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.

Freedom Gardens: Baby Steps closer to Victory

Our project building cold frames at the Fairgrounds has taken leaps and bounds this week.  Not only have we acquired a key building material (triple-walled polycarbonate), we also finalized our design, staked out the boxes, and became affiliated with a local non-profit (more to come on this topic).   By working with an existing organization, we hope to be able to secure grant funding to expand this project into a full blown demonstration of cold-season food production.  We hope to show that by applying simple, cost effective technologies, we can extend our growing season and increase access to locally grown foods throughout the year.  We also hope to utilize this space to demonstrate other useful technologies including a greenhouse, worm composting bins, and gravity drip irrigation by offering topical short courses to community members.

Freedom Bed #1

Freedom Gardens.  It does not look like much yet, but come May, we will have installed 90 linear feet of raised beds protected by polycarbonate lids, watered entirely by a gravity-fed drip system.  

We decided to lay out our beds to that they would face 30 degrees west of south (210 degrees on a compass).  We did this so that our boxes will capture the maximum amount of mid-afternoon sun in the winter - a crucial time of day to build up heat in the boxes to last the long night.  

After surveying in the box locations, we painted their outline in green.  By the end of the week, the ever helpful Fairgrounds crew will loosen the soil with a grater, and then excavate down 2.5 feet with a front end loader.  After excavating out the soil (a loam full of cobbles), he will back fill it with screened topsoil.

Looking North towards the racetrack

Out of the goodness of their hearts, and a bit of extra top soil, the Fairgrounds crew also constructed us a fabulous berm.  Not only will this berm serve as a privacy fence, we will put our water reservoirs on top of it to provide pressure for the gravity powered drip irrigation system.  The berm runs up to the horse racing track, where FG crew will be able to easily fill up our tanks from the water truck with a hose from this water truck.  The truck holds 3500 gallons, so they should be able to fill up our planned ~1800 gallon reservoir easily.

The polycarbonate.  We decided to use Thermaglass™ brand triple wall polycarbonate, partially because a greenhouse builder down in Darby ordered a crate of it so we could save on shipping. We loaded up four 6’ x 16’ sheets on a boat trailer, and made the slow drive back.  Today, we cut each sheet into four 4’ x 6’ pieces.  Each of these pieces will form the lid for a bed.  Heath and Mark did most of the work cutting the polycarbonate while I took photos.  We used a sheet metal blade on a circular saw to rip it, to reduce the potential for chipping compared to a wood blade. 

Close-up of the polycarbonate

Next steps.  Mark is in the process of drawing up the final designs for the cold frames, and we will be able to put together a list of lumber we will need.  After we draw up this list, we will make a visit to our local re-used building materials center  I'll post the plans when we get them drawn up.

The site should be graded and topfilled this week, and we will be getting soil test results back from the lab soon.  That means we will need to decide on what compost we are amending our beds with, and if we need to do any correction of soil micronutrients.  We still need to figure out exactly how our irrigation system will be laid out, but we plan on using 15 mm drip tape, with 8" spacing between emitters.  Within the next week or two, we will be able to build these suckers, and begin growing from seed and from transplant. . 

Heath and Mark observe tool safety

    And finally, our terribly ferocious mascot, Teddy Chompski: