Frame material typically comes in three forms. You will need to consider your options, and your needs, to make the best choice. Plastic or PVC greenhouse framing kits are inexpensive and easy to assemble. With a plastic framed greenhouse, you can move your greenhouse easily if the need arises. If you are considering this kind of greenhouse kit you will need to take into account how strong winds are in your area. If you live in a high wind area you might not want to use this framing, or at least will need to anchor it down for support.
Plastic bends and breaks when under duress from extreme weather, so it is ideal for milder climates. Plastic framed greenhouses are perfect, for the beginner, however. Wood is aesthetically pleasing, especially when combined with glass. It has the added bonus of being a natural insulator as it absorbs heat throughout the day and releases it throughout the night. Unfortunately, wood also absorbs moisture in the air, and will rot faster, requiring more treatment/maintenance than any other framing. Therefore it is ideal for very dry climates.
Aluminum will not rust and can withstand extreme weather. Aluminum framed greenhouses are designed as compatible with a variety of coverings from plastic, to glass, and polycarbonate. The channels in aluminum framing generally include details for you to add accessories to or to hang baskets from. An aluminum framed greenhouse offers ease in assembly for the do-it-yourself gardener. No matter which frame material you decide on, greenhouse gardening can be one of the most rewarding hobbies.