What are non-toxic methods for treating or encasing cedar fence posts to extend their useful life?
Alternatively, is there a wood more durable than cedar that might be available in Vermont (e.g., black locust)?
We are about to re-build a backyard fence that will run near an organic garden. We’d like to find an alternative to pressure-treated fence posts (where chemicals seep into the soil) or untreated cedar posts that rot in the ground.
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Untreated white cedar is the tried-and-true Vermont option for fence posts. They last a long time, but nothing lasts forever. Don't try to "encase" them with anything (polyethylene, for example), since encasing them might accelerate rot rather than retard it.
Ken Kern, the author of the classic book The Owner-Built Home, was a big fan of concrete fence posts. His book includes instructions for building forms to mass-produce concrete fence posts at home. Concrete fence posts are much more common in the Caribbean than in the northern U.S. They last longer than cedar, for sure.
Black locust used to be the standard fence post in New England and it's still available if you hunt around. But the classic way to "treat" fence posts before putting them in the ground was to char the outside and endgrain of the bottom. This makes the surface wood unpallatable to fungi and insects.
Concrete fence posts aren't exactly "classic" to New England, but an easy way to form them is with PVC schedule 40 pipe sections that are sliced on a table saw along one side (end-to-end) and then kerfed on the other side 90% through the wall thickness. That side will form a hinge to allow the form to be removed. The cut side is duct-taped together and then untaped to open after curing. This makes an almost glassy-smooth concrete post. How you're going to attach fencing to it, however, is another challenge.
If your soils are well drained, my suggestion would be to use untreated cedar posts, but backfill and tamp using 1/2 inch or smaller rock...no sand or fines. Put some rock at the bottom of the post hole as well.
Locust lasts one day longer than stone ( so they say).
Perhaps take a look at TimberSil http://www.timbersilwood.com/ It's a glass-infused wood product. It's not as pretty as cedar, but what I have seen has looked a lot better than pressure-treated posts.
I know it's available in the North East (my local lumberyard on the coast of Maine can get it).
Might also try: TMW (thermally modified wood) available from SYP 4x4 stock - Stellac kiln process seems to allow for some level of ground contact/ fresh water immersion; or perhaps Acetylated Wood?
Concrete fence posts? Sounds like a high embodied energy solution to me.