the frame in my father's garden, and I could only get by a few times a week to check on the seedlings.  I would occasionally prop the lid open, or close it, but it was mostly left propped about 1/3" open all day, to protect from frost in case my father forgot.

It turns out that the cold frame did an excellent job protecting against frost, as I expected.  However, I also learned that despite satisfactory results (greens to eat), I need to do a much better job of opening the box on clear, sunny days.

A summary of my findings:  Out of 30 nights where it got below freezing outside the box, it only froze on 9 nights inside. I just extended my season by 21 days out of the 45 that I ran the experiment.  The coldest it got in the box was 24.7 degrees, which did not kill my spinach.   The maximum frost protection provided was 22 degrees on March 17, when the lid was closed and covered with styrofoam and a rock. It got down to 13 F outside that night, and stayed 35 F inside the box.

Unfortunately, the box also got above 100 F on 13 different days.  Luckily, the spinach, and some lettuce transplants managed to survive these hot conditions, including a scorching 123 degree afternoon at the end of March. 

Stay tuned for more news about our cold frames at the Missoula Co Fairgrounds! We are in the process of construction preparation right now, and they should be up within the month.

If you are really interested....here is all of the data graphically instead of just highs and lows, or contact me for a spreadsheet.

Building a cold frame with a reclaimed shower door and lumber

The balmy, Pacific Maritime winter that Missoula has been experiencing has made me think about ways that I can grow, or harvest vegetables year round.  There are many basic technologies that gardeners employ to extend their vegetable growing, or harvest season.  Most season extending techniques utilize some sort of protective covering that conserves or traps heat underneath the protection, to raise soil and/or air temperatures. Season extender technologies include low hoop-houses (cloches), high tunnels, row covers, cold frames, and warm frames.  Each of these technologies has its advantages and drawbacks in terms of cost, durability, mobility, and scale.  Cold-frames are low cost, easy to install, and durable.  However, they are less movable than a row cover or cloche, but  are ideal for the home gardener who is planning to use the box year after year.   

Using a cold frame, gardeners will be able to extend the growing season for a few weeks in the spring and fall, and may use a cold frame to store or overwinter crops of carrots, onions, etc. until they are ready for harvest.  A cold frame is essentially a rectangular box, with a glass lid on top. Many gardeners in our lattitude transplant frost sensitive species (cucumbers or pumpkins) into cold frames to give them a leg up in the spring.  Cold frames can also be used to grow cold hardy greens (mustard, spinach, mache, chard, etc.) weeks later in the fall, and weeks earlier in the spring.

I was at my local re-claimed building materials center (www.HomeResource.org), a few weeks

 ago, and saw that they had a couple of dozen tempered glass shower doors sitting in the yard, begging to be used for some project.  Although you may think that the opacity of tempered shower glass might be a less than ideal material for a cold frame, the uneven and frosted glass is excellent at diffusing sunlight, so that all sides of the box receive even light, and to protect plants from harsh, direct sunlight as the season progresses.  Also, this glass is shatter proof, and will be much less prone to damage from a hailstorm, or a lawnmower hitting a rock.  

There are many styles of shower doors available at Home Resource, and I chose two identically sized shower doors (about 5' x 2.5') which already had a full length hinge attached to the door.   I wanted the shower door to be slightly pitched towards one direction (to better collect light from the southern sky), so I purchased reclaimed 2x10 boards for the back edge, and 2x6 boards for the front edge of my cold frame.  The sides were made out of short pieces of 2x10 cut on the bandsaw into trapezoidal shaped wedges.   

Here is a list of supplies I used for 1 cold frame:

Sides and back: 62" + 2*27" = 116" of 2x10 lumber

Front: 62" of 2x6 lumber. 

I planed a 1x4 down to a 1/2" width, and ripped it in half to fit inside the hinge.

4 metal brackets to stabilize inside corners of box (see second photo)

Some 3" deck screws

Former Shower door with full length hinge

I designed the box so that the metal frame around the shower door would lie flush on the wooden frame (ideally).  After cutting the front and back boards to the proper length, I used a table saw with the blade on a 10 degree bevel to cut off the front edge of each board, so that the shower door would sit flat.  

To make the sides into trapezoids, I used 2x10 lumber, drew the pattern on the board, and used a band saw to cut one corner off, to make a trapezoid.  Because I'm not skilled with a bandsaw, I used a planar attachment to get the side boards perfectly straight.  Taking your time getting these pieces right will improve the ultimate air-tightness of the frame. 

I then assembled the box using metal brackets on the inside, to stabilize the shape, and 3" deck screws to join the boards from the end.  After removing the handle and latching mechanism on one side of the shower door, I screwed the door, and hinge onto the box using biting metal screws.  

After assembling, there were still some air gaps so I used self-adhesive rubber foam to fill the gaps, and trimmed them down with a utility knife.

For a total cost of less than $15 dollars, I was able to make a cold frame that should help me grow greens in the spring and fall, and protect frost sensitive plants for years to come!  Look for updates as the season progresses - I'm planning on installing temperature probes to determine how effective the cold frame is!