12 key questions to consider before you plunk down greenbacks for a greenhouse.
1. Is my yard suitable for a greenhouse?
You’ll need a spot that’s as level as possible to avoid extensive soil excavation. It should be relatively close to your house to facilitate water, electrical, and gas hookups. An ideal site drains well, is sheltered from the wind, and has a southern exposure that gets at least six hours of sun during the winter. You’ll need enough space to accommodate the greenhouse length (ridge) oriented east to west. Larger greenhouses may be subject to city planning and zoning requirements or housing association regulations, so check with the appropriate authorities before you place your order.
2. How large should my greenhouse be?
Size is largely a matter of budget and gardening goals, but choose a model that will allow you to expand your activities in the future. Some greenhouses can be enlarged if you decide you want more space later on, but it’s more practical and economical to build one a bit bigger than you currently need. Besides, the extra space always comes in handy for storage—or even a quiet spot to sit and rest. Many gardeners quickly outgrow greenhouses smaller than 8 by 10 feet.
3. How much am I willing to spend?
Kit prices range from $20 to $50 per square foot. For less than $500, you can buy a tentlike, portable greenhouse of woven polyethylene that folds up for storage. The average good-quality, 120-square-foot kit will probably cost $4,000 to $6,000. One benefit: Because greenhouses are used for horticultural purposes, you won’t have to pay sales taxes or duties.
4. What framing material do I want?
Wooden greenhouses may be attractive, but wood has drawbacks. Greenhouses are typically damp inside and dry outside, setting the stage for warping, cracking, and shrinkage. Wood eventually rots, although you can extend its life with diligent maintenance. High-quality wood greenhouses are framed with redwood and cedar, not pressure-treated wood.
Aluminum-frame greenhouses generally cost less, ship for less, hold their shape better, require less maintenance, and are easier to handle. Most come painted white or green. They generally have rafter extrusions that accommodate hangers for lights, irrigation equipment, and plant baskets.
Galvanized steel tubing and PVC cost less, but they limit your window options, are more likely to be damaged by high winds, and sometimes make it more difficult to install automated venting.
5. What type of windows do I need?
Glazing choices include single or double-insulated glass, woven and non-woven polyethylene, Plexiglas, and twin- and triple-wall polycarbonates (Photo 2). In general, the translucent, ribbed polycarbonate glazing is the most practical. It insulates well, resists UV light, costs less to ship, and is easy to handle. It doesn’t yellow or become brittle like Plexiglas does, and it offers some privacy. Unlike polyethylene, it doesn’t need to be replaced after a few years.
Because polycarbonate glazing is translucent, it transmits less light than glass. This helps keep the greenhouse from overheating on warm, sunny days.
6. How do I provide venting?
Without ventilation, greenhouses quickly overheat. Excess humidity may also build up, fostering plant fungus, mold, and mildew. You can install vents almost anywhere, but roof vents perform best (Photo 3).
Although you can open vents manually, this means you have to be on call—and if you forget even once, you may lose a crop of seedlings. For automated ventilation, you can choose an electric exhaust fan or solar vent openers.
An exhaust fan needs a thermostatically controlled switch, which is generally installed on the back gable wall of the greenhouse. Automatic solar-powered vent openers gradually open roof vents as the interior greenhouse temperature rises. When the temperature cools, the rod retracts and the vents close.
If you’ve used polyethylene as glazing, you can manually roll up greenhouse walls during hot weather.
7. How should I heat the greenhouse?
Once you’ve determined how to cool your greenhouse, consider how you’ll heat it. If you plan to begin your greenhouse adventure by overwintering frost-sensitive plants and are willing to wait until early spring to begin propagating seedlings, you’ll have what’s termed a “cool” greenhouse—one in which minimum night temperatures are 40ºF to 45ºF. An electric heater and some insulation will maintain these temperatures in most areas.
If you want to grow veggies in winter, you need a “warm” greenhouse in which night temperatures stay above 50ºF. Tropical plants such as orchids need a “hothouse” temperature preferably 65ºF or above. Natural gas and propane are the best way to heat these types of greenhouses.
8. How much preparation work am I willing to do?
Before the greenhouse arrives, someone will need to lay lines for electricity, gas, and water; install electrical outlets and a spigot; and build a masonry or timber foundation. Although doing the work yourself saves money, hire a professional for jobs that require specific expertise.
9. How much greenhouse construction can I do?
Once the prep work is done, decide how much construction you’re able and willing to do on your own. Most kits claim they can be assembled in a couple of weekends by homeowners with minimal construction experience and basic tools.
10. How much light do I need?
Fluorescent and incandescent lamps are fine for starting seeds and growing seedlings. Metal halide lamps promote growth, while high-pressure sodium lamps promote budding and flowering. Some fixtures allow you to convert from halide to sodium lamps.
11. How will I water plants?
You can use a hose to bring in water or you can connect pipes directly to your water supply. For manual watering, coiled hose and wand systems work well—they prevent tangles and allow you to reach overhead hanging baskets and the back row of flats. Automated systems run on a timer and often include a reservoir of fertilizer for automated feeding.
12. What other extras should I consider?
Greenhouse benches are a must. They’ll give you a place to take a break and will also accommodate your pots, trays, and flats of plants. Pre-made commercial units generally come in cedar or redwood. Be sure to include shelving below the bench top for storing supplies, and consider installing a sink for washing hands, pots, and tools. Many greenhouse gardeners also invest in a soil sterilizer to help prevent soil-borne diseases.
Joe Provey is a freelance writer in Bridgeport, Connecticut.