Updated: June 23, 2019
4.0 Greenhouse location
To maximize sunlight, greenhouses should be constructed with long-sides facing southeast to southwest. For plants requiring less sunlight a greenhouse can face northeast to southeast, or northwest to southwest. Locations in order of preference to maximize sunlight:
1. South or southeast
4. North (a northern exposure usually does not receive enough light for most plants)
4.1 Amount of light you need
The amount of light your greenhouse will get is probably going to be the main factor in where you locate it. Typically, you’ll need a lot of sunlight in winter, and less sunlight in the summer (staying in line with Mother Nature’s wishes). It’s easiest to have it face south, to take advantage of the natural seasons. Using another direction (i.e. southeast, southwest etc.) will expose the greenhouse to fewer hours of sunlight per day, and you may have to use some shading in the summer and extra lighting in the winter. Choose anywhere between south to east if you lack choices of building sites.
Getting all-day sunlight is best, but getting sunlight exposure on the east side is good enough. Morning sunlight is excellent for your plants because it starts photosynthesis early (in case you don’t remember photosynthesis is the process by which plants convert sunlight into energy to fuel the plants activities) maximizing its growth. This is most important for winter light in the months from November to February. Ask yourself, “When am I going to use the greenhouse?” If you’re using primarily in the winter, make sure there is no sun blockage from trees, shrubs and buildings. If you are into summer growing alone, make sure there are no shadows forming during that season when the sun is high.
Before building /installing your greenhouse, figure out how many hours the sun will shine on it during the months you wish to use it. In order to get the most year round light, take the angle of the winter and summer sun into consideration. The sun in the winter is about 30 percent weaker, so make sure that the glass in your greenhouse is positioned at a 45-degree angle and it has no shading. Buildings can cause trouble for your greenhouse. The sun may be able to shine over a fairly tall building in the summer, but in the winter when the sun is lower it may block the sun’s rays from reaching your greenhouse.
Over all there are lot of factors concerning the amount of light that gets to your plants when housed in a greenhouse. Here are the main ones to consider:
4.2 Global location
The farther north your location, the less sunlight your greenhouse will receive in the winter.
4.3 The month
Days in June, July and August have more daylight than days in November and December.
4.4 Time of day
If you’ve ever been outside for an extended period of time, you know that noon during the summer is when the sun is at its most powerful. Orienting your glass to the angle of the winter sun at noon will give you less reflection and more sunny goodness. The glass should be at 90 degrees to the sun to enable light to get the maximum effect.
Weather is a definite factor. Cloudy days mean less sunlight to work with. Snow on the roof of your greenhouse blocks sunlight from getting inside, so be extra vigilant in the winter if you plan to grow anything during the season.
4.6 Go south
Setting up your greenhouse south facing is the best, but 45 degrees to the west or east of south is okay as well. Early morning sun is better than late evening sun (remember, “Photosynthesis” is the 5 dollar word for today). If you can’t get this orientation due to building constraints, then be prepared to make some changes or investments to get your plants to grow.
4.7 Building materials
Materials can make a difference—aluminum tends to be lighter than wood framing and it usually blocks light. A wood frame can be painted white to add to its reflective power but it still will be less reflective than aluminum.
Different types of glass will affect the amount of light that gets into your greenhouse. As we discussed, clear glass oriented at a 90 degree angle will give your plants the most sunlight. Tinted glass will allow less light etc. Fibreglass allows less glass but will filter the light in ways which are beneficial for some plants. See our section on glass (or glazing) for more info about this.