There are two main types of plastic greenhouses on the market today: polycarbonate panels and polyethylene covers.  Each has its own pros and cons, and quality can vary greatly between the two types of materials.  In this article, we will discuss the ins and outs of shopping for plastic greenhouses, the pros and cons of each material type, and how you can be sure you're getting a good quality product before you buy.

Polycarbonate is a type of engineered plastic that, unlike glass or acrylic, is virtually unbreakable.  Polycarbonate plastic is used in bulletproof windows, compact discs, and eyeglass lenses.  Polycarbonate plastic is also incredibly lightweight: it's only about one sixth the weight of glass.  The primary disadvantage of polycarbonate is that it's more expensive than glass or acrylic.  Polycarbonate panels are often used in constructing greenhouses because they can be made crystal clear to look just like glass, but with far less weight and much more durability.  Unlike glass, you won't have to worry about polycarbonate panels scratching, cracking, or shattering.  They are a very good quality choice in plastic greenhouses.

Polyethylene covers encompass another whole realm of plastic greenhouses.  Greenhouses with flexible covers that stretch over a frame can be either permanent outdoor units that are cemented to the ground or temporary use units designed to be set up and taken down, as needed.  The ability to have a greenhouse set up only part of the year can be very advantageous for some homeowners; particularly those who are limited on outdoor space and don't want to commit to a permanent structure that's set up year round.

The most critical factor to consider when looking at greenhouses with polyethylene covers is the UV stability of the material.  By its very nature, polyethylene is highly reactive to ultraviolet light.  This means that an untreated sheet of polyethylene left in the sun would rapidly break down, probably completely disintegrating within a couple months.  For this reason, most greenhouses with polyethylene covers are treated to make them UV resistant.  Keep in mind, though, that the UV inhibitor that's added to the plastic doesn't make the covers completely impermeable to ultraviolet rays, it only increases the material's resistance.  They will still break down over time, but the UV inhibitor greatly increases their longevity.  Before purchasing a greenhouse with polyethylene cover, ask the manufacturer or retailer what the expected life of the unit is.  Also be sure to have them qualify their answer by explaining if they mean for continuous use or temporary use.  For example, when a retailer tells you that a greenhouse is rated for 3-5 years of serviceable use, this may be based on only having it set up 3 months out of the year, which really means that the unit is only rated for 9 to 15 months of continuous use.  This is an important differentiation to make.

Lastly, if you do decide to purchase a greenhouse with polyethylene cover, it's critical to cinch the material down tightly over the frame.  If the cover isn't cinched down tightly, the wind will literally beat the plastic to death, greatly reducing the useful life of the greenhouse.  Since the cover is always the first part to wear out on these types of greenhouses, you may want to inquire if covers can be purchased separately at a later date, or an even better option may be to buy one complete greenhouse unit and an extra cover up front, so you'll have it on hand when you need it.

In summary, if you want a greenhouse that will stand the test of time, and assuming you have room in your yard for a permanent structure, then polycarbonate panels are the best choice.  On the other hand, if you want something that can be used on a seasonal basis, and taken down when it's not needed, then a unit with polyethylene cover may be just what you need.  Either way, you can now begin your greenhouse shopping adventure, armed with the information you need to make an informed decision about what's best for you.