All greenhouses, cold frames, and other structures built with the intention of growing plants within, while allowing natural light to enter, have some kind of glazing. Glazing is the term for the type of translucent material that, apart from the frame, the structure is made of. It can be likened to the skin of an organism. There are essentially three types of glazing: glass, rigid panels, and polyethylene film. Each of these is ideal for different applications, but all are intended to protect crops growing inside. Each material needs to be chosen for the type of climate it’s in. How each lets light in will also affect the photosynthesis of your plants. Most plant crops produced under cover do best when receiving about 70-80 per cent of available light. Factors such as snow load, orientation or aspect, and amount of wind will also help to inform the decision as to what type of glazing best promotes photosynthesis.

A typical greenhouse, with its support beams, gables, struts, spans, or any other structural component, coupled with its location in respect to the angle of the sun and the choice of glazing, may in some instances only allow 40-50 per cent of available light. So, the choice of glazing materials is critical in these situations to the amount of photosynthesis plants can undertake growing in greenhouses.

Glass greenhouse

Glass was the only option for indoor growing for hundreds of years. Because of its clarity, glass glazing is not an obstacle to photosynthesis; plants grown under glass receive almost as much light as if they were out in the open. Glass can also last for decades, if not centuries, and is arguably the most attractive of all glazing options. It would hardly be the draw of horticultural and architectural enthusiasts alike if the Temperate House at London’s Kew Royal Botanic Gardens was made of plastic film and bent pipe, or if the Enid A. Haupt Conservatory at the New York Botanical Garden was constructed of polycarbonate panels and 2x4s. Despite its beneficial attributes as a glazing material, glass is not necessarily the best choice in all cases.

Because of its clarity, plants grown under glass glazing are more prone to leaf burn and other light-induced stressors than plants grown under polycarbonate or plastic films. Glass greenhouse owners and managers have gotten around this by using shading compounds (see below).

Glass is more prone to breakage, is much more expensive to install and repair, and is less tolerant of extreme temperature fluctuations. It is also less efficient as an insulator and for heat retention than other glazing options. Modern glass greenhouses are usually constructed of tempered or laminated glass as it requires less structural support than comparably sized glass panels used in the past.

Rigid panels on a greenhouse

Rigid polymer panels, such as polycarbonate, have become the standard in greenhouse installation over the last few decades. They are much easier to install, easier to repair and, while not cheap, cost less than glass of comparable size. They can be manufactured in varying thicknesses to both reduce the amount of light entering the growing area and to increase insulation value. Many polycarbonate panels are manufactured with negative space between the layers, which bolsters their ability to retain heat. Plants grown in greenhouses with polycarbonate glazing are less likely to burn, but may not grow as full or fast as plants grown under glass since there is less available light to photosynthesize. Acrylic panels are sometimes used as an alternative to polycarbonate as they allow more light to penetrate and tend to last twice as long.

Be wary of panels that are not UV stabilized as they will discolor over time and then, despite being an eyesore, will impede the photosynthesis process even more. Polycarbonate panels are also prone to scratching, which besides negatively affecting the aesthetic value, impairs light penetration and thereby photosynthesis of those plants.

Poly film greenhouse glazing

Polyethylene (poly) film is an easy glazing choice for temporary structures. It’s cheaper than polycarbonate panels and easier to install. It is manufactured in a variety of widths and colors (usually clear or white) to fit the situation. When the sunlight is most intense and the grower wants to prevent burning, white poly film works well. To achieve maximum light penetration and therefore photosynthesis, clear poly can be used. Poly is not as efficient as polycarbonate panels for retaining heat, losing as much as 60 per cent, but many growers will add a second or even third layer and often inflate the space between the layers, or place spacer blocks or materials to create that negative air space to aid in insulation. It is important to note in greenhouses where an inflating fan is being used, outside air should be pumped in to reduce the amount of condensation. There is some debate, especially in northern climates that do not receive much sunlight during the winter months, as to whether the second layer of poly film’s added insulation outweighs the dilution of available light and subsequent photosynthetic value. Research on this continues.

Poly is the most versatile of the three materials because it can easily be tailored to the season, the climate, and the crop. However, of the three options, it has the shortest usable life span, sometimes only a single season, and is especially prone to ripping and tearing in high-wind areas.

Greenhouse material by the numbers

When deciding which material is best suited for your particular situation, an overview of the various glazes and their respective properties is useful, as per a 2008 Rutgers University publication.

*A 2010 article in the journal Computers and Electronics in Agriculture reports this number drops to 75-76 percent when using a double layer of poly films.

A somewhat dated article (1993) in the journal Hort Technology, stated it accurately when it reported “the geographic location of the greenhouse (because of microclimate), grower experience and/or preference, and the local industrial support all have an important influence on the design and selection of a glazing system for a greenhouse operation. The crop and its required heating and/or cooling costs also enters into the decision.” It is important to remember glazing, while vital to the overall system, is just one component.

Shading options for greenhouses

For growers who find glass or panels still allow too much light into their growing spaces for the crops being produced, shading options such as draping or hanging shade cloths, or applying a light-colored paint material onto the roof can be used. Paint-like substances known as shading compound or shade paint can be applied directly onto various types of glazing and serve to block out some light, allowing enough to penetrate for photosynthesis, but reducing the intensity to avoid burning of foliage tips and canopies. These compounds are sometimes brushed on, rolled on, or, more often, applied with a sprayer nozzle. Most of these materials generally wash off over the course of the season and then are reapplied the following spring or summer.

A 2010 study published by the American Society of Agricultural and Biological Engineers examined the effects of flowering times on crops when comparing different types of shading compounds. They looked at NIR-reflecting (NIR-R) paints and neutral (N) shading paint. They found in their study that “The NIR-R paint transmitted 67 per cent, eight per cent, 24 per cent, 30 per cent, and 29 per cent less ultraviolet-A (315-380 nm), red (R; 600-700 nm), far-red (FR; 700-800 nm), NIR, and SWR, respectively, than the N paint. Transmission of blue (400-500 nm) and green (500-600 nm) light was 4.7 per cent and 4.5 per cent greater, respectively, under the NIR-R versus N paint.” Salvia, pansies, and petunias used in this study flowered one to three days earlier under the NIR compounds versus the N compounds.

An alternative to shading compounds is to place a shade cloth over top of the structure as well during the months of peak sun intensity, or suspend one inside, over top of the crop. Light-colored shade cloths will block some light but not absorb as much heat, while darker cloths will block more light but can absorb and radiate more heat, which is not always good in an already-warm greenhouse.

Ultimately, the selection of glazing material used is determined by balancing your crop’s need for sunlight to photosynthesize with the other climatic or site-specific challenges and conditions that exist. Remember that you can always supplement with grow lights if the glazing material you choose obstructs too much natural light.