I'd go with the 2x thick boards if you you want them to last more than a few years. The new treated lumber doesn't hold up nearly as well to earth contact as the old arsenic treated stuff. Better yet would to use cedar 2x but the price would be about 50% higher.

I'd probably stick with no longer than 10' beds because the longer the edges the more warping. It can be difficult to find a 10' board that is not already warped. If you have a Menards near you I'd buy the lumber there rather than HD or Lowes because the quality is better, and there is less handling involved with their drive up lumber yard.

I agree with Jack- and cedar fence pickets are another less-expensive alternative to 2x cedar lumber.

We used untreated 2x6 southern yellow pine lumber in our raised beds and some of them are going on their 4th season now and have not rotted yet. I fully expect to replace them in another couple of years, however.

A more permanent and expensive option might be something like Trex lumber, but the price of that will be very expensive.

These Oldcastle blocks make building raised beds really easy, also

https://www.homedepot.com/p/Oldcastle-7-5-in-x-7-5-in-x-5-5-in-Tan-Brown-Planter-Wall-Block-16202336/206501693

I've only seen them at Home Depot. We have used them to make a few ornamental beds and will probably use them when it is time to replace our vegetable beds.

Yep. Had already decided to use 2x12 for ease of construction and durability. Also was surprised at how "relatively" inexpensive PT wood has become.

Thanks,

Jamie - I had forgotten about those. I have 2 beds that are starting to rot at the corners and need to be replaced this year. I think I'll buy some of those to use in the corners and it will make it much easier to replace the edge broads in the future.

And I have a $100 HD rewards card coming this week!

Jack- I love them. We stacked 2 on each corner and used 2x12 lumber for a couple of beds and 2x6 for another one. If you stack them, you do need to drive some 1/2" re-bar down through the hole to keep them stationary.

The other option is to build it out of cinder blocks - the standard 8"x8"x16" concrete blocks available dirt cheap at any big box or home improvement store. Will never rot, are easy to construct and equally easy to disassemble if you want to remove or relocate the bed. Or even alter the size.

We're on year 2 of our raised beds we made using untreated 2x4's. We didn't want any chemical leakage from treated wood. Our dimensions are 8'x4', 10.5" tall. My beds are on a slope, so the actual soil depth is much deeper. I haven't noticed any rotting yet, even on the bed I pulled out a few weeks ago to make room for some equipment.

gardengal48 has a good recommendation, as well. We used blocks our first year which worked well. The only downside is they get heavy after the first 20 or so! LOL Bonus was that we used the holes in the blocks to plant flowers and herbs on the perimeters. The only reason we took it down was our slope. I still have them and plan to use them for the outer beds in the greenhouse since it's on flat ground.