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Home Explore the Science of Hydroponics Greenhouse The Differences Between a Cold Frame and a Greenhouse

The Differences Between a Cold Frame and a Greenhouse PDF Print E-mail
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Green with Envy: What your plants would tell you if they could talk. (The differences between a greenhouse and a cold frame).

Recently a friend sent me a cartoon which perfectly described what having a greenhouse is about. The picture was of two small flowers growing in the ground outside peering wistfully into a greenhouse. The greenhouse is filled with beautiful blooming plants bathed in perfect light and watered by a bikini clad woman. One of the small flowers outside said to the other, "Oh man, that's the life".

One of the most common misperceptions of people looking to purchase or start using a greenhouse is that if it's called a greenhouse, it will grow plants. Often a structure is called a greenhouse if it can hold plants with no regard for the environment it will create. You could easily purchase a "greenhouse in a bag" for a few hundred dollars, but you would not be happy for very long zipped up in a plastic bag. Chances are your plants won't like it either. Several greenhouse makers recognize the need for some basic industry standards to help consumers make more informed choices. Until those standards are developed, however, gardeners must spend a little time to educate themselves or run the risk of making a costly mistake.

There is an important place for entry level products as long as you know what to expect before making an investment of money or time. Most products called greenhouses on the market today are actually large cold frames. The good news is that there are many good options. Some of the least expensive products on the market can provide good results if the uses and limitations are understood. Even a "greenhouse" in a bag can give plants a head start if used properly.

Understanding the difference between a greenhouse and a cold frame is the key to achieving the best results. Shane Smith, author of the definitive resource on greenhouse gardening; The Greenhouse Gardener's Companion, defines a greenhouse as a structure that is heated. A properly designed greenhouse will allow the gardener to control the temperature and environment within a specific range. A greenhouse is either operated as a hot house or a cool house, depending on the zone it's in and how much heat is added. A hot house maintains a minimum nighttime temperature of 55 degrees or greater. A cool house would be one that will maintain a minimum nighttime temperature of 45 degrees. In most cases, a maximum greenhouse temperature would be around 90 degrees.

In contrast to a greenhouse, a cold frame is any structure that is generally used to extend either end of the growing season by a few weeks. Cold frames are not designed for heat, so the temperature cannot be closely controlled. No external heat is added, so they depend solely on the heat generated from the sun's radiation. Many cold frames are covered with a single walled glazing. Without insulation, only a minimal amount of heat from the day will remain overnight. They are not a good choice for winter growing or wintering over in cooler climates. Cold frames are most suitable for the few weeks before and after the last frost. With a cold frame, you can get a jump on spring by a few weeks before the soil was fully warmed and before the threat of frost has passed. Warming the soil temperature by even a few degrees can drastically increase the success of early plantings. In fall a cold frame can guard against an early frost giving some winter crops an opportunity to develop strong healthy roots before facing a long cold winter. In an area with a short growing season a cold frame may be all that is needed to give tomatoes that extra few days of precious sun to ripen.

Many people think that if you can walk in a structure it's a greenhouse. They are often surprised to learn that cold frames come in all shapes and sizes. A cold frame can be small structures on the ground with a hinged lid or it can be a long tunnel structure used by commercial growers. Greenhouses are easily used as cold frames if they are not heated, however, just adding heat to a structure will not turn a cold frame into a greenhouse.

There are some types of structures that cannot be safely or effectively heated. Factors to carefully consider include: the structure's size, the structure's ventilation capabilities, the type of covering used, and the frame. The size of the structure is the first consideration. Small structures are more difficult to heat than larger structures because they overheat easily. Any structure with low overhead clearance or less than 64 square feet of floor space will heat very quickly with the addition of external heat. The overheating may cause damage to the plants if the air temperature rises above 90 degrees. Ventilation is the second consideration. Structures that do not have ventilation will not be able to expel excess heat. If the heat source is a gas heater, the air will be moisture laden causing more potential issues from the excess humidity. Controlling a greenhouse environment requires the exchange of fresh air and the addition of heat.

Another important factor to consider when deciding to add heat is the type of covering used. Single wall covering such as glass, fiberglass, film and single wall polycarbonate do not provide any insulation. Heating a single wall structure is not much different than heating the outside air. Most of these coverings are not compatible with ventilation so they temperature can fluctuate between hot and cold rapidly. The framing is the final consideration. Some plastic or resin framing should not be near external heat sources since some materials have a low melting point.

If you are interested in growing tropicals, growing outside your zone, starting seeds early, wintering over sensitive plants in cooler climate, or actively growing in the winter you may need a greenhouse to accomplish your goals. Some seeds will need minimum soil temperatures of 50 degrees in order to germinate and establish themselves, but many seeds will germinate at 65 degrees or higher. Many of our summer vegetables, tomatoes for example, are tropical plants which thrive in the warmer temperatures. Warmer soil and air temperature alone is not enough for some plants to flourish. Some plants are day-light sensitive which means that even if they have the heat they require, they will not flourish in months where there is little daylight without supplemental light. Many greenhouses are designed to support grow lights and can be used to grow even sensitive crops all year. A greenhouse can be used to grow in most places all year long, provided you are willing to add the heat, ventilation, and in some cases, the light necessary.

Once you decide what you want to grow and when you want to grow it, you can determine if you need to use a greenhouse or a cold frame. Since most products are called greenhouses, make sure to look at the covering, the ventilation, and for a sturdy frame. If you are unsure if a product is a greenhouse or cold frame, ask how and where you add a ventilation fan. If a fan cannot be added, then heating the structure will be difficult. Price may be another clue. There are few greenhouses available for less than $1,000, although there are also quite a few high quality cold frames that sell for thousands of dollars. Regardless of whether you use a greenhouse or a cold frame, the plants inside will be the envy of the garden.

Michelle Moore is the general manager for The Greenhouse Catalog. She is an Oregon State University Master Gardener and has nearly 20 years experience working with greenhouses. She lives in Oregon with her husband where they are gardening outside of a greenhouse for the first time. You can contact Michelle at michelle.moore@greenhousecatalog.com or your can visit their website at http://www.greenhousecatalog.com/garden/

This article was originally published in Garden & Greenhouse magazine. Garden & Greenhouse is written for small commercial growers & retailers, hobby greenhouse owners and indoor/outdoor gardeners. For a Free subscription (US addresses only) go to www.GardenAndGreenhouse.net .


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Last Updated ( Thursday, 15 February 2007 )
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