Any question answered...

Want to build a cold frame for my allotment.

I've tried to scavange as much as possible but haven't found anything appropriate to form the sides. What sort of wood would be best to use consuiderign it'll be out in all weathers?

Will any sort do and the important thing is to treat it? If so what with?

I've used Scaffolding planks for the sides of a cold frame before. Worked OK.