In a series of articles, we will help you explore how to choose your greenhouse options – siting, shape, covering material, growing systems etc. – in order to get the most out of your new construction.

This series will be aimed at helping you planning your first greenhouse but, even if you’re a seasoned grower, you may still find a few gems of wisdom along the way.

To begin, let’s define what a greenhouse is and how it works.  After that we can take a look at  what you will want to accomplish with the one you’re going to build. These two steps may seem trivial, but they are actually fundamental to placement, design, and installation of systems in the final structure.

My working definition of a greenhouse is: any structure that allows sunlight to shine on a crop inside it, while modifying the climate around the crop to increase growth. This definition includes anything from a simple “row cover” (a single sheet of clear plastic film supported by stakes or hoops over a crop) to a full-blown glass-clad greenhouse with active heating, cooling, and irrigation systems.

What you end up with should be determined by what you want to accomplish and the resources (space, labour and money) you want to dedicate to your selected structure.   Every greenhouse design has a specific purpose which it is designed to support. You’ll need to figure out which design fits your purposes.

So first, let’s take a quick look at how greenhouses modify the climate around a crop.

Most people assume that greenhouses use the same “greenhouse effect” that causes global warming: that the greenhouse cover, like an atmosphere with extra Carbon Dioxide, transmits visible light coming from the sun and absorbs or reflects infrared “thermal” radiation emitted by the surfaces in the greenhouse, leading to warmer air near the surface.

While this is true for the planet (thermal radiation is the only way our Earth, sitting in a vacuum,  can get rid of energy absorbed from the sun), it is not true for greenhouses. Some greenhouse materials (not all) do reduce radiative heat loss a bit, but it’s a very small part of why a patch of ground will be cooler if the greenhouse is not there.

Consider what would happen to that greenhouse-free patch of ground (for simplicity, let’s look at a black parking lot in bright sun and no wind) on a sunny day: As the surface heats up from absorbed solar energy, the air above it also warms, expands and rises. As warm air rises, it is replaced by cooler air, drawn in either from above or from the sides. While the air over the parking lot will be warmer than the next door soccer field (which is absorbing less light), it will not be nearly as hot as it would be if we were to surround it by transparent walls and a roof, preventing heated air from moving upward and escaping.

A greenhouse reduces the volume of air the solar energy has to heat by preventing convection. Wind is blocked and the air cannot rise out of the structure to be replaced by cooler air from the surroundings, unless we open a vent or window. Heat loss from a closed greenhouse occurs mainly by conduction through the cladding (which provides some insulation), infiltration (air leaking out around doors, windows and through gaps in walls), and conduction to soil through the foundations. Radiative heat loss, both from the crop and the cover, does occur, but it is small in comparison to these other routes and generally may be ignored for greenhouse heating system calculations.

Different aspects of greenhouse design (shape, how the walls and roof are constructed), placement (orientation, surrounding structures and terrain, soil type) and material (cladding, structural material, foundation, floor covering, colour) will result in differing abilities to trap and retain heat and differing abilities to maximize light getting to the crop. These abilities will vary with the constraints placed on the greenhouse by local climate, time of year, production system and type of crop being grown. What is best for one situation may not work that well in another location or setting.

In our next blog posts we will focus on how greenhouse shape, location, and production parameters affect optimization of use.

To prepare for that, you might want to think about the following:

• How big do you want the greenhouse to be, and do you have an idea where a structure that size could go?   Remember that the greenhouse will need enough internal space (walkways wide enough for a small wheelbarrow, for example) for you to work inside it as well as store and install equipment, as well as space for plants to grow.

• How important are the aesthetics of the greenhouse? If it’s located out on the “back forty,” looks probably don’t matter, but they might if the greenhouse will be in your front yard.

• What is the climate like where you want to put it (considering wind exposure, seasonal temperatures, snowfall etc.)?

• What do you want to grow? Are you planning for transplants and flowers in pots or full-season growth of tall plants (tomatoes, grapes, cucumbers etc.)?

• Why do you need a greenhouse?  Do you want to
  • extend the growing season (e.g. produce transplants)?
  • grow warm climate plants (e.g. peppers) in summer?
  • grow during the cold season (with added heat)?
  • grow cold-tolerant crops (e.g. kale) in cold months (without heat)?
  • All of the above?

• Is the greenhouse primarily a “hobby structure” for a bunch of possible crops, or are you planning to produce a limited number of crops for your family or for retail sale?

• What growing systems are you thinking about using (direct seeding in soil, raised beds, soilless media, hydroponics)?

• Are you planning to be in the greenhouse every day, or will you need some reliable automation for when you’re not available?

• Have you thought about budget?    No one wants to spend more than they have to, but, depending on your answers above, some options, even the “best” for your situation, may not be worth the extra cost, and you may have to change your specifications, if the cost is too high.

Over the next few instalments we will look at how your answers to each of these questions will affect and may limit your possible choices for greenhouse shape, materials and required equipment.