Nobody?

I would not consider anything thinner than the 2" planks. The weight of the soil against them will certainly cause warping or bowing on thinner material. Doug fir is moderately rot resistant so should last longer than just 3-4 years.

Yes, for the height you want you need thicker wood or concrete blocks or even gabions. Checks craiglist for materials first you might find something on the cheap.

Let me offer up an alternative. I live in the Pacific NW, so cedar is commonly available, but the quality has declined because much of the reasonably priced stuff is now from fast-growing tree plantations and is not as rot resistant. I have a large number of 12" high raised beds, and I considered making them from cedar, but didn't want to rebuild every several years. After thinking this over, I decided to go with "composite lumber," specifically Trex. It's a wood-resin product that is highly rot resistant, and the first beds I made out of it are 7-8 years old and show no signs of deterioration. Most of my beds are about 4' X 8' and 11" high, and are held together with coated, exterior deck screws. The material comes in a variety of wood-like colors, and is an actual 1" thick X 5-1/2" high, so to get the height I want edI used it two planks high. It has a bit more flex to it than equivalently dimensioned wood, so I simply ripped some 2-1/2" strips from a plank of it and used them as cross bracing: I mounted them transversely at the midpoint of the 8' sides, and halfway up so that they span the seam where the two lengths butt against one another. This prevents bowing of the sides, and also keeps the two boards on the long sides perfectly aligned. So, for a 4' X 8' raised bed, the structure consists of two 4' X 4' sections, but the cross bracing is hidden by the dirt. The thing I like besides the fact that the material is very rot resistant is that it never warps. It also comes in lengths up to 20' and right now I am actually making some 5-1/2" high by 2' X 20' raised tomato beds, with cross bracing about every 4'. The best price I've found on these boards is about $2.50 a running foot, so they are more expensive than wood...but never having to buy materials again or needing to build them again in the future makes it economical in the long run. If you decide to go this route, be aware that there are several board designs: some have a wavy back and some have slits in the edges for concealed fasteners when you use them for decking, and some are just a plain board shape, which is what I use; they look like this: Trex composite decking

The other thing is that you can special order these in 1" X 12", which I didn't know when I built mine years ago. The obvious advantage is that construction would be easier because you wouldn't have to stack two of them to get a 11-12" height.

No need for brackets. I had 2-1/2" long deck screws and I drilled holes directly through the sides the same diameter as the screw shaft, with smaller, pilot holes in the ends of the cross braces. I used two screws on each end...one above the seam and one below. I got screws with color-matched heads, so the fasteners aren't even noticeable. I do recommend pre-drilling because this material is very dense.

A couple of other tips:

1) Like regular lumber, be selective in choosing your pieces of composite lumber: even though this material is very uniform, it's not uncommon to see a slight bend on one end of a board, which I think is a result of non-ideal storage. I've never seen any warpage afterwards when the material is in use.

2) You may prefer to use an impact driver rather than a regular drill driver to drive fasteners due to the density of the material.

3) I've used two techniques to reinforce corners:

a) I've attached a 2" wide vertical strip to each of the long ends on the inside faces so that I could drive screws through the end pieces in double rows an inch apart. (Also, for strength, have the end pieces extend at least 1/2" past the sides of the pieces they connect to).

b) alternatively, I've routed out a 1" wide by 1/4" deep groove on each end of the end pieces that the sides fit into and are then screwed.

4) Find a flat place to assemble the raised bed frame that is as close to its final location as possible, because these raised beds are really heavy when made out of Trex.

great tips.

One question, Do you think if I stack 2 12" boards with cross beams for both every 4', this would still be stable? Alternately, I can use cedar posts at the ends(& at the 4' for the long ends). Will either of these work for a 24" bed?