leaching
I designed and built several larger planter boxes for my mom last year (3'x 5' and bigger) and considered using pressure treated wood. One look at the warning sheet which is prominently posted next to the lumber and that was that for me!
Among the many dire threats and disclaimers, the manufacturer also warned that the chemicals might well leach into the surrounding earth.
Went with plain old red-wood. Not cheap, but at least I don't have to worry about someone planting vegetables in one of them a few years down the road and poisoning their family.
Regardless of whether you use metal or wood, it would not be difficult to design the system so that you could stick the posts into holes in the ground and then remove them at the end of the drying season, respectively when you need the area for other purposes. Indeed, with a bit of fore-thought, those holes could be used to anchor other outside constructions such as a sun-roof or mosquito-netting, etc.
You'd have to give some thought to safety - but the need to keep water and ick out of the hole when not in use would require a cap, anyway. Or a flagstone over it...
Just a thought.
(Personally, I think wood looks nicer, properly taken care of there is no reason why you can't get 20-25 years service out of redwood.
