Where did you buy these type of benches, and how have they held up to the weight of your plants?
I bought some of them through a local garden/farm supply company. The rest I bought in different parts/combinations from Greenhouse megastore and Structural plastics corporation depending on which had the best price at the time.
The brand name is Benchmaster.
They have held up very well. As long as you have a full set of legs to support the wider and longer benches, they don't warp at all.
I love them. There is no wood, no painting / staining and they don't rot. They are made to be in greenhouses and in garden center display area so the hold up to the UV of the sun