galvanized will rust, aluminum cuts better since you only need 18 inches of height. You could use the lexan for sides which might look cool, and invisible! The polycarb panels would resist flexing and torsion better but side staking would also provide support. The rules still apply, metal conducts heat so this is good for early and late season growing.

If I can find the panels I am thinking of, they are 15" wide. Aluminum would corrode in contact with the ground and the oxides of the alloys might prove toxic to the plants. Lexan/Polycarbonate would be great, but very expensive. Now, if I could find windows removed from buses I used to work on.......................

On this subject, I can only tell you what I did... I got raw oak lumber from a rural lumber yard. Oak will last a long time, and will leach no bad chemicals into the soil.

You really only need 6" of good dirt. I resisted this, but I got the same sized plants in 6" and 12" beds.
I get a lot of information from http://squarefoot.creatingforum.com/forum.htm

yeah galvanized will last a long time / many yrs

It would most likely outlast me.

One advantage to wood that I can think of is that you can lean on the edge as you work in the bed. I think with metal that would not be a good idea. As a bit of a klutz, I find myself doing that from time to time and at least I don't end up with lacerations. I think with metal, I might. I do think the metal looks cool (gardener's supply has some in their catalog), though, and would undoubtedly last a long time.

I used rough-cut 1'' x 12'' western red cedar when I did mine around 10 years ago... back then it was around $7.95 for 8ft lengths. They are still standing & in use today except the pressure treated 4x4's to tie the corners together rotted on some beds!

I also extended my original beds using stacked 2x6 and 2x8'' PT painted with redwood paint/stain. This gave me 13'' deep beds.

{{gwi:1340631}}

Hope this helps.

Garf,
You might like to look under square foot gardening. We all use raised beds and grow tomatoes up a trellis. Most use untreated lumber, but there is a kit for reasonable at Costco that some people are using. You could get some ideas there.
Keski