Greenhouses are very good for planting crops; they increase temperature, help keep moisture in, help keep frost away, all of which help keep plants growing great.

Glass is the best covering for a greenhouse but it can be expensive, and easy to break etc, therefore plastic sheeting secured well is very good; it is cheap, easy to repair, and relatively strong.
Greenhouses do need to be anchored well to stop wind from damaging them, and the soil surface intact.
On steep slopes, you could even prop one up on 'stilts', having platforms to grow seedlings and small plants, making use of otherwise unused land.
Plants can be grown just as normal inside a greenhouse, just make sure they get enough water as the covering excludes rainwater depending on how it is built.

A good and relatively simple design is shown below, but use your intuition, it's not set in stone.

The large 4x4 posts are to keep the greenhouse anchored in wind. Use Schedule 80 Pipe if possible, other pipe will be weaker. The mid rib PVC pipe is cut into about 22 ½ in long pieces and rejoined with the cross joints. EMT tubing in the mid-rib (number 14) or spine strengthens the rejoined tube. Use two or more people when bending and anchoring the PVC ribs into the EMT strap loops(number 4) to prevent damage to the joints and ribs. Stapling down the plastic cover is the best way to attach it, allow some extra plastic to extend and be covered with dirt to help make a seal. Find some old garden hoses and punch drip holes or use drip irrigation tube, it will stay under the ground sheet for drip irrigation. Lay down ground sheet plastic and punch holes for your baby plants, or a few inches of sand or gravel, these limit mud and weed problems. Bury the edges of the greenhouse plastic with soil or sand, it is important that the edges be sealed to keep rainwater and moles out. Even in the coldest weather be careful that a small heater or heat lamp doesn't overheat the greenhouse, in summer open the door if overheating. This greenhouse design will support four inches of snow, if there is more snow add additional prop supports Connect twine hanging from the ribs for vine and soft plants. As the plants grow help them twist around the twine for easier vertical growth. This works great for cucumbers, beans, and tomatoes. Flowers and food for sale can be grown year round in most areas. If you need more room extend the length of the greenhouse following this design, a wider frame will be weak in wind and snow. If you need to save money leave out the door but keep the door frame and use overlapping plastic flaps.

Illustrations and design courtesy of North Carolina State Cooperative Extension Service

Bill of Materials

* Item Qty. Description
* 1- 16- ¾ in. PVC Schedule 80 Pipe, 10 ft long
* 2- 6- ¾ in. PVC Cross Joints, Schedule 80
* 3- 2- ¾ in. PVC Tee Joints, Schedule 80
* 4- 32- ¾ in. Galvanized electrical metallic tubing (EMT) Straps
* 5- 2- 2”x6”x14” Outdoor Treated No. 2 Pine Boards
* 6- 2- 2”x6”x12” Outdoor Treated No. 2 Pine Boards
* 7- 4- 2”x4”x7” Outdoor Treated No. 2 Pine Boards
* 8- 4- 2”x4”x6” Outdoor Treated No. 2 Pine Boards
* 9- 4- 4”x4”x2” Outdoor Treated No. 2 Pine Boards
* 10- 2- 2”x4”x3” Outdoor Treated No. 2 Pine Boards
* 11- 2- 1”x4”x12” Outdoor Treated No. 2 Pine Boards (to cut up for door parts)
* 12- 1- Set of door Hinges
* 13- 1- Sheet of Plastic 24 ft x 20 ft, 4 mil thickness
* 14- 1- ½ in.x 10ft. Galvanized Electrical Metallic Tubing
* 15- 1- PVC cleaner
* 16- 1- PVC cement
* 17- - misc nails, screws, and staples