If you want to stain them, do it. The things that used to go into stains many, many years ago(which were not good) are now not even allowed to be used as a general rule. It is strictly regulated and enforced. These laws went into effect in the early 90's. You can use all those things that you stated, but it's just a lot of extra work. You will want to put something on them if you want them to last.

Whatever you do, keep in mind that "scientific" opinion regarding chemicals in stains, paints, and wood treatments changes over time. I have a small construction company that specializes in decks, fences, and patios, a few years back all pressure treated lumber switched from one chemical treatment to another. They reduced or eliminated the arsenic and increased the copper among other things. You never know when or if a product you use could be deemed "dangerous". Also, when the lumber industry made the "voluntary" switch to a new treatment chemical the EPA released a statement noting that the change was voluntary and that the original chemical CCA was never linked to cancer or anything harmful. Talk about covering your behind! Bottom line, chemicals are chemicals are chemicals...who you work for may determine if you think they are safe or not!

I wouldn't be too worried about health effects of the materials you mentioned. Turpentine is volatile, and the VOCs will quickly dissipate. Linseed oil is safe enough, as is paraffin. However, for protection, those are a waste of time and money. They won't add any life to your wood. Linseed oil has virtually no water resistance, and paraffin will only slow water penetration. That can help protect indoor furniture from spills till you wipe them up, but won't help where there is constant water exposure.

The only products I know will protect wood outdoors in even the worst climates is Smiths Penetrating Epoxy and/or Epifanes varnish. Fine Woodworking did a test of outdoor wood finishes, and these were the only ones that worked. My husband has used Smiths for over a decade on the spearguns he makes, and they still look like new. However, these products are very expensive, and I'm not sure about the toxicity of anything in the finish that could leach into the soil.

We just built boxes from the same material, 2x8x12 to be exact, and we are not using any of our Smiths/Epifanes because of the extra work, expense, and questionable toxicity. I considered painting with lime paint, but didn't bother. I figure that we will get several years out of these boxes, and if we still want to garden with raised beds at that time, we'll build the next ones out of redwood.

But it wasn't voluntary though. Its mandated and regulated. The EPA did get some teeth into it through heavy penalties and fines. I did use pressure treated wood, and I've protected my boxes every year. Im now working on year 13, and there's a lot of life left in them. You're simply not going to get that if you don't protect them with something.

The EPA framed it as voluntary initially...similar to how you voluntarily pay your taxes if your self employed I suppose. They did issue that statement regarding the safety of the CCA material though, I spent a few months showing it to customers assuring them that the remaing CCA material we were still receiving was safe according to the EPA. At the time using it was still ok, manufacturing it was and is another matter. My point was simply that times change, information changes, and so does opinion regarding any chemical. Mosquitoe spraying by municipalities comes to mind as a recent example.

I think we all, myself included, need to start looking for alternatives to wood to make our raised beds from.

I use douglas fir as well and figure ill get 5 years or so out of them.

If you think about how long it takes for those trees to grow big enough to become the lumber we use... 100 years... it's just not sustainable to continue doing so. So, when mine wear out I will probably replace them with something altogether different, perhaps stone.