The Cold Frame Corner (Part II)

Posted by: Susan Perry, Master Gardener in Larimer County

Well, a lot has happened since my last blog post.  All seemed to be going well in October/early November, although we noticed that the temperature in the lettuce/spinach box was no warmer than the outside air temperature.  Further study revealed the first (of several) flaws in our plans – the R-value of all the materials used in the cold frame really matters.  So, culled (bent) wood that doesn't have tight corners plus polycarbonate tops weren't going to do a darn thing toward keeping temperatures in the cold frames warm enough. 

 

Lots of good spinach and lettuce

So toward the end of October, we decided we had to revisit the use of rigid construction insulation to line the boxes.  We did that, but still no joy because (we finally figured) that all the heat was escaping via the polycarb.  Even though it is twin-wall polycarb, it just doesn't have any insulating power.  Yikes!  By now, it was Monday November 10^th & the forecast said weather was moving in – a “polar vortex” that would cause daytime temps in single digits and nighttime temps below zero.  Uh oh ….. when I left the house for a salvage company east of Loveland that had some Styrofoam on Craigslist, the temperature was in the 50s.  By the time I was loading a few pieces of Styrofoam into my car, the temperature had dropped at least 15 degrees and the wind had picked up, and by the time I got home, it was even colder. 

 

Beets inside and outside the cold frame.

We shifted into almost panic mode – we needed to do whatever necessary to keep the lettuce/spinach alive.  The ground around the carrots, beets, & leeks would not freeze in a week of cold temps so we focused on the two lettuce/spinach boxes.  We lined each box with two strands of incandescent indoor/outdoor Christmas lights, covered the lettuce/spinach w/floating row cover, put the polycarb tops on, put a flannel sheet, then a quilted moving blanket, then a sheet of plastic weighed down with rocks.  Then we went inside.

 

Using Christmas lights to increase the heat inside the frame.

The two strands of Christmas lights per box worked.  At the highest, before we figured out how to make best use of the lights, the temp in the box got to 70 degrees.  We needed to be sure the air temp didn’t drop below about 30 degrees.  With the lights, it didn't seem like a precise approach, so we just made sure to keep the cold frame temps in the mid-30s.  There were a couple of middle-of-the-night temperature checks.  Finally, we noticed a pattern:  the box would heat up rapidly till it got close to it’s maximum temperature, then even with the lights still on, it just wouldn't get warmer; and when we unplugged the lights, the temp would drop to 50 degrees, then take 8 -10 hours to get down to 39 degrees.  This led us to conclude that we could plug the lights in before going to bed & unplug them in the morning.  Even better was when we got the lights on a timer, because we could have the lights turn on an hour or two after we went to bed & go off an hour before waking and all would be well.

The brutal cold finally moved east so we were able to make some additional modifications.  We’re not keen about using rigid insulation, but we made tops for the beet & carrot boxes just until we got something better worked out.  And the leeks …. well, we never got them protected at all & despite having the most cold tolerant variety possible, well they were toast (or should I say mush?).  We've salvaged what we could, sautéed & froze it for future use in soups.  But that was a disappointment, but not really a surprise.  The height of the leeks, even if we trimmed the tops off, was still an obstacle to constructing a decent cold frame that would work.

 

Root crops during "polar vortex".

Where are we now?  We’re testing space blankets in the lettuce/spinach boxes instead of floating row cover plus flannel sheet plus quilted moving blanket plus plastic sheeting.  So far, it looks like it’s a great alternative.  The space blanket reflects the heat from the (now) single strand of Christmas lights in each box.  We've devised wickets for each box & have tried to make tabs on the space blankets so we can attach them to the wickets and slide them open & closed in morning and evening.  Attaching the space blankets to wickets makes them easier to deal with on windy days, and means we can just leave them in the box, slid open, all day before sliding them back in the evening when we close the cold frames up.  We also were able to purchase proves for our wireless thermometers so that the foil on the rigid insulation would not disrupt the signal. 

 

Adding wickets and space blankets to the cold frames.

We still need to make wickets for the beets and carrots so we can use space blankets in those boxes.  We also need to get a better sense of what combination/number of Christmas lights are necessary in the lettuce/spinach boxes to keep the temps between 35 – 45 degrees regardless of how low temps go in the next “polar vortex”, which is sure to come back.  We also need to harvest all of the remaining root crops (carrots, beets, & leeks) that were outside of the boxes – always planned to be harvested first while the cold frames would be harvested last.  We’re just hoping that there’s something to salvage …. we’ll see.

 

We survived the cold!

So, except for the leeks, the experiment is working.  And we already have ideas for future modifications to improve the process.  Just too bad the “polar vortex” arrived so soon, before we were really ready, but that’s the way it goes sometimes.

Next week:  home-made ravioli using spinach from our garden!  I’ll let you know how it turns out.