I am about to create a number of raised planter-boxes where i can't separate the wood and soil with plastic. So, in order to create something that will last for as long as possible I have been trying to collect information about which wood species will last the longest with direct soil contact.

I have found a fair amount information about Robinie, oak and some hardwoods like Cumaru, Jatoba and Ipe as wood species which will last a long while with direct soil contact.

But I expect that someone must have made some structures comparisons and tests, that I just have been unable to find. Can someone either tell me which species of wood will last the longest in direct soil contact. Or, even better, provide some structured study that I can take a look at?

  • Other rot resistant species include Osage Orange and Black Locust. They're both commonly used for fence posts. Teak has a long history in the marine industry. You can also double down and use a "penetrating epoxy sealer". May 12 '20 at 20:07
  • Did you search previous Q&A? This may also not be quite on-topic for WW.SE. Much of the information being asked for would fit closer to DIY.
    – jdv
    May 12 '20 at 20:08
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    Your location will determine what woods are available at what cost... redwood and cedar are prominent here. But I would also ask if you're planting food or flowers... I'd never plant food in a box with treated wood. (Yes, I know the manufacturers claim that it's perfectly safe; they claimed that about the last chemical formulation, until they stopped claiming it.) May 13 '20 at 3:04
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    I welcome you and your question. That said, my immediate thought would be to go to the local sawmill (you're not finding those species at a big box store!) and ask the proprietor. They'll know the local species and soil conditions and should be able to give you localized recommendations. They've got a reputation to protect, so they're not likely to steer you wrong. Of course, they've got lumber to sell, too...
    – FreeMan
    May 13 '20 at 18:23
  • In my part of Maine we use hemlock, and find it lasts much longer even than cedar. It's a well-kept secret, though, typically if you buy garden boxes at the hardware store they're made of cedar. Or maybe that's because hemlock is heavy as [expletive].
    – workerjoe
    May 19 '20 at 14:39

What is the best wood species for direct soil contact?

The Forrest Products Laboratory has a lot of information on wood durability.

Here is a chart from one of their reports titled "Above and in-Ground Performance of Naturally Durable Woods in Wisconsin"
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Based on this data Eastern Red Cedar is a winner.

By way of comparison to treated lumber here is a chart from a report titled "Long-Term Durability of PRessure-Treated Wood in a Severe Test Site"
enter image description here

The AWPA is the American Wood Protection Association. They have arrived at a set of standards for treated wood for specific situations. This standard is used by many lumber providers.

Here is a graphic illustrating the applications and the rankings.
enter image description here

UC4B rating would be the winner for treated lumber.

  • It's possibly worth noting that much reduced longevity is being noted in American "red cedar" products used outdoors in recent decades, and this is even without the direct soil contact.
    – Graphus
    May 14 '20 at 12:40
  • @Graphus - What is this reduced longevity attributed to? May 18 '20 at 18:14
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    @GregNickoloff, I think the faster growth of the trees. They have less time to built up the compounds within the wood that give rot (and insect) resistance if the wood is being laid down faster. Old-growth stuff can be almost unbelievably long-lived for this reason, because in general the trees grew so much slower due to e.g. high competition for resources.
    – Graphus
    May 19 '20 at 5:04

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