One of the easiest and cheapest ways to put up a serviceable greenhouse for mild cold weather is to use plastic sheeting, PVC tubing risers, and metal stakes into the ground. The cost is minimal and if you pay a little extra for the UV resistant plastic sheeting, it will last a few years before the sheets must be replaced. The PVC tubing is inserted into a 4 way T-joint and the opposite side of the joint is slipped over vertical metal stakes made from re-bar or other small metal tubing which is driven into the ground at each corner and along the sides at intervals of a few feet. The distance from one side to the other will change the height of the greenhouse so a bit of experimentation and planning is in order. The top of each PVC tube along the side is bent over to meet at the top along the center, where it is attached to the tubing from the other side by a 4 way T-fitting.
Short lengths of tubing run between the T-fittings at the top of each rib to join them and provide stability and a center line for hanging misting hoses or whatever. Similar lengths are run between the T-fittings at the bottom to provide a place to anchor the greenhouse to the ground (with tent stakes or wire to the driven stakes) and to provide an anchor for the sheeting.
The plastic sheeting can be held to the PVC pipe by binder clips (which hold very tightly but will tend to rust over time) or by a small clamp made from a short (less than an inch) length of the same or slightly larger PVC pipe which is cut open along one side so that it can be snapped over the sheeting at each pole. A commercial version is called "Snap Clamp"^
On the ends, the sheeting can just be allowed to hang down, weighted by rocks to keep from blowing up, or a separate sheet can be cut and clipped around the last rib, then a flap cut in the plastic and secured behind you with binder clips or a glue on tent zipper.
If a "real" door is desired, a strong frame must be constructed first. Metal posts can be cemented into the ground or large metal fence posts can be driven and then a wooden lentil fitted to the top between the poles by banding strap or other means. The top of the posts should connect with the end rib and be strapped or zip tied in place. Sheeting is then placed between the posts and the rib, and a gate with sheeting made or purchased and hung to one of the poles with a latch on the other side. Fencing gates are excellent and easy to mount and cover. PVC doors (and hinges^) are generally not long lasting, but may be useful if care is taking when opening and closing.
The top of the door posts can also be used to mount vent panels; either above the door, or to the left and right, depending on the height of the greenhouse and the height of the door.
Any plan that involves wood touching the ground is going to rot in a year or two. Even "pressure treated" wood (wood which has been infused with toxins to prevent rot) will not last more than a few years, and how stupid is it to bring chemicals into contact with the earth in which you are going to grow food?
Openings trap air and allow wind to fill and potentially lift all but the most solid greenhouse. Be very careful when entering or exiting your greenhouse on windy days. Vents should be mounted facing away from the wind.
A basic household thermostat can easily be connected to a 12V fan and powered from an AC adapter, or other power source. Depending on the thermostat and power source, several fans can be used, but may not be effective in a large greenhouse. The thermostat should be mounted in the shade, near the plants, but not enclosed so that normal air flow produces a representative temperature at the sensor. Fans should be mounted in sets at either end of the greenhouse; one evacuating air, the other pushing in fresh air.
The more traditional method is to build a "door" at the top of the greenhouse and open or close it manually or using a automatic roof opener such as the gigavent.
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