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Four Best Greenhouse Plastics

Greenhouses covered with glass can be beautiful. But for the DIY gardener, greenhouse plastic is a more practical option.

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The small greenhouse with growing tomatoes and cucumbers in the garden with green vegetation a sunny summer day.VERONICA VISKOVA/GETTY IMAGES

Buying Greenhouse Plastic

According to Shelby DeVore of Farminence, glass used to be the most common covering material for greenhouses. Not anymore.

“It’s still used, but glass is more expensive than plastic and more easily damaged by wind and hail,” she says. “Plastic is much more accessible to the average person and easier to work with.”

Here’s what you should consider when shopping for greenhouse plastic:

  • Light transmission: Greenhouses let in light that’s vital for plant growth. They control the climate by keeping in the ideal amount of humidity and keeping out excessive heat and cold. Polyethylene has the highest light transmission. The thinner it is, the higher the transmission. Look for products with dust-resistant and anti-drip coatings that keep that light pouring through.
  • Durability: A greenhouse is a major financial investment. As a year-round gardener, you want a plastic covering thick enough to last for several years, yet thin enough to allow good light transmission. Plastics are typically 6- to 12-ml thick, and thicker is more durable but more expensive. DeVore says 7- to 9-ml thick is typical for backyard greenhouses.
  • Color: Greenhouse plastics are predominantly clear, white or opaque. In general, the clearer the better because clear plastics let in the most light. White or opaque plastic coverings provide diffused light that promotes balanced plant growth inside a greenhouse. In colder climates, clear plastic is typically the choice. In areas of intense summer heat, opaque plastic is used because it reduces light transmission and diffuses the sun’s rays. “Here in Texas, we use a shade cloth or the sun will cook our plants alive,” says DeVore. “It’s a plastic net that filters out 20 to 40 percent of sunlight. We take it down at the end of the summer.”
  • Cost: Clear plastic is cheaper than opaque, and thin is cheaper than thick. Paying more now for a thicker, more durable covering will save you money in the long run.

The following are the four major types of plastic to consider if you’re planning to build a greenhouse.

inside a Polytunnel greenhouse under constructionPlazacCameraman/Getty Images

Polyethylene

This is the most common plastic used for greenhouses. It’s thin (6- to 12-mil), durable and the least expensive of the four greenhouse plastics. A single layer of polyethylene works well, but a double layer is ideal.

“Blowing air between layers creates an air pocket that acts as insulation,” says DeVore. “With a double layer, there’s a lot more efficiency in trapping sunlight and heat.” Also, more condensation occurs with a single layer of plastic than a double layer.

Polyethylene can only cover wood or metal frames. “It destroys a PVC (polyvinyl chloride) frame, so if you must use PVC under polyethylene, brush a protective layer of paint over the PVC,” DeVore says.

Building greenhouse with polycarbonate and metal framepashapixel/Getty Images

Polycarbonate

This common plastic is more expensive than polyethylene. “There is better light transmission with polycarbonate than polyethylene, so this is a consideration if you worry you will not get enough natural sunlight in your greenhouse,” says DeVore.

Because polycarbonate is rigid, it’s used for end walls and to create rooms in a greenhouse. “Polycarbonate lasts 10 years or so versus two to seven years for polyethylene,” says DeVore. The drawback? It’s can’t be used to cover domed or arched greenhouses.

greenhouse made of carbonate on a plot next to the house. Greenhouse for growing vegetables on a plot next to the houseOxana Medvedeva/Getty Images

Polyvinyl Carbonate

This is the least common and most expensive of the greenhouse plastics. DeVore says it’s good for greenhouses in areas with high winds because it comes in firm corrugated panels. The milky white panels let in less light than other plastics, however.

Polyvinyl carbonate is typically used to diffuse light evenly and protect plants from excessive exposure to harmful rays. It’s bendable for curved roofs.

The trick here is finding it. As of this writing, we couldn’t find an online source.

Woven Plastic Film for a greenhousevia merchant

Woven Plastic Film

As expensive as polyvinyl carbonate, woven plastic film is reinforced polyethylene that can be double layered. It has reduced light transmission and is good for domes and arched roofs.

It resists tearing, which DeVore says is an issue with greenhouse plastics. “Wind and hail rip plastic, and small tears become big,” DeVore says. “The reinforcement in woven plastic film helps stop or minimize tearing.”