Before I had a whole big enclosed garden, I built an L-shaped planter box out of Trex (4x6x8 wings, 4x4x12 center box). 7 years later, it looks as good as it did on Day One, no noticeable wear-and-tear at all. The other boxes I built (four 4x4x10s) are out of cedar, only because Trex wasn't available in that width and I wanted one solid piece. The cost of Trex vs cedar was almost dead even.

Here's a pic of part of the box.

Thank you for that wonderful suggestion!

How is the cedar holding up?

Be careful with Trexlike product. It bends and warps when not supported with minimum 16 inch joists (how they were intended to be used). That said though epxpensive they are almost permanent.

I use pressure treated wood for my beds, which are 16 inches tall. My brother's used the stuff for 10 years and it's still holding up great.
Good luck finding what suits you.

Mine are cinderblock, painted to "match" the house, no warping or rotting, we paid a few nice men to stack them 2 blocks tall, like legos, and to fill the beds, and off we went. Most likely the roughest way on your back, but we'll be here 20 years untill retirement,so it was worth it to never do it again.

I know it has been awhile since you posted but I am considering building my boxes out of galvanized sheet metal. I believe that these should hold up the longest in all climates and should not "Bake" the plants any more than a regular box should because the walls of the metal are so thin and should dissipate heat quickly. I have gotten a bid to be custom built for around $250. What do you think?

Stone, or decorative cinder blocks will last a long long time. My mom has some stone flower beds she built before my sister was born a little over 21 years ago. Whoever told you they'd last 10 years or less is full of it. Those beds will probably last longer than you or I.

I have a 300 sf raised bed made out of Allan Block. I suspect it will outlive me (40-something).

Dan

Mine are steel with a nice rusted patina. They were welded and have 1-inch "angle iron" along the top edges, facing inward, to give a nice-looking lip. The steel looks great and, from how long derelict car bodies last, should last a very long time. The steel has a nice functional aspect, too. It seems to warm the soil quickly and I can readily secure the edges of cloche covers with magnetic strips. Two of my beds are rectangular, one is square, and the other two are pentagons.

I think on average in an area not near the ocean and doesn't get tropical amounts of rain steel will loose about .005-.007 inches per year. Of course if it's painted, plated, or powder coated it'll last much longer.

Jeremy - You got me wondering, so I tracked down the info you posted. NASA has a site that lists some test results in the link below. Our climate is probably pretty close to that of Esquimalt, Vancouver Island, BC, which has a corrosion rate of 0.5 mils a year. I like that alot more than 5 mils a year! Thanks for raising the issue.

Here is a link that might be useful: Steel corrosion rates