In order to choose the best greenhouse cover, you will need to consider what type of structure your greenhouse has, the types of plants you want to grow in it, and the climate where you will be placing your greenhouse. Greenhouses can be large, permanent structures or small, temporary fixtures, and the type of greenhouse you have affects the choices you have for covers. The type of plants you grow is important because different plants have different light and temperature needs, both of which can be affected by the cover. For the same reason, you should consider the climate and season in which you grow plants.
Any greenhouse cover is meant to be used over some kind of greenhouse structure. Very large, professional greenhouses are typically sturdy, permanent structures and will require rigid panels as a cover. There are also covers that are meant to be used with pop-up structures. These structures and covers are more likely to be temporary fixtures that are good for do-it-yourself projects. Make sure you are aware of what kind of greenhouse you have or would like to build before you determine exactly which greenhouse cover you are going to buy.
Greenhouse covers are used to control a variety of factors such as light, moisture, and temperature. Make sure you know what conditions you need to control and what the desired conditions are for the plants that you grow. Some covers provide shade for plants that shouldn't be in direct sunlight, while others optimize sun exposure and solar heat for tropical plants. For example, some fabric covers are solid or tinted to provide shade, while polycarbonate or polyfilm plastic covers simply diffuse the sunlight and insulate the plants. Some plants need protection from UV rays, and a greenhouse cover with UV protection is the best choice.
The material of a greenhouse cover can also accommodate different climates and seasons by controlling those same variables. If you are building a greenhouse to use during fall or winter or in a colder area, choose a material like polycarbonate, that provides insulation. You might also consider a cover that has two layers, sometimes referred to as twin-wall. You can find these covers in flexible polyfilm plastic, in which the material is made thicker with multiple layers. You can also achieve this effect with polycarbonate or glass panels, which often have a space between the two layers to increase the insulation with a pocket of air.