Over the past months we have posted a few blogs on a new sustainable greenhouse prototype we are developing.  We have received comments from a number of traditional greenhouse users with concerns that this new prototype will not be capable of plant starts because of inadequate light levels.  The prototype does push the envelope of greenhouse design but through the use of careful daylighting strategies not typically implemented it is capable of effective plant starts.  The Research and Development greenhouse that is the basis for our prototype started over 1,000 tomato plants last spring without the use of supplemental light.

To understand how this works a typical all glass or plastic glazed greenhouse has no reflective surfaces and the glazing reduces light transmission into the greenhouse by up to 40%, or more.  This means that the light levels entering a traditional greenhouse are first significantly reduced by passing through the glazing and then any light that does not immediately hit a plant surface is either absorbed by the ground or passes back through the glass and out of the greenhouse.

The sustainable greenhouse prototype uses some of the clearest glazing available and a number of reflective surfaces to increase light levels inside the greenhouse.  First, reflective surfaces on the outside of the greenhouse, i.e. light shelves and reflective roof surface, bounce additional light into the greenhouse.  This reflected light allows more sunlight through each window than would direct light alone increasing the effective aperture size without more or larger windows.  Next, the clear glazing allows more of this light to pass into the greenhouse compared to a similar area of glazing on a typical greenhouse.  Finally, although the greenhouse is made up of a number of opaque walls and roofs each of these is covered with a white reflective surface.  The reflective surfaces inside the greenhouse mean that any light that does not immediately hit a plant or the ground is bounced around inside the greenhouse until it does.  Through a combination of thoughtful daylighting strategies our prototype greenhouse achieves adequate light level for plant growth and plant starts with fewer windows than a traditional greenhouse.