Updated: October 11, 2019
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.
The Best of Both Worlds
Whether you decide that a glass or a plastic-covered greenhouse would be best for your needs, there are fantastic options for either choice. Keep in mind that the glass greenhouses are typically more expensive than the greenhouses with plastic covering. Both have their pros and cons though, so purchase what fits your needs as well as your budget.
Exaco Royal Victorian
The Exaco Royal Victorian Greenhouse is hands-down the best glass greenhouse available on the market without getting into custom designs or sizes. It is small enough to fit comfortably on most properties where a greenhouse may be warranted, and large enough to accommodate a variety of plant life for any avid gardener.
This greenhouse does have some customizable features, such as the location of the glass sliding door. Since all of the vertical glass panels are one solid piece, you can place the glass sliding door panel and runners in any location on the greenhouse: front, back, or sides.
The foundation frame is six feet high and does not include the roof or peak of the greenhouse, so almost anyone can comfortably walk into this greenhouse and tend to their plants without worrying about low-clearance or feeling cramped.
This greenhouse also comes with a misting system at no extra charge, as well as the connector. All you have to provide is the hose and water source. This misting system not only helps to keep your plants hydrated but it also helps in maintaining humidity levels within the greenhouse. It also saves you a bit of money that would’ve gone on to invest in a misting or sprinkler system, if you were planning to go that route.
The quality of the materials used in this Victorian greenhouse are high as well. The heavy aluminum frame is thick, durable, and resists rusting and corrosion with an attractive and functional powder coating. The laminated glass panels are 4mm thick and will stay clear and resilient in any weather conditions. This greenhouse is more than capable of handling heavy snow, high winds, and intense heat without fail.
Additionally, each glass panel is fixed into the aluminum frame and held in place with heavy rubber grommets. The rubber allows enough flexibility for the aluminum and glass to weather high winds safely and it also provides additional, superior insulation around the glass. This helps keep the temperature and humidity consistent within the greenhouse, avoiding intense fluctuations that might otherwise damage your plants.
You should install it over a thick, sturdy foundation like a poured concrete slab with some more fibrous materials mixed in well. This will help keep the surface from becoming slick after misting.
All things considered, this is an amazing greenhouse that is well worth the investment. The customer service team for the manufacturer has a great reputation of being punctual and polite as well, and there’s a 15-year warranty on this greenhouse, so you’re definitely getting your money’s worth out of it.
Traditional Glass Greenhouse
Available in a variety of frames, colors, and materials to choose from, the Traditional Glass Greenhouse available through the Greenhouse Megastore is the runner-up for best glass greenhouse.
With several length and width combinations available, you can order exactly the size of the greenhouse that you need to comfortably house all of your plants. The side wall height measures at five-and-a-half feet tall, and the roof is pitched at a 24-degree angle. This ensures that you have plenty of headroom and clearance to maneuver easily around the interior of the greenhouse.
These greenhouses come with a single standard door with an adjustable glass-screen combination for added ventilation. Optional double doors are available as well. Each one of these greenhouses comes with anywhere from two to six automatic vents, depending on the size ordered, so feel free to contact customer support if you have any questions about the number of vents in the size you’ve chosen.
This traditional greenhouse is structurally designed to handle up to a 30-pound snow load. It can also withstand winds up to 80 miles per hour, so it’s fair to say durability is not going to be an issue for most customers. You can choose from single or double-paned glass panels to fit into the heavy-duty aluminum frame.
With this greenhouse, they provide the option for future expansion if you decide you need more space as time goes on. All lengths are extendable in two-foot increments, and every structure comes with an easy, step-by-step assembly guide and video so that you can assemble and expand your greenhouse with ease and understanding.
The Greenhouse Megastore offers a lifetime warranty on the frame of this greenhouse, and a ten-year warranty on the glass paneling. Their customer service team is always ready and willing to help with any questions or concerns that you might have, so you can feel secure that you’re investing in a quality product through a great company.
Polycarbonate Plastic Greenhouses
Rion Grand Gardener 2
The best polycarbonate plastic greenhouse is the Rion Grand Gardener 2 Clear Greenhouse. This greenhouse features crystal-clear polycarbonate paneling with full UV protection and high impact resistance.
Available in four different sizes, from 8’ by 8’ to 8’ by 20’, you should easily be able to choose a greenhouse that fits the space available on your property that also suits your needs and expectations.
This greenhouse is also available in two polycarbonate options. The crystal clear polycarbonate has all the visual appeal of glass but offers much higher durability and safety. Alternatively, you can choose the 4mm thick twin-wall side paneling.
The twin-wall panels come standard for the roof of this greenhouse, but using them for the side panels as well will diffuse any harsh external light by approximately 90 percent, providing a soft, even light penetration into your greenhouse. This is great if you have a combination of low- and high-light craving plants because you can easily add grow lights for the plants that prefer more light.
The frame of this greenhouse is weatherproof, meaning you don’t have to worry about rust, corrosion, deterioration or any other problems that come with lower-quality materials. The frame of the Grand Gardener 2 is easily assembled via a pin and lock system, and the roof panels easily slide into place for your convenience.
The roof has more benefits than just being easy to assemble. Its barn shape means you have plenty of headroom to easily move around and take care of your plants appropriately while optimizing the given space for as many plants as you can fit. The roof also features a fantastic vent, which will allow for adequate ventilation even if the greenhouse doors are closed.
It’s best to secure this greenhouse to a poured concrete slab or other heavy foundation for added stability. An optional base kit is also available for purchase and provides even more stability and height for your greenhouse.
Overall, this is a fantastic polycarbonate plastic greenhouse that will serve you for years to come!
Palram Hybrid Hobby Greenhouse
Available in a wide range of sizes, the HG5508PH Hybrid Hobby Greenhouse is another great option for a polycarbonate plastic greenhouse that you should consider if this is the kind of greenhouse you intend to buy.
Opaque, 4mm thick twin-wall panels make up the roof of this greenhouse and diffuse light, while still allowing up to 70 percent light transmission to ensure the healthy growth of your plants. Crystal clear side panels allow up to 90 percent, by contrast, and will permit you to view your plants and flowers in a gentler but still beautiful light.
For over thirty years, Palram has manufactured high-quality polycarbonate panels. They only use the highest-grade polycarbonate in their manufacturing process. They also add an extra layer of UV protection into their panels to minimize any potential for discoloration or fragility over time.
The frame of this particular greenhouse may be either powder-coated silver or green aluminum, whichever you prefer. Both options are attractive and durable, promising to withstand years of use. The polycarbonate plastic panels slide right into the frame without the need for clips, making assembly much easier for the customer. While the frame is a rust-resistant aluminum, the base is made of a heavy-duty, galvanized steel that offers stability and durability.
All Palram greenhouses feature hinged doors rather than sliding, and this model is no different. It ensures a tight seal, especially in inclement weather, and is structurally sound for all intended use.
This greenhouse also comes equipped with rain gutters to safely channel water off of and away from the greenhouse and its foundation, a magnetic door catch, and a roof vent for ventilation and moderation of air flow and temperature.
Overall, customers that purchased this kit are happy with the quality of the greenhouse that they received. Assembly has been noted as being very easy when following the included instructions, but if you do find something to understand, the customer service team for Palram will be more than happy to assist you over the phone or by e-mail correspondence. Most customers found they could put the greenhouse together completely and without any compromising problems within five to ten hours.
To conclude, this is an amazing greenhouse for people that would prefer the safety of a polycarbonate plastic material for their panels over glass, and is well worth the price. With a great, US-based customer support staff and an included five-year limited warranty, you can’t go wrong!