I wouldn't use treated for food space, but that's my preference.
I used cheap plain pine 2 x 4s, and they have lasted several years--I finally had to replace a few the third year, but most are still fine. I found some used 2 x 12s and 2 x 8s and they have lasted much longer...so I will go with bigger boards as I replace them over the years (figuring the larger mass took longer to break down, but it could just be that older construction lumber was a lot better, lol).
We skipped the boards and used landscaping bricks. The cost wasn't much different (it was on sale) and we butted it against our privacy fence on one side so we didn't need as many bricks. 4 years and counting
Our resident master gardener swears by pine 2x12's though. They do rot in time, but for what they cost you can replace them easily while the rotting timber is beneficial in the garden.
The 12 inch board brings the garden up higher and saves on the back too.
I used untreated 2 by 12's they were used from the factory farm chicken houses payed $3 each I made 12 4 by 10 foot raised beds they are growing greens now under plastic or atleast 10 are I ran out of plastic.
What kind of wood do I want to pick up from Home Depot / Lowe's for a raised bed garden? Do I want something that will rot and break down naturally (untreated) or do I want pressure treated wood?
We use 1 x 8 x 8 cedar and it lasts for years whereas other types of wood only lasts 2-3 years in our rainy climate. The cost differential for cedar is about 1-1/2 times more initially however in the long term, using cedar is the better choice financially wise for us.
I'm using 2"x8"x16'--yellawood, treated, supposed to be for gardens, is $12.06
Same size, cedar is $49.00
Yowza! That's pricey for cedar. I picked up 2x6x8 pine for under 3 bucks each at menards this week. Untreated. I can see if you lived somewhere extra damp you might go with a cedar or non-wood product though!
I used to just have mounded raised beds--you don't even need an edge really. But, I do think they look neater and you can plant right up to the edge and not worry about erosion.
You can also ask for wood that is not suitable for retail sale, such as a small split, or something and get it for 50% off. I did that at Lowes. I did make raised beds out of old tractor tires (work great) and used concrete blocks. Both work great and are free!
I cut down about 20 trees and have more wood than I need, so I am cutting a couple of trees into 3" lengths. Then I'm going to put some landscaping fabric down and place the 3' tree pieces in a circle with a 4'-5' diameter. If necessary, I will tie a rope around them to keep them in place.
I'm going to create several of these and grow flowers in them. I already have plenty of 4' beds made with pine 1x8's for vegetables.
The 6' length kind of thru me as I wanted to build 4x8 beds. No biggie, I build 6x6 beds instead...couldn't beat the price of ~$7.50/ bed with tax. I cut some scrap 2x2 to use as corner braces and used galv screws to put the beds together. Took less than 90 minutes to put together 7 beds which is around 15 minutes / bed to build. Not bad. Have to wait to see how they hold up.
IMHO, don't use pressure-treated.
I think best wood is Western Red Cedar - same species used for shake shingles. Rot resistant, yet relatively light in weight, so construction won't kill you. Chain stores (here in the East, at least) don't carry. So check with mom & pop lumber yards.