I've wanted to build a few large planters for a while now, and I've considered using concrete, wood, etc. I don't know what other cheap material to use.
Initially, I wanted to make them out of pre-mixed concrete, but realized that a square 24" concrete planter would just be too much weight for my rooftop. Each paver would be minimum 60-lbs each, so that's 300 pounds for one planter. Four planters would easily be over 1,000 pounds.
I then considered using wood, but untreated wood would rot and treated wood would poison the plant. So that's also out of the question.
So my question: for someone like myself with very extremely limited DIY skills and tools, how do you recommend I build a lightweight large planter?
I would like to try and build it because the cheapest 24" x 24" plastic planter is $35, and they're very ugly.