Cold frames extend growing season

By Ted Schaaf
Gardens on Spring Creek

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Cold frames were originally placed on the south sides of greenhouses, mainly to provide a location to acclimate greenhouse tender plants to the outdoors before planting them outside.

Today, many gardeners are discovering that these mini-greenhouses allow them to extend the growing season by two to three months. Imagine the joy of harvesting your own fresh greens and serving them for a Thanksgiving dinner.

Building a cold frame

I recently spent an enjoyable morning at the home of one of our garden volunteers, Linda Vrooman. She and her husband, Bob, welcomed me to tour their raised garden plots.

I was especially interested in the cold frame that Bob built. Bob's cold frame measures 5 feet by 8 feet. The bed consisted of four 2-by-8 boards held together by lag bolts to create a frame. A second frame was placed on top of the first frame. Half of the bed is filled with amended topsoil, thus avoiding our heavy clay soils.

The biggest expense, according to Bob, was the corrugated polycarbonate that he purchased at a local home improvement retailer to be used as the cover.

Bob also added a very helpful, solar operated vent opener to automatically open the lid of their cold frame. While not absolutely necessary, gardeners who have just once forgotten to open their cold frame lids and experienced the total loss of all their seedlings will tell you a vent opener is a worthwhile investment. These openers can be purchased for around $50 from Charley's Greenhouse & Garden (www.charleysgreenhouse.com) and other greenhouse suppliers.

This cold frame was so simple to build that I can't wait to pull out the circular saw and build one for myself!

Growing greens

Before planting greens, I like to add organics to my soil. At the Gardens on Spring Creek, we use both alfalfa pellets and cottonseed meal. Both products are high in nitrogen and are available at local feed stores.

Greens, with lettuce topping the list, are by far the best choice for a cold frame. The early seeding of lettuce, carrots, kale, spinach, radishes, beets and Swiss chard in March produced over eight bags of greens that Bob and Linda donated to the food bank.

From my own experience with kale, I can tell you this is one tough vegetable. Even when planted outside, it's the last one to give into the arctic cold fronts. You might also consider planting turnips, arugula and cabbage.

Bob will begin reseeding into his cold frame in September. This will give the seedlings a head start before the cold weather sets in.

Watering plants in a cold frame will take a bit of experimentation to determine frequency, which changes from day to day. As a general rule, though, the colder the temperature is outside the less water your cold frame plants will need. It's usually best to water in the morning, as this will allow the plants to dry off before heading into the evening.

In addition to growing vegetables, a cold frame is also an excellent place to start cuttings. Take your cuttings anytime between September and February and remember they will root better with bottom heat from a submerged heating cable.

There are a number of greenhouse flowers that would benefit from completing their growing cycle inside a cold frame. Pansies come immediately to my mind. I always struggle with keeping this plant from stretching in my warm greenhouse. Snapdragons, alyssum, and nasturtiums would also welcome the cooler temperatures found inside a cold frame.

Most large-scale nurseries over-winter plant material inside large sized cold frames called hoop houses. It's a real joy to walk into them in early February and see perennials beginning to show signs of new life several months before anything else outside is waking up.

It's clear to me that these mini-greenhouses are a must for the avid gardener, especially in a year like this one when Mother Nature has handed us several setbacks to our gardening season. If you're not ready to give into our short season, now is the time to start building your own cold frame. Don't let your lack of carpentry skills discourage you. Many Internet companies offer them for sale for under $100.

Why not make your cold frame a fun family project? The small size makes them an ideal way for small children to experience the joy of gardening.

Happy gardening.

Ted Schaaf is a horticulturist for the city's Gardens on Spring Creek, located off Centre Avenue about a half mile south of Prospect Road in Fort Collins.