How Greenhouses Work

A greenhouse can extend a plant's growing season by a few weeks, or it can create a complete microclimate that's a successful substitute for the plant's native environment. From its origins as an indulgence for the wealthy and privileged, greenhouses are now used in many important and unexpected ways to help man understand and make use of the natural world.

Since World War II, there's been a huge increase in the commercial use of greenhouses to feed the world's population. In temperate climates, greenhouses extend the growing season and protect plants from harsh weather conditions. In higher latitudes, greenhouses increase plant production by optimizing the available light. Even in hot, arid regions, specialized greenhouses have been developed that can help lower temperatures and manage water loss in plants due to transpiration.

­­Cultivating attractive and hardy plants for commercial nurseries and florists is a thriving industry that relies on greenhouses to create perfect, pest free plants.

In many areas, controlling the environment inside a greenhouse is easier and more reliable than trying to manipulate all the variables of growing delicate plants outdoors.

Universities and research facilities around the world, like the Eden Project in St. Austell, England, are using the controlled environments in greenhouses to recreate specialized ecosystems in order to understand plants better. Greenhouse technology is making it easier to study the potential value of medicinal plants and explore ways to increase plant yields and make plants more disease resistant. Greenhouses are even being used to preserve plant species whose natural habitat is threatened. So let's take a look at how they work.

Greenhouses create a sheltered environment for plants by using solar radiation to trap heat. This system of heating and circulating air helps to create an artificial environment in a greenhouse that can sustain plants when the outdoor temperature is too cool or variable. Heat enters the greenhouse through its covering of glass or plastic and starts to warm the objects, soil and plants inside. The warmed air near the soil begins to rise and is immediately replaced with cooler surrounding air that starts to heat up. This cycle raises the temperature inside the greenhouse more rapidly than the air outside, creating a sheltered, warmer microclimate.

In temperate climates, the sun might do all the heating in the greenhouse, but where the temperatures plummet, artificial heat may be necessary to maintain temperatures above freezing. Where some greenhouses have access to central heat from the main building, others have to rely on natural or bottled gas, heating coils or heating fans. These will usually work in conjunction with a thermostat. Because heat is one of the biggest expenses of keeping a greenhouse, other sources of energy are always being explored, like the use of solar batteries or animals as heat sources.

There are also other processes acting on the air inside a greenhouse. The sun's energy can travel through greenhouse glass easily, but the radiation emitted by the plants and soil that have absorbed the heat doesn't get out as easily, helping to trap heat inside.

This makes it possible to keep a greenhouse warm, but it also can cause problems with overheating. In order to keep plants from getting too hot, some method of heat control is necessary. Vents that allow the lighter, hotter air to exit the greenhouse near the roof and cooler air to enter closer to ground level act as air conditioning. Proper ventilation keeps the air in a greenhouse circulating. This helps maintain a stable temperature and also cycles the carbon dioxide (CO2) that plants need for photosynthesis [source: Martell]. Generally, greenhouses have at least two vents, one on or near the roof and one on the lower half of the structure. Mechanical ventilators can also help maintain good airflow and heat control by opening and closing the vents automatically when the temperature in the greenhouse changes.

And of course, all of the plants inside a greenhouse need some sort of water. Whether you use a garden hose, watering can or a sophisticated automated system with water sensors, water is essential in a greenhouse. Because watering is the most time-consuming greenhouse chore, the use of some type of automated system, like wicking, capillary matting or drip irrigation can make the process more consistent and reliable. Even if feeding water directly to the greenhouse via an underground pipe isn't possible, placing a greenhouse near water is a practical necessity.

In the next section, we'll explore different types of greenhouses and the relationship they have to the plants they contain.

Since greenhouses use the sun to create a warm environment for plants, when the temperature drops, there's less sun-generated heat in the greenhouse. For plants that need more heat than a greenhouse can provide naturally, heating systems are necessary to make up the difference. Greenhouses are divided into categories based on how much supplemental heat they need to produce in order to keep plants at a certain temperature.

Cold Houses and Cold Frames: Cold houses provide protection for plants, but the temperatures inside can still dip below freezing during the winter because they have no supplemental heat source. Cold houses can help start spring crops a few weeks early and extend the growing season in fall, but they're limited by the weather.

Cool Houses: Warmer than cold houses, cool houses keep plants above freezing and in a temperature range of between 45 to 50 degrees Fahrenheit (7 to 10 degrees Celsius) [source: Britannica Online Encyclopedia]. Keeping the temperature above freezing will protect frost sensitive plants, like geraniums and hibiscus, which would be impossible to keep year round in areas that experience freezing temperatures.

Warm Houses: A warm house will allow a broader range of plants, but requires slightly warmer temperatures too, around 55 degrees Fahrenheit (13 degrees Celsius). Although the temperature range doesn't support many tropical plants, some varieties of orchids and ferns can over-winter in a warm house environment.

­Hot Houses: These greenhouses are designed to house tropical plants like caladium, dieffenbachia and gardenia, which need a temperature range of 60 degrees Fahrenheit (15.5 degrees Celsius) and higher. They require the most supplemental heat and insulation and can be expensive to maintain.

Conservatories: Conservatories are designed to display plants, not just maintain and propagate them. They often have finished floors, ornate window treatments and space for furniture. Window greenhouses and small tabletop greenhouses are also considered conservatories because they're used primarily for display.

In the next section, we'll take a look at some interesting styles of greenhouses, and you can see if any would work for your house.

Working greenhouses are built with function in mind and usually lack the style and detail of conservatories and sunrooms. Functional greenhouses are designed to take advantage of the available sun and heat, provide ventilation and allow enough area for planting and maintenance. The structures are typically lightweight, simple and efficient.

Lean-to: One of the least expensive greenhouse styles, lean-tos are sometimes called half-greenhouses because they look like a house that's been cut in half, straight down the middle. Lean-tos are attached to other buildings for support and share a common wall, which provides additional heat and shelter. Although lean-tos usually have lower material costs and may have the advantage of being close to electricity and water, they're limited in size by the supporting wall of the adjacent building. They also need to be carefully situated in order to take full advantage of the available light.

­­Freestanding: Positioning a greenhouse away from other structures can be a sensible way to maximize the light from all sides. Depending on the area, this can be important in autumn, or when the sky is overcast. Although freestanding greenhouses come in a number of different styles, like a-frames, span roof and Quonset, material costs and access to water and a supplemental heat source are important considerations. Commercial growers often erect large freestanding greenhouses end-to-end to maximize efficiency and limit heat loss.

Decorative: Decorative greenhouses or conservatories have a long and romantic past. In the 19th century, they were widespread and represented wealth and refinement. To this day, decorative enclosures that bring the outdoors in, like sunrooms and enclosed decks, are very popular for their aesthetic value both in private homes and public buildings.

Cold Frames and Other Structures: You can make a greenhouse wherever you have light and can circulate the air. This can be convenient because it's easy to make temporary greenhouse-like structures that can get seeds started long before the last frost and then be put away until the following spring. Greenhouses can be small or large, incorporate a deep window, a jar or even an inverted plastic tub -- with a little modification.

Next we'll look at the different parts of a greenhouse and why they're important.

Although greenhouses look like simple structures, there's more to them than meets the eye. A reliable frame, covering, flooring and ventilation are all necessary for basic operation. To sustain the environment, a heating system and some automated processes, like irrigation via a dedicated water supply, may also be necessary.

The Frame: A sturdy frame is necessary to maintain the plastic or glass panels that let in precious light and capture heat in the greenhouse. Larger greenhouses also need a foundation. The frame can be made of any number of materials, the most common of which are aluminum, wood, rigid PVC and galvanized steel. Aluminum lets in more light and can also support clip-on panels, making it the most common choice.

The Coverings: Often referred to as glazing, the panels that cover greenhouses are specially designed to let in as much of the sun's radiation as possible. Ideally, they also provide insulation, are impervious to deterioration from ultraviolet radiation and are shatterproof. The panels can be made of heavy glass or any of a number of synthetic materials designed to maximize light exposure and help reduce heat loss. Glass lets in about 90 percent of the sun's radiation, helping to retain heat and hold up to ultraviolet light [source: Hessayon]. Synthetics, while cheaper and sometimes stronger than glass, let in less of the sun's rays.

The Flooring: Greenhouses can have very simple flooring made of wooden slats, or even a pressed dirt floor. Where conservatories usually have finished floors, work-a-day greenhouses will often have dirt around the perimeter and pea shingle, wood, concrete or stone walkways.

Exploring plants as a hobby has never been easier. Greenhouses are now less expensive to buy or build than ever before. From prefabricated kits that can be purchased on the Internet to lighter and sturdier materials for building your own, owning a greenhouse is no longer a distant dream for the average gardener. Keeping and cultivating plants is a popular pastime worldwide, and greenhouses can extend the seasonal growing time for amateurs and professionals. So what's involved in building a green house of your own? Read on to find out.

If you want the biggest tomatoes in your neighborhood next season, need somewhere to over-winter your orchids or love the idea of having your own blooming flowers year round, a backyard greenhouse may be just right for you.

Building your own greenhouse used to be a difficult process requiring complicated building plans, expensive materials and professional assistance. Now, with the widespread availability of prefabricated kits, the process is easier and more affordable. The most popular greenhouse size is around 8 feet by 6 feet (2.4 meters by 1.8 meters), probably because it doesn't require a poured foundation. Whatever the size, placing the greenhouse so that it receives enough strong light and is in proportion to the rest of the property is an important consideration when considering a greenhouse addition.

When planning for a greenhouse, one of the first questions you should ask is about the types of plants you'll be keeping. The varieties of plants you choose will determine how warm the greenhouse will have to stay in winter, and to a degree, how large it will need to be. That coupled with the weather in your area will give you an idea of how much supplemental heat the greenhouse will need to maintain your plants at a safe minimum temperature.

­Another concern is the amount of time you'll have available for maintenance. Automated ventilation, heating and watering systems can make keeping plants much easier, but at a cost. For automated systems to work, a power supply will be necessary, and any water delivery system needs access to a water line or water holding tank.

Some greenhouse styles are modular and can grow with you. That way you can start small and expand. Modular units are also well-suited to mixed applications. You can keep plants in different sections of the greenhouse separated by a divider.

If displaying your plants to advantage is one of your primary goals, a conservatory or sunroom may be the best choice. They're more expensive than bare-bones greenhouses but can be stunning additions to a property.

For lots more information on greenhouses, cultivating plants, getting your hands dirty and related topics, dig around the links on the next page.