Greenhouse Vegetable Gardening (2015)
LOCATION, STRUCTURE, AND MATERIALS
Many of us dream of owning a greenhouse, and happily, in many cases it’s a wish that's easily granted. If the greenhouse is used to its full potential, the initial cost will be recouped within a few years, even though its true value—the pleasure of growing things—is immeasurable.
The greenhouse tucks nicely into the garden, and provides both privacy and shelter from the wind.
The purchase and/or building of a greenhouse need not be complicated at all. If you’re looking for a fully functional outside living space or a pool house, however, be aware that those structures are not considered greenhouses, so you’ll need to get in touch with contractors who deal specifically with those types of buildings.
The simplest way to acquire a greenhouse is to buy it as a prefabricated (prefab) kit, of which there are many manufacturers with retail outlets in Sweden and in the US. Prefab greenhouses are typically cheaper, without being inferior in quality, than custom designed structures. If there are special requirements regarding height and size, or color preferences, you can often make arrangements with the manufacturer at an additional cost. The advantage of going the prefab route is that important details such as the roof angle, water drainage, and ventilation system, which can be major headaches to deal with, have already been seen to by the manufacturer and are fully functional.
If you’re planning on building the greenhouse yourself, you can tackle this project in different ways. First, check out existing prefab structures to see how technical issues have been addressed. A greenhouse built from old windows or similar materials is fine for cultivating plants, but you’ll need to keep in mind that the structure needs adequate ventilation as well as sufficient light exposure so that it doesn’t get too dark inside—the ratio of glass to frame structure must be balanced.
First and foremost, you must select a good site in your garden for the greenhouse. There are good areas for this, and then there are some that are less ideal.
♦ A greenhouse needs sun. It should never be in the shadow of outbuildings, hedges or fences, especially during the winter when the sun hangs low in the sky. In summer this may not be a big deal, but a shaded greenhouse will be cool and dark and may turn mossy as a result.
♦ It’s best to place the building’s short ends to the east and west, and leave one long side facing south. If the greenhouse abuts the wall of a house, a garage, or similar structure, the opposite long side should face south.
♦ The greenhouse needs to be sheltered from the wind. The windier the site, the more heat the greenhouse will lose—and it’s important to preserve as much of the sun’s heat as possible if the greenhouse doesn’t have its own heating system. And if the greenhouse is heated, you’ll end up having to turn the thermostat up higher and keep it on longer, thus incurring higher utility costs.
♦ Avoid placing the greenhouse under big trees, since the roots can interfere with the foundation of the structure. Tree roots tend to find ways under the greenhouse and use up the nourishment in the soil. The greenhouse will also get soiled from bird and insect droppings, as well as from leaves and plant debris falling onto the roof.
A multi-paneled greenhouse brings to mind a gazebo.
♦ Opt for a site where the greenhouse can be integrated into the garden and anchored by stone slabs, paths or flower beds leading up to it. It should look as though the greenhouse has always been there, as if it’s an integral part of the house and the garden.
Adapting your ideas to reality
While the above recommendations on choosing a site for your greenhouse may seem easy enough, reality often brings up a set of challenges, making it necessary to adapt these general guidelines to the specific characteristics of your property. If you only have one location available for the greenhouse in the garden, then that’s the space you’ll have to use. However, being aware of the pros and cons of your particular environment makes it easier to work around them and try to amend the situation. A new hedge a few feet away from the greenhouse can provide protection from the wind; you can remove a tree branch that throws off too much shade. If the greenhouse is under a big tree, roof and gutters will have to be cleared of leaves regularly; if it gets a lot of shade, then the windows will need to be scrubbed free of moss.
In Sweden, according to the public regulations for garden sheds and cabins (‘Friggebod’), you do not need a building permit for a greenhouse measuring up to 10 m² (107.64 ft²), as long as you follow all current applicable rules (see page 144). This is the norm for when the structure is situated 4 1/2 m (4.5 yards) in from the lot’s borderline. Permits are also easily obtained for slightly larger greenhouses—builds up to 15 m² (161.46 ft²) are currently under review, and might be applied for in 2008. A greenhouse built mostly from glass is not as blocky as a house; it doesn’t cast shadows on neighboring properties like a wooden greenhouse might. In several counties you can apply for a building permit over the phone, but that will depend on the location. In environmentally sensitive areas where it might otherwise be difficult to get a permit, it’s of utmost importance to select a greenhouse shape and color that don’t clash with their surroundings. Blueprints of the project will need to be supplied along with the application, which should not be a hurdle since greenhouse manufacturers keep very detailed plans of their products, including all measurements, outlines and specs.
Choice of color and design
Once you’ve chosen a site, next are the options of color, design, and size of your greenhouse. This doesn’t really affect the foundation you’ll need, since that will depend on the greenhouse’s primary use (see page 150).
As you select your greenhouse, you should make sure that it’s compatible with your house and surroundings. A Victorian, hothouse-style structure will clash with a midcentury modern house; a modern aluminum greenhouse will be jarring next to the long and narrow lines traditional to southern Swedish Skåne cottages. Both structural and exterior materials make up the greenhouse’s overall look.
Most greenhouse models have their own particular advantages and drawbacks. The most common structure is a rectangular house, which can vary in both length and width; small builds are almost square. When a rectangular greenhouse is bisected along the roof ridge it becomes a half-roof house—also referred to as a ‘mural’ or ‘lean-to’ in English—which is placed with its glassless side along a house wall, a privacy fence, or a garage.
There are also round multi-frame greenhouses that consist of five to twelve sides—the number of frames increasing in relation to the size of the foundation. There are variations on this style where the greenhouse is built more like an oriel or a bay window, a winter conservatory or a pool house.
In the past, a traditional Swedish greenhouse was rather long, narrow and low, with a steeply pitched roof. At times it was dug down in the ground so deeply that the pitch of the roof started at ground level, which was an efficient way to conserve heat. If you’d prefer your greenhouse to have a traditional look, opt for a design that’s long and narrow—it also happens to be the most efficient building in terms of available space for cultivation. In a round greenhouse, cultivation is done along the sides of the structure and the middle area might be reserved for a seating arrangement.
A simple homebuilt construction suited to spring and summer cultivation, but not for overwintering.
A greenhouse built as an annex to an outbuilding where one of the greenhouse’s long sides is the house’s wooden wall.
An annex to the house
A lean-to structure, with one of its long sides abutted against a house wall, gleans much of its heat from the main house for free. The house wall acts as an insulator; if it’s made of brick or stone it will absorb and retain a lot of daytime heat that will then be given off during the night. The greenhouse will also become more energy efficient, as one of its main walls is protected from the wind. From the standpoint of heating and cultivating, placing your greenhouse along a wall of your house is the best way to go. If it also happens to have an entrance from inside your house, being able to use the greenhouse as a lovely spot for your daily cup of coffee in spring and fall is definitely a plus. The main drawback, however, is that the greenhouse might become too warm, so you’ll have to make up for that by installing roof vents and vented windows along the roof line. You can purchase prefabricated versions of lean-to’s; if you decide to build your own, you’ll have to pay special attention to the angle of the roof—or near-flat or flat roofs seen on many garden room structures rarely manage to provide enough ventilation.
The structure of the frame
Today’s greenhouse framing material is often made of aluminum, which is excellent for this type of structure—it’s relatively sturdy and can be spray-painted or left as is. Aluminum will last many years—even unprotected—its color merely a cosmetic consideration. Along the seacoast, with its wet, salty winds, aluminum structures may have a shorter life, although my neighbor’s greenhouse, on the Swedish island of Hven, has spent twenty-five years a short 50 meters (160 ft) from the sea and it’s still holding up very well. Its aluminum structure’s untreated surface is matte and has turned grey over time, but that’s the extent of the change.
A wood framed structure is beautiful, but far less common in modern greenhouses. As aluminum is used more frequently, wooden versions have become expensive, even though the raw material itself may be cheaper. A wooden frame also requires a lot more maintenance than aluminum.
Red follows white, green and black as common greenhouse color. It can be very attractive in the right site.
Even larch wood, which is touted as tough and virtually maintenance-free, needs some looking after from time to time.
Old greenhouses were mostly built out of wood, which meant scraping a lot of old chipped paint and sanding the wood before applying each new coat of paint. The ratio of structural frame to glass surface was different also from today’s measurements—older greenhouse construction used more wood and less glass. In order to maximize exposure to daylight, the wooden structure was painted white to reflect the light from the structure into the greenhouse. This is critical for growing plants during the fall, winter and spring, but is a completely unnecessary step if you are only interested in cultivating in the summer months. The same applies to the ratio of glass surface to frame, as the proportions aren’t as important in summertime as during the darker time of the year. Larger greenhouses in the past were also often made of wrought iron—big heavy buildings—and even they were painted white. Steel frames aren’t used for greenhouses any more except in arch-shaped Quonset frames covered in plastic sheets.
Most aluminum frames can be painted for a fee—and you will need professional help for this because the frame is powder-coated with special paint, and it has to be done before the structure is assembled. The cost of this service can be a tad high compared to the cost of the whole setup, but a fresh coat of color adds a lot of panache to the structure.
When choosing color for your greenhouse, it’s important to remember how much of the structure will be on display. On certain models the glass is held in place by wide silicone strips that cover quite a bit of the frame, which means that your preferred color might not end up having the look you were after, whether inside or out of the building. You can also have the aluminum structure painted white to give it a more traditional look.
A wooden frame can be painted white, but stains containing iron oxide, like the classical Swedish Falu red, are easier to maintain than white. Take care to waterproof any wood that’ll be in direct contact with the soil to make it last longer.
A greenhouse built from old windows. An angel’s trumpet is in the foreground.
Old or new
There are prefabricated wooden greenhouses available on the market for the traditionally inclined, or for someone living in a particularly ecologically sensitive area. If you’re thinking of building your own greenhouse, a wooden structure is the easiest to work on, as aluminum and steel require special equipment and building skills.
If you’re hankering for a greenhouse with that old-fashioned look, remember that the size of the glass panes is an important consideration. White small-paned greenhouses were commonplace up until the 1950s, at which point larger panes were introduced and became common. In today’s greenhouses you’ll find either oversized glass panes, or small overlapping panes without any wooden frame or putty. However, if you prefer the greenhouse to have a real old-fashioned feel, use small panes and putty. If you build the greenhouse from old windows, you might need to refinish them first to get rid of the original lead paint.
A special heads up about the Victorian decorative roof finials for sale with some pre-fabricated greenhouses: they are not a Swedish tradition, and neither is the dark-green color that is common to greenhouses in England. To fit in here, the location would have to be rather unique.
For someone who’s only interested in growing plants and not in decorative details nor in cozy nooks for leisurely coffee breaks, a hoop greenhouse is the best way to go. Steel hoops covered in plastic sheeting are a common sight at commercial growers and nurseries. Their advantage is that this type of greenhouse is moveable and provides a substantial growing area at comparatively little expense. Their main drawbacks are bad insulation and poor ventilation.
An important decision to make involves the cover material for the greenhouse. There are glass panes or rigid plastic sheets made of poly carbon; there’s a substantial difference in looks, in how well you can see through them, as well as in insulation capacity and cost. A less common option is plastic sheeting that is stretched or hung over hoops.
Glass is the most common and also the cheapest cover material. Glass is clear and translucent; it’s an ineffective insulator but lets in the most light. Glass is heavy, so the structure must be able to support its weight. Glass comes in different thicknesses that will influence the potential size of the panes.
Glass panes measuring 3 mm (0.12”) in thickness are typically 60 cm (23.6”) wide. Naturally, there are more joints in a greenhouse with 3 mm (0.12”) glass panes than in one with 4 mm (0.16”) panes. If the joints overlap, condensation will build up and moss will start growing between the panes. The advantage of small panes is that they’re easy to change out and install. Also, if one pane breaks, it’s cheap to replace.
Glass panes that are 4 mm (0.16”) typically measure up to 75 cm between the aluminum bars. The panes can be as tall as the side of the greenhouse, or there might be a joint at the midpoint of the window. They’re very dramatic and easy to care for, but also very heavy to install and expensive to replace.
A glass roof requires a steep pitch—quite high and pointy—to prevent snow from collecting onto it, since wet snow gets very heavy and could break both the glass panes and the frame. This is very unusual, however, even in extreme winter weather, but it’s better to be safe.
Glass can be covered with a thin layer of metal for insulation, although it’s an expensive treatment and there are better ways to insulate a glass greenhouse. Tempered glass is good in a public setting because non-tempered glass shatters into large sharp pieces, while the tempered breaks into small, safer square pieces.
A wooden structure with small glass panes in French doors. The doors are recycled from a recently demolished house, and have gone through a thorough refinishing.
A homebuilt structure made with polycarbonate plastic sheets and wood.
Easy to install
A greenhouse with plastic frame, covered in plastic film.
There are several different plastic materials that can be used for covering a greenhouse. The most common of them are sheets of rigid double-layer, fluted insulating plastic. These sheets are more expensive than glass per square foot, but they insulate much better. The sheets come in different thicknesses and widths between the flues. Width and thickness impact the insulation efficiency. Even three-layer sheets with extra good insulation properties can be had. Instructions with the greenhouse will state which kind of plastic covering is needed. All thicknesses do not suit all structures, so it is necessary to choose the type specified for the particular structure.
The rigid plastic sheets are made out of polycarbonate and are not see-through—everything looks a bit fuzzy or indistinct. The width of the fluting will influence its appearance; the wider it is the more translucent the plastic sheet will be, although it’ll never be as clear as glass and the sheets age with time. One side of the sheet is protected with a UV-inhibitor acrylic layer, but it still doesn’t prevent the sheet from wearing out. That aside, polycarbonate is an excellent material and is definitely recommended for anyone who wishes to cultivate the year round.
Another advantage of polycarbonate sheets is that they’re lightweight and thus don’t require a sturdy structure like glass does. It’s just a matter of fastening the sheets to a wood frame by drilling a hole in the sheet and then installing them with a washer and screw. The sheets must be upright with the edges perpendicular; the edges need to be covered to prevent water and dirt from running down the fluted ribs. Special polycarbonate sheet edge trim can be bought at the same time (and at the same retailer) as the sheets.
Thanks to its being lightweight and a capable insulator, polycarbonate sheeting is also often used for roofs. It can handle more heavy snow than glass can, so the roof’s pitch doesn’t need to be as steep. Since the sheets insulate better than glass, the snow on the roof might settle instead of melting. Wet snow becomes heavy and can break the greenhouse frame, so it’s very important to sweep the snow off the roof on a regular basis.
A greenhouse made of aluminum and glass, with double sliding doors.
FOOD FOR THOUGHT
Width between posts
Standing height/side height
Simple and inexpensive greenhouses can be built out of greenhouse plastic film, stretched over a frame structure similar to a tent. Often the structure is a steel frame anchored in the soil, and a reinforced, UV-inhibitor treated plastic sheet is stretched over the top.
This is an excellent trial solution, allowing you to do some temporary cultivation. The frame is hardy and can last many years, but in time the plastic cover will turn yellow, become brittle and begin to crack in about five to ten years. The light source diminishes year after year due to the discoloration of the plastic, so even if the greenhouse is not falling apart physically it gets darker and darker inside, which is not good.
There are even plainer versions of this type of greenhouse, where plastic film is simply stretched across PVC tubing. In a greenhouse where light and heat are not a priority, plastic film will do the job just fine—for growing lettuce, dill, or pansies, for example, which don’t need heat under the plastic film. So even if your greenhouse is bare bones, plants will still start growing earlier than if they were out there without any protection whatsoever.
There is no clear-cut answer on how to choose the cover material for a greenhouse, so when selecting the design and the size of your greenhouse, make a point of first establishing its main purpose.
♦ Polycarbonate sheeting is recommended for year round cultivation.
♦ In greenhouses located further up north, polycarbonate can add an extra month to the growing season during spring and fall.
♦ In the south of Sweden, glass often provides enough coverage for the majority of the year.
♦ For an inviting outside room or cozy corner for your coffee break, utilitarian polycarbonate is not as inviting as glass.
♦ Polycarbonate in a wall and on the roof can be combined with panes of glass on the side of a preferred view for the best of both worlds.
A small greenhouse made of aluminum with low sides and wooden floor, and a simple sliding door.
Commercial growers often use polycarbonate walls but glass in the roof to maximize the light and to get better ventilation in their greenhouses. Those of us who grow plants simply for the enjoyment of it can compromise by having polycarbonate sheeting on the roof and glass panes on the sides instead. Heat rises to the ceiling, and then most of it seeps out through the roof; you can stop this heat loss with insulating fabric. Glass walls, in turn, can be insulated with detachable bubble wrap in winter. It does hinder visibility somewhat, but it’s removed quickly in springtime.
The size and shape of your greenhouse is contingent upon how you plan to use it. Most recommend that you buy as big a structure as you can afford, because soon enough you may find yourself outgrowing whatever you have. While this is sound advice, there are limits to consider. Building a greenhouse without a permit is limited to structures measuring only up 10 m² (107.64 ft²), although an increase of the maximum size to 15 m² (161.46 ft²) is under consideration. 107.64 ft² to 161.46 ft² is more than enough if it’s only to be used for cultivation. What requires more space are seating arrangements and the workspace itself. If you wish to include a place to relax, you should mock up your seating plan and then measure the space before selecting a greenhouse. Compare it with a room in the house to get a feel for the right size.
More important than the square footage of the greenhouse is its height. Even if the greenhouse is on the small side—only about 5 m² (53.8 ft²), say—it’ll still work perfectly well for growing plants as long as the ceiling level is adequate. A small greenhouse will often have short walls, so the only place you can stand upright is at its center, which makes for not only an uncomfortable space but also an overly warm one. If possible, raise the roof ridge so that the head clearance is adjusted, and so that even your small greenhouse is fully functional and enjoyable to use.
So it isn’t the dimension of the greenhouse floor area that determines your available cultivation space, it’s the total dimensions of its interior. A long, narrow 1.5 m (4.92 ft) wide greenhouse placed alongside a house wall is perfect for growing plants even if it’s only 3 m (9.84 ft) long, as long as the ceiling height is adequate. However, if you’d like to include some kind of seating arrangement, you’ll have to look into wider greenhouses. If at all possible, select a building where the standing side height is actually ‘standing height’, i.e., 180 cm (5’9). This is the measurement taken from the level of the soil of the long side, to where the top of the wall meets the ceiling. Smaller greenhouses usually measure only 135 to 155 cm (4.43’ to 5.09’) high, which can be very uncomfortable. If for some reason you can’t manage a taller greenhouse, one way to circumvent the height issue is to dig down and lower the smaller greenhouse into the ground, thus gaining extra height that way.
Heat can become a problem in the greenhouse. A glass house heats up much faster than a regular house, and too much heat is not good for plants—quite the opposite. When temperatures near 30°C (86°F) many plants really take it hard, and when a greenhouse is short and squat, its environment heats up very quickly indeed. However, the more air circulates in the greenhouse, the longer it takes to heat up; to avoid overheating, the greenhouse needs to have tall sides along with proper ventilation. A steep roof also holds more air than a flat roof.
The door is an important part of the greenhouse, but in small structures the height of the wall is usually so low that the only place to install an opening is on one of the short ends, under the gable. It can be a sliding door or a swinging door (i.e., a regular, hinged door), which can be divided in two sections like a stable door. Its height can vary: in small houses, doors measuring 180 cm (5’9) are common, but ideally they should be 6’5. The width of the door varies also, and here again—as with height—many tend to be too conservative in choosing its size. In a small greenhouse you typically won’t need an opening to accommodate a wheelbarrow, but you should at least be able to enter the area carrying a plant tray.
An older lean-to greenhouse made out of wood.
Sliding doors sit in frames that are part of the greenhouse structure. If the house is small, the doorframe might protrude outside the gable, which puts you at risk of injuring yourself, and is not especially attractive to boot. Plus, gravel and soil collects in the tracks as you walk in and out of the greenhouse, eventually making it difficult to open and close the door.
A swinging door or a screen door opens and closes like a normal hinged door. It needs to be latched securely to a wall to stay open because it can be a real wind catcher, so while this type of door is easier to install and more attractive, it might be a bit less practical.
The house and door are integral parts of the construction, so your options are limited in your choice of foundation and laying a base for the greenhouse. The base will be under the door and thus becomes a doorstep to negotiate. If you’ll be pushing a wagon into the greenhouse you’ll need a ramp, which you can buy as an accessory to the greenhouse; however, if the ramp is in place it’ll be impossible to close the door.
You can opt to place the door on one of the long sides of the greenhouse if the wall is tall enough. This makes it more convenient to enter and exit the space, and improves the ventilation somewhat; the downside is that it’ll encroach on some of the growing area. One solution is to place a door at each narrow end of the building, or one on each long side, as this enhances the airflow yet further, in addition to decongesting the traffic inside the greenhouse. This is an especially efficient way to create more room and ease of movement for those with mobility issues.
Roof windows/window vents
Ventilation is controlled through roof windows (also called window vents), louver windows, and through the door. There are usually one or two window vents included in a prefabricated greenhouse, and more can be ordered for an additional fee.
You’ll want more windows than are included in the kit. A professional grower should be able to open a roof area equivalent to 25% of his ground area, which means that a 10 m² (107.64 ft²) greenhouse should have a 2.5 m² (26.9 ft²) area on the roof that can be opened. If the greenhouse windows measure 75 cm (2.46) wide and long, that gives each window only a 0.6 m² (6.46 ft²) opening; this means a 10 m² (107.64 ft²) greenhouse needs at least four such roof vents.
Roof ventilation is preferable. To ensure adequate ventilation, make sure to purchase and install the extra vents while you’re building the greenhouse, because it’ll be nothing if not frustrating to install those extra windows once the structure is finished.
Ventilation window, window/vent opener and shadow fabric. Notice how the joints indicate where the glass panes overlap.
An aluminum frame structure with hinged door fitted on the gable end. The house is low to the ground but can be properly ventilated with several roof window vents that face in two directions.
Add as many window vents as the structure will safely allow. For maximum effect, place them so that every second window vent opens in the opposite direction of its neighbor’s. If the house is in a very windy location, place the vents so they open towards shelter.
Greenhouses with sides made entirely of glass are difficult to ventilate through the side. One way to deal with this is to swap out one glass pane for a louver window or a window vent. The advantage to this is that you can create a cross-breeze if the window vent is placed on the narrow wall across from the door—but this is something you need to plan for while the greenhouse is being put together. Louver windows have narrow glass slats that can be angled to different openings; they’re difficult to close tightly to shut out the winter cold and thus are not an entirely satisfactory option.
Window vent opener
Ventilation windows are opened manually by lifting up a catch and fastening it with a peg. A more sophisticated system involves gears and a chain, a bit like on a bicycle; these are wound up and down. The windows are opened in sunshine and closed when it clouds over and turns cold, usually in the morning and at night. This means you have to check on the greenhouse twice a day, and that might not always be practical.
The best ventilation accessories are automatic openers—thermostatic bars. You don’t need to install them for all your windows, but you should consider putting at least one on each side of a small greenhouse, and two for each side for a 15 m² (161.46 ft²) sized building.
Automatic window vent openers (thermostatic bars) react to ambient heat—when the temperature in the greenhouse increases, the window vent opens up. These openers often contain an expansion wax that warms up in high temperatures. You can’t be absolutely precise when regulating temperature settings—there’s usually a few degrees wiggle room—but you can make the opening happen predictably around 20°C (68°F). All things considered, it does a good job of preventing the heat in the greenhouse from reaching 50°C (122°F), which it can easily do on a sunny April day. Heat rises, so when it’s 20°C (68°F) up near the ceiling, it’s substantially cooler at lower levels in the greenhouse.
Window vent openers are fitted to each window. The wax cylinder responsible for opening and closing the window is removable from the mechanism and kept inside in storage over winter, and then put back into place in the spring.
Automatic window vent openings are necessary to help cool the greenhouse when temperatures climb. The heat rises quickly in greenhouses, and temperatures higher than 25°C to 28°C (77°F to 82.4°F) are stressful for plants.
During the spring and fall, window vents open and close at controlled temperatures. If need be, you can open more windows in the middle of the day if you happen to be at home. In spring you can make the windows open earlier at a lower temperature in order to prevent too wide a swing between daytime and nighttime temperatures.
If the nights are chilly—at a few degrees above freezing, say—daytime temperatures should not be allowed to rise too far. Either you have to open more windows during the day or you must add heat to the greenhouse at night. As the air becomes warmer both at night and during the day, window vents should be left open more frequently. The warmer it is, the longer the window vents should be left open, and at the peak of summer the window vents can stay open both night and day. Keep in mind that the door is another vent, so leave it open during warm days, and on warm nights, too.