Greenhouse Vegetable Gardening (2015)

Your greenhouse needs a foundation, and how much work you devote to it will depend on how you intend to use your greenhouse, and if you plan on heating it year round. The size of the greenhouse is not important, as it’s quite a simple task to assemble it. For a pre-fabricated greenhouse you would typically buy a base, which is then set down and anchored to the ground of the site you have chosen.

The simplest option

A very small greenhouse can be placed upon a simple wood frame of landscape timbers, untreated railway ties, or other sturdy timber. The wood is placed on a layer of gravel, on a perfectly tamped down, leveled surface. Dig down 10 to 20 cm (4” to 8”) into the ground, fill the space with gravel, pack it down and lay the wood frame on top of the gravel. This foundation model has been perfectly serviceable for several 10 m² (107.64 ft²) greenhouses set in heavy clay soil, in Swedish zone 4, -36°C to -29°C (-30°F to -20°F), for over 20 years.

Piers (sometimes called plinths) with well-balanced wooden supports, where the greenhouse frame will be fastened.

To make your structure even sturdier, lay your foundation in deeper. The further north you live, the further down you’ll need to dig to provide adequate support. Frost, once the soil freezes, can make the ground heave and push the greenhouse askew.

A greenhouse made of glass is heavy, and this in turn makes it quite stable. What can happen if the foundation isn’t laid out carefully and level, however, is that the frame can warp and make the glass panes shatter. A greenhouse made out of polycarbonate is much lighter, but it will need solid anchoring to withstand strong winds and inclement weather.

A greenhouse anchored by concrete piers

Some additional foundation work is required for slightly larger greenhouses, as well as if you want to be absolutely certain that the foundation won’t budge due to ground frost. You’ll need to dig deeper wells for piers—down to below the frost line, which will vary according to where you live. The well for the pier is first lined with gravel; a cylindrical cardboard mold (called a sonotube) is inserted into the middle of the well; concrete is then poured into the sonotube. Some areas require that you reinforce these anchors with a rebar, which is inserted into the piers. You can buy pre-cast piers that you place onto the gravel layer, to which the greenhouse base—or frame—is then attached with anchoring bolts, or to a wood sill that the greenhouse is then affixed to.

For a small greenhouse you’ll only need one pier per corner. A bigger greenhouse with sides longer than 3 m (9’8”) will need an additional pier at the midpoint of each long side, too. If the size of the greenhouse exceeds these measurements, then you might need a pier in the middle of the gable, or perhaps placed on each side of the door.

A stone-tiled floor requires digging down into the ground of the greenhouse.

Wood sill anchored with bolts. Time to raise the frame.

This type of foundation is excellent for a greenhouse that is not going to be heated year-round. It’s also a great option for a half-roof ‘lean-to’ against a wall.

Foundation for the larger greenhouse

A larger greenhouse with a poured concrete foundation is more complicated to install, since a ditch has to be dug down to below the frost line. Local builders can tell you how deep you’ll need to go. Cover the bottom of the ditch with a layer of macadam or gravel, and pour a layer of concrete on top. Onto this Leca blocks (medium weight concrete blocks), concrete blocks, or brick pavers are cemented into place until the foundation is level with the ground. The greenhouse base is then attached to the foundation.

If the greenhouse is to be kept frost-free through winter, you’ll need to line the outside floor area with insulation boards, or you can also use insulating cement blocks.

The ground in the greenhouse

Usually the ground of the greenhouse is a plain dirt surface, except at paths and seating areas. If the whole floor is to be covered and heated during winter, this will need to be taken care of concurrently with the foundation work.

Dig out the ground in the greenhouse to below the frost line. Fill the area with drainage gravel up to about 25 cm (10”) below the final, finished level. The coarse gravel at the bottom will block the capillary capacity and prevent the ground water from rising to the surface.

Leveled, flush surface with braces, sill plate, and studs. The blocks are placed onto a layer of sand.

The completed greenhouse, featuring budding spring plants. You can provide shade in several ways—with bamboo blinds, for example.

Put down a 5 to 7 cm (2” to 2 3/4”) layer of construction grade sand (for slabs, brick pavers, concrete stone) on top of the drainage gravel. On top of this lay a level, 3 to 5 cm (1” to 2”) layer of fine sand, before finally setting down stone or brick pavers, or concrete stone. Calculate each level’s thickness so you reach right height when everything in place. The slabs can vary in thickness—thin slabs are fine, and thick slabs can take on the weight of a car. One way to better insulate the greenhouse against the cold rising from the ground is to add rigid foam insulation boards between the layer of drainage gravel and the layer of construction sand. Adding sub-floor heat-coil mats (like under the tiles in a bathroom, say) is another option. The heating mats are covered with a level of fine grit sand, and the stone slabs are placed on top.

Another way to deal with the foundation is to pour an all-in-one foundation, similar to ones found in houses without a basement, although they don’t offer up any real benefits to the greenhouse since you’d then also need to install a floor drain to stop water from pooling in spots. Normally, excess water seeps down in the ground and through the slab joints. However, if a whole continuous slab is poured then the water has nowhere to go.

An in-ground greenhouse built in the old-fashioned style. It works very well, even for keeping plants through winter.

Professional horticulturalists often choose to pour concrete paths and leave the rest of the ground bare, as this makes it easy to sweep and rinse off the paths and keep them clean. Dirt and moss can make paths very slippery, which can be dangerous, so the debris is rinsed off onto the side of the path and into the open soil. This is a very crafty solution that’s worth copying. Using gravel as ground cover is not as good, as it’s a challenge to keep clean and the roots of plants will grow through the pots’ draining holes and down into the gravel. Pots need to be placed on saucers if they are to be placed on top of gravel or sand.

Wood as ground cover, in the form of grating, is another option, although the main drawback is that it gets very slippery when wet, which is often the case in a greenhouse. Plastic mats can also become slick, and so should be moved inside in wintertime.

Bark mulch and wood chips are not recommended, nor are cocoa mulch and coconut fiber. These materials break down over time, insects and pests make it their home, and it’s too difficult to keep clean.

Sand might work, as long as it’s kept free of weeds and soil debris, and is replaced with new weed free sand regularly.

Heated soil

In order to grow plants directly in the ground over winter, the soil needs to be heated. For this you’ll need to build simple, permanent growing beds, and include an insulated foundation for the greenhouse according to the preceding description. Inside the greenhouse, the ground soil is left intact. Dig out the soil from the area in which you wish to cultivate, lay down heating coils in a layer of sand, and refill the area with the soil that was dug up. You’ll need to place a barrier (a ground cloth made of thick fiber, for instance) between the sand and the growing soil so you’ll know where the heating coils are when the time comes to dig up and change out the soil.

Another option is to build a high foundation wall out of cement, and let the greenhouse glass connect to this wall at about sixty to seventy-five cm (2’ to 2 1/2’) above the ground surface. Raised growing beds can then be built inside the surrounding foundation wall, and a path along the middle of the ground surface makes for a more convenient height to work on the plants. This is a good way to increase air circulation, and it produces a sturdy and cozy greenhouse. As the glass doesn’t go all the way down to the ground the space might be a little bit darker than in other greenhouses, but by the same token the risk of panes shattering due to stones or other impact will be lessened.

Walls also help keep the temperature steady within the greenhouse—the heat doesn’t plummet at night since the walls radiate the heat they’ve absorbed during the day; likewise, heat doesn’t soar in the morning because the walls have had a chance to cool down overnight.

In-ground greenhouses

A slightly older variation on this theme is to sink the greenhouse into the ground a little, because the ground doesn’t get as cold as the open air during winter. Excavate the entire area where you’ll place your greenhouse down below the frost line. Line the bottom of the space with macadam, cover it with a slab of poured concrete, or set down some other ground cover such as gravel or stone slabs. Pour footers for stone/brick walls, and build the walls up to ground level. Insulate the outside of the walls with rigid foam insulation boards, or build the walls with insulating concrete blocks. Make an opening for a point of entry by starting all the way down at the in-ground level and add steps up to ground level. Position the greenhouse onto the wall by making the glass start at ground level, but leave the dug-out depth as it is inside the greenhouse.

Cultivate at ground level by removing yet more soil, and replacing it with new dirt, since at 50 to 80 cm (20” to 32”) deep the soil is probably of pretty poor quality—hardly suitable for growing anything. Beds can also be built up against the wall—they should be at least 30 to 40 cm (12” to 16”) deep, but if this measurement is off by an inch or two it doesn’t really matter. The shallower the beds, however, the more careful you need to be with irrigation and fertilizing. If you choose to grow plants in pots or containers, they will have to be placed on the ground.

This type of greenhouse gets pretty warm during the winter because the ground provides snug protection against the elements and stores heat. It’s great for overwintering plants, but its utilitarian aspect will never make it a cozy spot for a coffee break or an outside living space you could set up in a more traditional greenhouse. Nevertheless, if you add a heating mat for the plants to stand on and tent the lot with plastic bubble wrap, your plants will be able to survive many degrees below freezing.

While this kind of greenhouse is rare in today’s manufacturing, it’s still a good option to consider and a very accessible DIY project. With this model, your greenhouse will not be the dominant structure in your garden but it will have an attractive, vintage appeal, and be economical to maintain and heat to boot. If the glass panes are insulated during winter it will retain heat even better. You can also make a complete lean-to of the northern long side, which will then act as heat storage facility. During sunny spring and fall days, the wall absorbs and stores a lot of residual heat, which is then radiated into the greenhouse at night. It becomes a sort of in-ground half-roof or lean-to greenhouse, which is especially suited to the northern Swedish climate.