You could just put the beds together and put the slats on them and put on the trays. If they are too low, put up on cinder block. Then you would have a greenhouse full of flower beds for real. I do not think they need to be welded together. You would want to be able to rearrange them if necessary...

Here is a picture of the garden bench my DH made for me. It was made from a metal toddler foot and head board. You could make benchs like that just use a full, queen or king. size frames. He welded the sides to the back. Thesides were the foot board.
Good luck halley_rose

http://i174.photobucket.com/albums/w106/debjean13581/IM000562-1.jpg

That little bench is just darling halley_rose! In our area, we have some recycled lumber places. One sells cedar 2X4s at 40 cents a foot, Or, you could paint the wood you have with a nice solid color deck stain- it would look nice and be more durable. Bed frames do work for such things. I've gotten them for $3 a set at my favorite thrift store. They do have to be supported so they don't twist and your slats pop out tho. Have fun!

A friend has a greenhouse made out of PVC pipe which was put together as a box. The PVC was also used inbetween the top and bottom pipes for support and again in the middle of the top. She then covered the whole thing with thick plastic to block the cooler air. She puts it on her deck in early Spring to get her plants/veggies growing. I'm estimating the size of it at 8' X 8'.

Sal

When we built my greenhouse, We used pressure treated lumber for the frame and cross pieces only. Then we stapled
Hardware cloth to the top of the frame. No collection of water or soil, because it would just go through the hardware cloth. I loved that!