Greenhouses

Greenhouses  are little oases for your plants. If you live in an areas with cold winters, a greenhouse can help you extend your growing season by at least a month on either side; you can start seeds  sooner and keep your plants growing longer by sheltering them. Greenhouse are also great for tropical plant  or orchid  enthusiasts who want to maintain or expand a collection but have run out of space in the house. Some gardeners just need greenhouses to start trays of seeds or protect a few large plants, while others maintain extensive collections or grow large amounts of food. Whatever your intent, there is a greenhouse out there for you! 

Greenhouses range from small, movable, tentlike structures to full-sized, all-season structures with year-round cultivation of tropical plants. They can cost as little as $100  for the small ones or as much as $30,000  for the large ones. Before you start collecting materials and breaking ground, you might want to read a book about greenhouse gardening  to get some ideas and figure out what options are best for you. 

Covering Materials

Glass  Glass  is beautiful, traditional, and durable -- lasts longer than any of the plastics. Does not retain heat well in single panels; insulated panels retain more heat and are stronger. Glass can be difficult to install unless you are an advanced DIYer, since the frame needs to be perfectly level for correct installation. Glass is heavy; the framework of your greenhouse needs to be sturdy enough to support it. Transparent, which can be dangerous -- too much direct sunlight can burn some plants.

Fiberglass  Fiberglass  comes in rolls or corrugated sheets. Translucent rather than transparent, creating even, diffuse light ideal for plant growth. Not very pretty -- gets dirty easily, and dirt accumulates in ridges. UV-protective coating will disintegrate in about 6 years, causing the fiberglass to yellow.

Polycarbonate  Polycarbonate  is clear like glass, but much stronger. (This is what they make "unbreakable" glasses out of, too.) Available in several thicknesses, including single,  double  and triple-walled.  Multiple walls provide greater insultation and help diffuse light. Can last as long as 15 years.

Polyethylene Film  Used in commercial farming operations because it's cheap and low-maintenance. Doesn't last that long, but most greenhouses that use polyethylene  can simply be fixed with a new layer. Excellent light transmission; translucent, so the light is even and diffuse. Can be used in a double layer; when the airspace between the two layers is inflated, it improves the insulation dramatically. UV coating is important; make sure to buy "outdoor use" poly film.

• Consider a dual-material  covering for the best of both worlds.
• You may be able to get a greenhouse with clear glass sides and a polycarbonate roof; this will give you excellent insulation where it matters most along with the classic look of glass.

Frame Materials

• Steel 
• Very strong.
• Require less framework to support the greenhouse, so there is less shadowing.
• Make sure to get a weather-resistant steel -- stainless,  galvanized,  or powder-coated. 

• Aluminum 
• Lightweight and rustproof, aluminum  is another excellent frame material.
• Not as strong as steel, so the frame will have to be more robust.

• Plastic 
• Inexpensive and portable, plastic  is a good choice if you're only using your greenhouse temporarily.
• Not as strong as metal or wood, so usually reserved for smaller, lighter greenhouses.
• Poor conductor of heat, so there is less heat loss through the frame.

• Wood 
• Wood  is an attractive choice for greenhouse frames, but not the most durable.
• Remember that it is going to be hot and humid inside your greenhouse -- these conditions will accelerate the decomposition of a wood frame.
• Treating the wood is a good idea. Also look for rot-resistant woods like red cedar.
• Wood is also a poor conductor of heat, like plastic.

Size

The size of your greenhouse depends on a few factors.

• How much space do you have?
• How much money can you spend?
• What do you plan to use the greenhouse for?

If you plan to use the greenhouse in the spring to start seeds and maybe in the fall to extend the growing season, you probably won't need more than a basic model, about 6'x6'  or thereabouts. There are many small, inexpensive greenhouses that will probably suit you.

If you plan to garden through the winter, on the other hand, you might want a larger, permanent structure. Once you're decided on a permanent greenhouse, free space and price are the main factors to consider. Also keep in mind whether you will need a building permit to have a large greenhouse on your property.

One last word on greenhouse size -- get bigger than you think you need, if you can afford it. It's harder to expand a greenhouse later, and you'll probably need more space after just a year or two (once you realize how much you enjoy using it!).

Greenhouse Equipment

There are some things you need for your greenhouse beyond the greenhouse itself.

• Thermometer 
• The most important of greenhouse gadgets is a thermometer  that can record the high and low daily temperature inside the greenhouse.
• You need to know if your plants are getting too hot or aren't warm enough.
• A thermometer with a built-in hygrometer  will help you establish daily humidity levels. (You can also buy one separately. )

• Ventilation 
• Once you have your thermometer, what do you do to keep the temperature within an ideal range? You need to circulate cool air and let out some of the warm air.
• Ventilation is also necessary so that the plants can breathe -- you wouldn't want to be shut up in a plastic house without a fan or a breeze either.
• An exhaust fan  will change the air inside the greenhouse every 1-3 minutes.
• You can also automate your vents to open and close automatically according to the temperature you set.

• Benches 
• Another absolute necessity if your greenhouse is going to be for transplanting seedlings or repotting plants. Your knees and back will thank you.
• Look for a work bench that is a comfortable height for you. 

• Heating
• During the day your greenhouse will be nice and toasty, even in winter, but when the sun goes down it will be cold inside.
• Small greenhouses may not need more than an electric space heater.
• Larger greenhouses are often heated more efficiently with a gas-powered heater.
• For starting seeds in the spring, you probably won't need an extra heater.
• You may need a heater if you plan on year-round growth and harvests, depending on what you plant.
• Make sure the heater is adequately vented, as the gases given off by the heater may be toxic to both you and your plants.

• Misting/Watering
• Misting  the air will keep your greenhouse humid and cool the air on hot days.
• You might also want to invest in either an overhead sprinkler system  or a drip irrigation  system.

• Shade
• Shade covers  will keep your plants from being scorched during warm months by blocking much of the incoming sunlight.
• Make sure to get shades that can easily be removed or rearranged.

• Flooring
• The flooring of your greenhouse should have good traction, be easy to clean, and be comfortable for you to stand on for long periods of time.
• Gravel,  concrete,  brick and paving stones are all popular choices.
• Drainage is extremely important -- standing water can breed all kinds of pestilence.
• The floor under your benches can be another material that will absorb water. Sawdust,  bark,  and other mulches may be a good idea.
• Also consider the aesthetic appeal of your choice. Concrete has many values, but gravel is more attractive. This greenhouse is where you want to spend your free time -- shouldn't you like the way it looks?
• If you are planning to use the greenhouse to protect vegetables and fruits to extende their growing season, you may not need a floor; just plant right into the dirt. 

• Lighting
• Grow lights can give your plants a root up if they won't get enough natural light during the early part of the growing season.
• There are a number of grow light types available.
• Metal Halide  (MH)
• Lots of blue wavelength light.
• Encourages lush, leafy growth.
• Ideal for plants that get little natural light.
• High Pressure Sodium  (HPS)
• Mostly produce light in the red-orange band.
• Encourages flowering and budding, as well as tall growth.
• Best used in conjunction with natural sunlight.
• If used alone, especially for young plants, they may grow very tall but not fill out.
• Fluorescent 
• Newer bulbs produce a good spectrum of wavelengths for plant growth while using less energy and giving off less heat.
• Not strong enough for flowering/budding, but great for starting seeds and and shade plants.
• Incandescent 
• Incandescent are mostly used as a supplement for greenhouse or indoor plants.
• Less expensive than other grow lights, but not as effective.
• They will do as a supplement or for over-wintering plants, however.
• Provide a full spectrum of light.
• You can read more about the different types of grow lights here.

External Links

• LittleGreenhouse -- A retailer's Web site, excellent guide to greenhouses.
• CarryOnGardening -- A UK Web site with simple discussion of greenhouses, geared towards people with disabilities.
• GreenhouseBuyersGuide.com -- A very thorough website devoted to all aspects of greenhouse selection and usage.
• AdvanceGreenhouses.com -- Another extensive guide; scroll down a bit for some useful equations to calculate how much heating power you'll need.
• DesertGreenhouseGuide.com -- Website devoted to greenhouse gardening in dry parts of the country, with good tips for everyone.