Linseed oil generously applied gives some protection and is non-toxic. Some woods like cedar have built-in preservatives. The plastic wood they sell for decks may be non-toxic, but I will have to check into that. (I plan to make some raised beds next year and don't want any toxic wood preservatives involved either.)

Another approach for your hoophouse would be to not have wood in contact with the soil. You could put down a row of concrete blocks, or just pour a concrete footing and put your wooden structure on top of that. If your concrete blocks had the right sized holes, you might be able to thread your hoops directly into their holes.

MM

hi try eco wood treatment . we bought some at sherwin-williams its a great non toxic wood preservative,also turns the wood old looking , its also available at home depot i understand best to you all , fred