I think treated lumber is not good for food crops due to the leaching of chemicals into the soil. I think it is OK if you line the bed with a thick plastic sheeting as someone said above. There are inexpensive plastic boards that look like wood, or are part of a kit for a raised bed. I have found reasonably priced ones at Sam's Club, possibly at a regular Walmart, and at Gardener's Supply online. Some of gardener's supply are expensive and there are shipping costs to consider. If you use untreated wood, the thing is it will rot over time, but can still last a few years and you just replace as you need to. You might want to do a search for "lasagna gardening" which is a way to smother the grass before you start a raised bed, and also add organic matter and nutrients to your bed. You can easily kill the grass by laying just some bags of soil or compost or mulch down and very shortly the grass underneath will be dead. The size should not be too wide, becoz then you have to walk on the bed to harvest, so usually 4 ft wide is the most that's recommended. There are different depth recommendations depending on what you want to grow, like tomatoes and carrots vs lettuce and radishes. Deeper is good, but why waste soil esp if you have to buy bags of it, so I think most people do 8 to 12 inches. Have fun!

I don't see any downside but 14 or 20 feet is long enough that I don;t think end caps will prevent outward tipping. If I were using standard dimension lumber, I would make the caps 4 feet even so I could get 2 caps from an 8 foot board. Caps on the outsides are how I made my 12 3x8 foot beds. I also used 6 inch landscape screws rather than galv spikes to attach them. It was easier to drill and drive than nail since I had no one to hold the boards steady during assembly. I figured that since I was drilling (or nailing) 3/4 of an inch in from the ends of the cap board (into the solid end grain of the 8 foot side)the wood would be weakened somewhat and overtime pull apart. If that time comes it will be easier to replace the 4 foot end rather than the 8 foot long side.

I would not line with weed fabric, particularly the bottom of the bed. It will prevent roots from extending into the native soil, will hinder the movement of air and water (as per Michele Derviss's comments) and will also prevent soil biota from migrating into your beds for healthy soil. At a previous home I had raised redwood beds similar to yours used for a large veggie garden. The wood sat at ground level and were anchored by 4"x4" posts. We loosened soil below the level of the beds for several inches, and mixed a combination of native soil, plant compost, and composted cow manure to fill the beds, with the majority being native soil. The beds produced very well and were virtually weed-free when mulched with chopped leaves, and when we moved after about 10 years, the beds were still holding up well. We did mulch the paths between the beds so that we didn't have to worry about grass and weeds invading the beds.

I guess I was confused by the same thing as Dave. But now that I looked at your blog I can see that you are using land scape timbers stacked 2 high for your frame so about 6" which is shorter than the typical raised bed of 10-12" or more. Did you till up the native soil at all yet before piling on the organics? I think even with fluffing up the existing soil and mixing in with the grass/leaf compost in spring the soil still won't be to the top of the frames without bringing in some additional but over time you can keep adding compost and that should raise the level. As long as your native soil is not rock hard clay down past where your tiller is able to reach you should be OK, but you might look into double digging to get your amendments a little deeper. One other note...landscape timbers warp and twist pretty quickly so you might have to replace them every few years. Good luck with the Okrie next season! Okra roots do a good job themselves of breaking up soil to a depth of a couple feet or more so you have that going for you too.