I'm working with the local elementary school to make a square-foot garden for the students. A nearby sawmill is donating lumber--6" x 6" rough-cut cedar logs--for construction of the beds. Naturally, we're hoping that the beds will last for several years. Should we seal or otherwise prepare the logs with a surface coating as a preservative? Any such coating must be nontoxic and environmentally friendly.

Thanks in advance for your help!

Jeff