Using Polythene to Build a Greenhouse
While glass is the best material to use for a permanent greenhouse, an inexpensive polythene greenhouse is perfect for winter gardening. This greenhouse can be dismantled in the spring and summer quickly.
The greenhouse should be placed over level and well drained ground. Make sure that no trees or large bushes will overshadow the greenhouse.
You can use treated lumber or PVC pipe to make the frame over which the clear polythene will be stretched. Be sure to take a measured drawing into the home center when you buy your materials. It is easier to make a greenhouse out of PVC pipe, but it will only last for a few seasons. If you paint the PVC with a good grade of spray paint, it can help it last longer. You will also need enough polythene to completely cover the frame, as well as the bottom perimeter. Purchase the best grade of plastic that you can, the thicker, the better.
Step 1 - Set Up The Frame
Lay out the bottom frame of the greenhouse on a two foot wide rim of polythene. One foot should be on the inside, one on the outside of the frame all the way around the perimeter.It is a good idea to seat the lower frame into the soil or gravel to keep out pests. This will force the plastic sheeting to "tent up" around the perimeter.
Step 2 - Add Upright Sleeves
Depending on what type of bottom frame you decided on, you will need to add appropriate upright sleeves at least every 24 inches. These can be "t" joints for PVC, or pieces of rebar vertically inserted into the ground. Slip the arching cross-members over the rebar, or into the sleeve joints on each side. This should form an arc of PVC pipe about 10 feet in height at the apex. If you live in a windy area, you may want to put a cross member across the top of the structure to make it more rigid.
Step 3 - Attach the Plastic
Attach the perimeter plastic sheeting to each upright using duct tape, pulling it fairly taught.
Step 4 - Adjust the Trim
Tent the structure with the polythene plastic sheeting, trim it so that it is about inches off of the ground on all sides. It is a good idea to use one section of plastic to do the main structure, and separate pieces to do each of the ends. This will be easier to repair if necessary. Tape the tenting down to the perimeter sheeting using duct tape. Do not draw it tight over the frame, it should have some "sag" in between each of the arches.
Step 5 - Set Up The Entrance
One of the ends can be attached to the upright frame with tape. One end will have to have an opening for access. The easiest way to do this is to simply install the end, and using a razor knife, cut out a doorway flap that can be resealed with Velcro tape. Simply cut from the ground up to a height of six feet, and over to the left 20 inches. Cut another piece of plastic that is seven feet by 24 inches and tape it over the opening on the left hand (hinge) side on the vertical length. Attach Velcro tabs on the inside of the "door" and to the main plastic sheeting. This gives you a double sealing entrance to the new greenhouse.