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I was gunna build a green house not sure how big yet just deppending on the supplies but at least 12 by 15

i live in south texas near gulf coast and it get pretty hot the sun thrashes down all the time normaly in summer it dont get under 90 and it get 100 a lot.

So i was wondering what would be the best desighn to build

and should i build it in the shade under the trees

would just cheep plastic work or would i have to have uv protected plastic
and also would i have to put a vent with a fan on it

Uv protective plastic/film is a must. Stong sun will turn non protected plastic/film yellow and useless rather quikcly. You can get decent amounts of the good film from http://www.greenhousemegastore.com

Also some shade cloth will help keep the temps down. Vents are always a good idea, but do not need fans, of course fans will help but it is not needed right away. If you have a good location for it, meaning good sun and not many wind blocking obstacles (homes, tree line, hills) it should vent normaly with out fans provided there are enough vents on the structure.

In my opinion, always position it with as much sun exposure as possible. You can always add shadecloth.

I've ordered stuff from the site Dustin posted, good prices and service.

I agree, the more sunlight, the better. If you have some plants that don't want that much light, a little shade cloth does the trick.

I second Dustin - UV protection is a MUST! Other plastic will fall apart in less than a year.

I think you biggest challenge will be dealing with heat. You definitely will want to have major ventilation. Even to the point of removing the sides partially or completely. Otherwise, you might end up cooking your plants. The larger the greehouse (higher roof, etc.) the easier it will be to ventilate and avoid fast overheating.

I think the type of your greenhouse should depend on what kind of cacti would you like to grow in it. If I lived in Texas, I'd build only a roof (=no problem with heat). (BUT I grow Ariocarpus and Lophophora alone, other genera may need a greenhouse).

i might just make a top to start with them put side on it later
i will probly grow mostly differnt types of cactus i wish i could fill it up with loph but i dont think my local officers would like that even tho its native to texas but i'm just getting started with growing cactus seeds