Dirty Business: Hardening Off, Part 3: Cold Frames

OK, so let’s recap our hardening off process:  First, we used a fan to strengthen the plants by imitating the wind they would have had to deal with if they were growing outdoors.  Next, we did the first transplant into larger pots to bury the plants up to the leaf line, which gives them a stronger foundation.  Now, we’re going to put them outside on a limited basis while still in the somewhat controlled environment of a cold frame.

A cold frame, in concept, is really nothing more than a box of some sort with a clear lid that can be adjusted.  They can run the spectrum from super basic to unreasonably elaborate, and really the only considerations for what type of cold frame to use come down to practicality and affordability. 

By placing plants in a cold frame, we’re able to give them much greater exposure to the outside environment while still affording them a fair degree of protection so they don’t get shocked and die.  Plants are like people in that they need to get acclimated to environmental changes.  For example, plants get sunburned just like we do.  Ever put a plant outside and at the end of the day you see that the leaves have turned a sickly yellow color and maybe even shriveled a bit at the edges?  That’s sunburn.  Likewise, they can’t be allowed to get too hot or too cold because that will also shock and kill them just like it would a person.  So, when using a cold frame, we are able to control the amount of exposure the plants are getting to the outside.

In the past, our cold frames have been as simple as a square made of cinder blocks covered with a couple of glass shower doors.  You can also use bales of straw to create the “walls”, and a lid made from a wooden frame with clear plastic stretched over it if you like.  Our current cold frame is somewhat ambitious, because Mrs. Butcher is the ambitious one.  I, on the other hand, am the lazy one.  I will not stand when I can sit, and I will not sit when I can lie down.  Anyway, she built a wooden box that is wide enough for our seed starting trays, with the walls slanting downward so that when we prop the lid up, we don’t have to reach too high.  The lid is a repurposed French door, which we prop up with two different sized posts.  We have a tall post to open the door far enough to be able to get the trays in and out, and a short post for allowing ventilation.  We also repurposed some Styrofoam sheets and stapled them underneath the glass to diffuse the sunlight.  It’s heavy as hell, so not a side-effect that I’m particularly happy with, but it is sturdy and has held up well over the past four years.

Here’s what it looks like when the tall post is used:

Here’s the short post.  For the first few days, we won’t use it like this because it’s too much exposure too soon, but we will eventually prop the door up after the plants get acclimated and the weather warms up more.

Here’s how we’re using the short post right now.  It is open just enough to allow some ventilation, but is closed enough to keep the plants from getting too cold.

The past few days haven’t been too sunny or too warm so we’ve been able to put the plants out in the morning and leave them there for the day before bringing them back inside around sunset.  If the sun had been blazing, then we’d only put them out for a few hours and then bring them back inside.  However, because the weather has been gentle so far, the plants are able to acclimate well.  After about a week, we’ll be leaving the plants in the cold frame overnight and just closing the lid all the way to retain some heat.  By Memorial Weekend, this first round of plants will be ready to go in the ground.

I did some searching for other cold frame designs so you can see the range of possibilities.  Check these out:

This one looks expensive as hell.  Plus, I’m not going to waste my hard-earned money on a cold frame for useless plants.

This one is also pretty elaborate but also looks homemade and therefore more affordable.

Here’s the super basic straw bale method.  It’s not pretty, but it works very well and it’s cheap.

Here’s one that looks kind of like what we did in the past with cinder blocks.  One important note here is that, like the one we used, this one also doesn’t actually mortar the blocks and bricks together.  This is something that you want to be able to dismantle at will because otherwise it’s just too heavy to move anywhere when needed.

So, for those of us who don’t have the luxury of a greenhouse (COUGH-Loveshaq), cold frames are a very useful way to get a good head start on getting your plants ready for the ground, and literally anybody can make one, no matter their level of expertise or comfort.