The Cold Frame: Underappreciated Friend of the Rose Grower

By Cliff Orent, American Rose Society (ARS) Consulting Rosarian & Certified Horticulture Judge

What exactly is a cold frame?

Historically a cold frame, or cloche, was a structure, most often made of brick or wood, with a hinged or removable transparent top, used to protect plants from cold weather. They were constructed in such a way that the clear top would allow heat in during the day, creating a "greenhouse effect," trapping heat that would otherwise escape at night. These structures were unheated and used primarily to extend the growing season in late fall or early spring, to protect plants during winter from the most severe weather, and, with covers removed, for hardening off.

Today, the term "cold frame" is often used interchangeably with "greenhouse" for a cold frame with a heater can quickly become a greenhouse for all intents and purposes.

Why did I purchase a cold frame?

Until last year, I had always grown my roses in the low desert, and purchasing a cold frame was the farthest thing from my mind. My objective was always to keep the roses cool, given that summer high temperatures often reached and exceeded 115° F. But with my recent move to the high desert, I was quickly faced with the fact that my tropical plants (e.g., plumeria, exotic hibiscus, certain types of cacti, etc.) would not be able to withstand the below freezing temperatures of the high desert winter, and without protection, would die. With little time to deliberate, I made the decision to purchase a cold frame, along with a space heater, that would hopefully allow my tropicals to get through the winter. Little did I suspect that I'd soon find more than a few uses for the cold frame for roses. I would imagine that most purchasers of cold frames have a specific use in mind at the time of purchase and that they, like I, soon find multiple other uses for the structure.

What size cold frame should you consider?

The three primary considerations when deciding what type of cold frame to purchase are space availability, intended use and financial resources.

Cold frames need not be large nor need they be fancy. If you have but a few plants to protect and winters aren't all that harsh, then a home-made cloche or a small cold frame will likely do the trick. However, once you see the potential benefits of a cold frame, assuming you live anywhere other than southernmost Florida, coastal or low desert areas of southern California, or perhaps a few of the warmer areas in Texas or along the gulf coast, you'll likely want to spring for something somewhat larger than you might at first think.

My first cold frame

I simply estimated the space required to house my tropical and semi-tropical plants for the winter and opted for a 30' long x 10' wide by 7' high structure from TekSupply, which arrived requiring assembly. With a number of manufacturers from which to choose, I decided to go with their EZ-Build & Gro Cold Frame line — I liked the EZ-Build part! These cold frames range in size from 5' long x 6' wide x 7' high to 50' long x 12' wide x 7' high, and film and end frame kits are available. Even the largest structure in this line is a far cry from the many thousands of dollars that can easily be required for a custom greenhouse, and these cold frames can be expected to provide several years of use, even in tough climates, when properly assembled and anchored. Plastic covering may have to be replaced every three or four years, depending upon climate, but the basic structure should last far longer.

Doors for cross-ventilation

While most cold frames are slightly cheaper without the door kits, it didn't take long to learn that cross ventilation was critical, given the heat that accumulated during the day, and doors at both ends were well worth the relatively minimal additional expense.

Jump starting bare root roses

Once my most tender tropicals were safely tucked away inside the cold frame, I started to experiment with various rose-related uses. One of first things I tried was starting my bare root roses inside the cold frame. Bare root roses are available in the low desert from Thanksgiving through January, and plants started during these months would be off to a rough start in the high desert, where lows dipped through the 20°Fs and even into the upper teens. Protected from the cold and wind, my bare root roses were blooming inside the cold frame at least two to three months before that could have happened outdoors. As the temperatures began to moderate and the roses became established, I moved the plants from the cold frame into an area covered by shade cloth to transition them into full sun and wind.

Magma, a new bare-root rose

This photo shows Magma, a new hybrid tea from Kordes, which arrived as a bare-root plant from Palatine Roses (Ontario, Canada) in November, already out of the cold frame and blooming in the shade structure in March. It's the end of April as I'm writing this, and by comparison the hybrid teas which spent the winter unprotected outdoors have yet to come into spring bloom.

Protecting young own-root roses

I learned the hard way that there's tremendous benefit to be gained from protecting young, own-root roses in a cold frame over the winter. A number of my fall arrivals succumbed to the below freezing temperatures coupled with strong desert/mountain winds, and those that did make it through will now need several months just to recover to where they were before winter hit.

Here's an example of an own-root rose that arrived from Vintage Gardens in November that spent the winter outside, without protection:

Own-root plant outside for winter

And here's another own-root plant, that arrived at the same time, after spending the winter in the cold frame:

Own-root plant in cold frame for winter

Both of these own-root plants arrived at the same time in November, and both were similar in terms of appearance and maturity upon arrival. And remember that the plant in the first photo was one of the lucky ones — several others that weren't brought into the cold frame didn't make it through the winter.

Other uses for cold frames

Only the imagination limits the possible rose-related uses for cold frames, but another that quickly became important for me was in rooting rose cuttings. Add heat, a fan and a misting system or fogger, and you have an instant greenhouse for rooting cuttings (see right image).

Rooted cuttings in cold frame

The cuttings shown in this photo were started in January and the photo was taken roughly twelve weeks later.

Summary

In conclusion, for the majority of climates in the U.S., cold frames can be of an enormous benefit to rose growers. They can take the form of a small unit on an apartment balcony, or can be the equivalent of a fairly good-size greenhouse, space permitting. Cold frames can be used to start bare-root roses, to protect young own-root roses, to root cuttings or for other purposes, limited only by the rose grower's imagination. I'd urge you to give it a try and would wager that once you've experienced some of the benefits yourself, you'll soon be looking for a larger structure for a variety of rose-related uses.

About Cliff Orent

Cliff Orent has served as president of the Desert Rose Society, is a certified horticulture judge and co-owner of the website Rosarians Corner and also runs Euro Desert Roses. Having planted his first rose in 2001, Cliff's collection now numbers close to 6,000 roses, including about 2,000 seedlings from his hybridizing efforts. He is co-chair of the 2009 ARS Fall National Convention which will be held in Palm Springs, CA for the first time. Active in preservation efforts, he has worked with the World Federation of Roses Societies' Specialized Conservation Committee.

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