Wednesday, March 28, 2012

Article: Colorado Cold Frame

A cold frame built from recycled materials protects herbs and greens after a heavy snowfall

At 40 degrees north latitude, Boulder, Colorado gets the same amount of sunshine as places like Valencia, Spain; Naples, Italy; and Istanbul, Turkey.

Valencia has garnered fame for its oranges. Naples is considered the birthplace of pizza, spaghetti, gelato, and a number of fine wines and olive oils. Istanbul is famed for its cotton, fruits, and olives (as well as a bit of confusion over what the city should be named).

Valencia, Naples, and Istanbul are a gardener's paradise.

Boulder is not.

This habanero plant was among the lucky ones
in this hail storm. It lived to produce a few peppers
before the fall frosts killed it
Here in Boulder we routinely see 90+ mph winds, golf ball sized hail, and 3-foot snowfalls. On top of that, we're a mile above sea level... so the sun shines brightly, the air is dry, and even summer nights get cold in a hurry.

At our house in Boulder, our vegetable garden is a 15 by 40-foot spot that used to be "lawn." No doubt in 1962 when the house was built it was a luxurious carpet of Kentucky blue grass. In 2004 when we moved in, however, it was two-foot-tall crab grass and weeds.

Our second summer here, we covered the grass with mulch to kill it, roto-tilled it under, and began work amending the soil for a garden.

I don't know how many tons of manure and compost we mixed with our native rock and clay before we had something resembling "soil" in our garden. Let's just say it was a lot. Each year we've continued to add organic material: spent grains from the local brewery, a prodigious supply of yard and kitchen compost, chicken coop manure and straw, and just about anything else we can get our hands on to help dilute the clay. Seven years later, we've got a healthy population of worms... and what I'd actually call soil.

But the dirt has been the least of our worries.

Because we can get blizzards and heavy frost into late May, most folks recommend waiting until mid-May (Mother's Day) to plant anything.

A spring thunderstorm drops golf ball-sized
hail on south Boulder
The first year we planted our garden on June 6th (yes, D-Day) just to be safe. We congratulated ourselves for being patient and avoiding a late frost that had struck two nights before. Hours after the last tomatoes were lovingly tucked into the soil, a thunderstorm dropped 4 inches of golf ball sized hail in about 10 minutes. The garden looked like someone had taken a machine gun to it. We had to replant everything the next day.

The next year on the exact same day a windstorm came up and literally blew all of the plants out of the ground. All that was left were a bunch of square holes in the ground.

Over the years... we've learned to cope with the wind and hail. Sometimes, just before a thunderstorm hits, I'll put on a ski helmet and go out to cover the plants with buckets, plywood, or even canoes. Other times, we have to just suck it up and replant after mother nature has finished slapping us around.

In those lucky years when we sneak through to late June with no hail or wind damage, we then confront the searing Colorado heat. Boulder is very dry in the summer. And really hot. Temperatures in the upper 90's are common, and we can have a week or more where we break the century mark.

Our poor plants just don't have enough time to grow a sufficient root network between mid-May and late June... and typically wilt flat every afternoon no matter how much we water them. The strong plants perk up by evening... just in time to gird up for the 60-degree downslope wind that flows out of the nearby snow-capped peaks almost every night.

An early fall heavy snow decimates the yard
Most years, just about the time the tomatoes really start producing in September... we get a hard frost or a heavy snow (or both), and it's all over until the spring.

While the weather is unpredictable, our neighbors are a steady source of wisdom and shared experiences. One evening at the Southern Sun brewery, a few of us were chatting about the challenges of Colorado gardening. Kevin, from a few doors down, offered to lend me his copy of Eliot Coleman's book entitled Four-Season Harvest: Organic Vegetables from Your Home Garden All Year Long. In it, I was re-introduced to the concept of the cold frame.

When I was a cook at an adventure lodge in Alaska, we used summer cold frames to ensure a steady supply of fresh herbs for the restaurant. During the Alaskan winter, of course, the cold frames were stored away along with the deck chairs, rafts, and hip waders... but Colorado is a different story.

Here in Boulder, I resolved to reproduce the subtropical climates of Istanbul in my winter back yard—armed with leftover fence posts and two dumpster-reclaimed sliding glass patio doors.

My first task was to find a good spot for my cold frame. Our house faces northeast, so my two choices were the southeast-facing side yard, or the southwest-facing back yard. While I eventually plan to have cold frames on both sides, I decided to start on the southeast side since the landscaping was a little more defined on that side of the house. 

My next task was to figure out a design that would make the best use of my salvaged materials, but also handle these basic requirements:
  • Must be cold hardy. In the past 7 years we've had a few winters with a week or more of -20 degrees F nighttime temperatures.
  • Has some headroom. I wanted a cold frame that could accommodate taller plants like eggplant, jalapeno peppers, and perennial herbs like rosemary. 
  • Is able to blow off steam. Boulder routinely gets 75 degree F days in December, January, and February... so the cold frame had to be able to handle these surprise "summer" days without undue human intervention.
  • Looks sharp. Our side yard has a landscaped walkway that's the main entrance for my wife's tutoring business. I wanted a cold frame that blended handsomely with the existing landscaping. 

Our house is made of brick, so I decided to use the house as the back wall of the cold frame to provide a heat sink. The sliding glass patio door panes I had were 80 inches tall and 32 inches wide. I figured this would give me a cold frame that was roughly 7 1/2 feet long and 2 1/2 feet wide. As luck would have it, I had just enough leftover cedar from a fence project to build two cold frames of this size. 

6" x 6" posts overlaid at the corners
Thanks to a somewhat over-engineered fence project, my fence posts were 10-foot 6" x 6" posts. I stacked the 6x6 posts on top of each other, which gave me a raised bed roughly 11 inches tall. You could use just about anything to build a simple raised bed approximating these dimensions.

From there, I used a combination of 4" x 4" and 2" x 4" leftovers to complete the framing so that one long side of the patio door pane would rest on the outer edge of the raised bed, and the other side would sit close to the house wall. The patio door pane sits at roughly a 45-degree angle when the cold frame is closed. The patio door rests in place through gravity alone. When I want to prop it open, I simply tilt it forward and prop it with a stick.

A lot of cold frame designs call for glass that sits nearly flat, but I've found that the extra head room is good for accommodating the taller plants, and also helps give excess heat somewhere to go. I have a 2" x 2" that sits across the top of the window pane, which seals off a gap of about 1 1/2 inches. In warmer weather, I leave remove the 2" x 2" so that extra heat can escape. 

Once completed, I filled the base of the cold frame with a mixture of 1/4 horse manure and 3/4 soil from the garden. We've planted it with all sorts of things in the past year and a half: romaine lettuce, swiss chard, eggplant, jalapeno peppers, kale, spinach, cilantro, rosemary, Italian parsley, and more. 

Heads of romaine lettuce reach maturity in February
next to perennial Italian parsley and spinach starts
The rosemary and parsley plants are perennials, but we usually plant hot weather stuff like eggplant and jalapenos in early spring for a summer or fall harvest, and then cold-hardy plants like kale, spinach, lettuce, and chard in early fall for a winter harvest. 

What I've found works best is to plant the winter crop in September, so the warm days of fall help germinate the seeds, lead to strong growth, and get the plants established for the winter. Once late November hits, the plants effectively stop growing. These nearly full-grown plants will hang out in suspended animation in the cold frame all winter long... and are tasty and "spring fresh" when you pick them in January or February. 

It's all about experimentation... and this summer I'll be trying cherry tomatoes, tomatillos, and habanero peppers. This fall, I'm looking to break into endive, escarole, and carrots. 

When the winds start to howl, the hail falls, or the mountain snow piles up, I always sleep a little better knowing that some part of my garden is cozy and warm under the protective blanket of my neighbor's patio door, and some leftover fence posts. 


  1. There's an old saying I just made up, if you want something to complain about, start a garden. Even here in sunny SoCal, my winter seed experiment was pretty much a flop. I like your idea of a cold frame, and I think I can build one on a smaller scale for starting seedlings. Now I just have to keep my eyes open for a scrap window.

  2. Let us know how it goes! We had 84 degrees in Boulder on Sunday, and snow today...



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