Glass or plastic? It sounds like the kind of question a clerk would ask you while you’re in line at Walgreen’s. When it comes to greenhouses, plastic refers not to bags or credit, but coverings that the house is made of. Glass, the traditional choice, has a long history of being used for greenhouses whereas plastic is newer to the scene and has had to prove itself out over the years. With plastic technology changing so much over the years, there’s a lot that has changed. Some questions to ask when selecting a greenhouse cover, according to the University of Arizona Department of Agriculture & Biosystems Engineering:
How much energy (light) does it let into the greenhouse, and how much energy (heat) will go out?
What are the purchase, installation, and maintenance costs?
How well can the grower manage the environment which is imposed by the glazing to produce a quality, salable product for profit?
Both glass and plastic transmit light over the visible light spectrum, which is the kind of light that plants predominantly require, called Photosynthetically Active Radiation (PAR) transmittance. When considering transmittance of a greenhouse covering consider the following questions:
- Is the glazing is single or double-layer?
- Is it made of rigid panel or flexible film?
- Are special additives included in the material?
Single-layer glazings generally transmit 90 percent and double-layer glazings about 80 percent of light, no matter whether it is plastic or glass, rigid or flexible. A few percent differences in transmittance aren’t a big deal since other factors that interfere in the transmission of light anyways, such as shading by the structure, the location of the greenhouse, its orientation to the sun, season, etc.
Direct or diffuse light? Light that enters the greenhouse directly without any prior reflection is called direct radiation. Diffuse radiation is when light gets scattered by anything in the air, such as dust or clouds, or the greenhouse cover itself, in the case of a translucent or double-layer glazing.
Intensity being equal, direct and diffuse light both provide photosynthesis equally, but it’s believed by many that diffuse light is advantageous because the diffusion of the light means that the lower plant leaves that would otherwise be shaded by upper leaves in a direct light situation, would receive light, improving plant growth. According to this line of thinking, plastic glazing technically has the upper hand, as does double-layer glazing, though in practice it may not matter much since light gets diffused in the atmosphere already. If located in an area such as a desert with plenty of direct sunlight then this could be an issue.
Certain plastic glazings are able to manipulate the transmission of particular sun wavelengths that favourably affect plant growth.
A drawback to plastic glazing is that it yellows and fades, resulting in 10 percent or more loss of light transmission over their short lifespan of a few years.
Cost and efficiency
Plastic is cheaper than glass, making it an ideal choice for those on a low budget or those who want to maximize their profits. Plastic is also easier to install, particularly an issue in areas with high labour costs.
Lighter glazing demands less supporting structures, which means less cost and less shading, which affects light transmission.
Glass can break and plastic glazings can tear. A break in glass is easier to patch up, but more costly to replace. Since glass is fragile it is more likely to break in certain circumstances, like closing a cold frame.
Glass is more durable and lasts longer, which translates to less time having to purchase, transport and install than with plastic which needs to be replaced every few years.
The greenhouse heating effect means that glass has traditionally been an excellent barrier to infrared transmission, which means that the greenhouse lets light enter through the glass and manages to the heat in better than plastic does.
Most people like the look of glass better. It is clearer, looks more solid and has the traditional look of a greenhouse. Plastic on the other hand is translucent and looks cheaper. When damaged, glass and plastic both have a particular look to them. Glass can crack and break, whereas plastic rips and gets cut open.
Where is the industry going?
Plastics have transformed the greenhouse industry from the advent of cost-effective irrigation systems to the development of cheap glazings. While the use of plastic in the greenhouse industry for glazings is dominant, it’s growth is stagnant, whereas the growth of glass is increasing at a greater rate. This table shows the growth rate of glass and plastic greenhouses (in sq m) both in Canada and its largest province, Ontario, between 2008 and 2011. Since businesses aim to operate as efficiently as possible it’s helpful to look at industry trends, but at the same time, what is good for an industry as a whole may have no relevance to your particular situation. source: Ontario Ministry of Agriculture and Food
image: Maria Gurina (Creative Commons BY)