Is "untreated" lumber treated?

April 18, 2020 12:34 PM

I'm trying to build raised garden beds. Just as I was about to purchase some redwood boards, I remembered a tour of a lumber mill where I heard about "dip tanks" with fungicides (?) so that the lumber could be stacked outside without discoloring. Is this still happening, and will these chemicals leach into the soil? Also, I have a one-year-old who licks and chews pretty much everything. Is this an actual thing to think about and if so, what's the workaround?
posted by slidell to Health & Fitness (11 answers total)

Usually, the 'dipped' cedar and redwood is labeled as such and that's a sealant similar to what they use on pressure treated boards, but without the pressure. If you cut those pieces, you'll need to seal them on the ends where the cuts are, or you'll get rot problems like you would with untreated lumber. Unfortunately yes, oftentimes lumber is treated with anti-fungals and not labeled as such.

Pressure treated wood doesn't have anything supernasty in it, and since 2003 or 2004 the EPA banned the use of chromated copper arsenate (the really nasty stuff) and since then pressure treated wood is considered relatively safe for use in garden beds. If that still swicks you out (understandably so!), I'd suggest building beds like these (we build some of this style, and the soil doesn't touch the wood, just the metal...they require more labor, but are cheaper and sturdier than entirely wooden ones), or full metal clad ones, or using concrete, which is fairly inert once set.

Alternatively, if you've got access to wood rounds or logs, you don't need a border to do raised beds, hugelkultur is a german style of raised beds that makes more of a soil hill around wooden filler that breaks down and makes the soil more productive over time. It's pretty rad!

But, none of these are great options for a one year old to chew on though. If that's your criteria, you're gonna be hard up to find a good solution.
posted by furnace.heart at 1:06 PM on April 18, 2020 [2 favorites]

Clients often ask me to design outdoor planters and in the last five years increasingly everyone is stating they do not want treated timber - which is fine by me as I was making noises about this when I studied over 15 years ago.

Here NZ the treatment is CCA - Copper Chrome Arsenic - and despite industry assurances it does leach to harmful levels, link is re fenceposts but principle is the same. Correction - I see the US is 15 years ahead of clean green NZ, so you're a bit safer.

There are timbers that do not need treatment, redwood (altho' heartwood\planks cut from inner part of tree, from older trees is most durable), some eucalypts and even some timbers that are normally not seen as durable.

I have used macrocarpa sawn in to boards when green and with a drainage liner - the black in the image is a natural rubber paint, an inner drainage liner goes on next and then it's filled with soil. The boards are also not allowed to touch the ground, using cinder blocks or similar. These actions keep the wood and water separate to prolong durability.
posted by unearthed at 1:11 PM on April 18, 2020

Response by poster: Thanks. I appreciate the information so far. To add to the discussion, I also just found this helpful article on what chemicals leach from lumber, including some of the newer approaches, but I can't tell if any of that applies to the random "untreated" redwood 2x6s you'd find at Home Depot. It seems focused on treated lumber. It does have this scary sentence: "Another 2014 study found that exposure to copper that was released from the surface of [copper azole] treated wood could result in an acute toxic exposure for children less than 8 years old IF the child ingested most of the copper residue on her/his hands and feet." (Eight!)

This idea of using galvanized metal is intriguing. A quick Google leads to the question of, does it leach zinc? (I'm also intimidated by cutting it, but maybe it isn't that hard?)
posted by slidell at 1:37 PM on April 18, 2020 [2 favorites]

All my wooden beds are untreated pine or cedar. The do weather- but they won't leach. I also have a galvanized metal bed that came pre-cut so I could just fasten it together. I'd get a kit- and the advantage of the kits is the wooden kits are all untreated.
posted by Homo neanderthalensis at 1:56 PM on April 18, 2020 [2 favorites]

You could go with cedar, you may also think about lining the planter/raised bed walls with plastic sheeting, though the kind of plastic is important and you may not be comfortable with any plastic whatsoever.
posted by Alvy Ampersand at 2:07 PM on April 18, 2020

The issue with galvanization is that the zinc layer is thin and once it chips, the steel rusts. You wouldn't want to cut it for this reason.

If you have enough space you could use straw bales as borders. The straw will break down, but you can just compost / mound it up and put in a fresh set.

ETA: probably annually or maybe every two years at a stretch you'd want to replace straw bales.
posted by momus_window at 2:13 PM on April 18, 2020 [3 favorites]

I bought old untreated redwood fence slats and posts to build my raised beds. They were already weathered, but redwood can withstand a lot. I've had these beds for 2 years and they are fine.

I would be more worried about splinters than chemicals with this solution if a toddler wanted to chomp on it tho.
posted by ananci at 2:54 PM on April 18, 2020 [4 favorites]

How high do you need the beds to be? If it’s just for better drainage, not gardening while seated, you don’t have to edge the beds at all. Slope the edges a little below the angle of repose, firm them up with a long board or the back of a shovel, and mulch them thick with organics or rocks or cardboard, depending on what and where you’re growing. Old fashioned, thrifty, nothing to leach.
posted by clew at 4:05 PM on April 18, 2020 [3 favorites]

Lots of people use straw bales as raised beds, just enriching them a bit in the middle, and the soil underneath gets sooooooo delicious.
posted by clew at 4:07 PM on April 18, 2020 [2 favorites]

For what it's worth, I think a lot of redwood's reputation as being durable, anti-microbial, good in contact with the soil, etc., is based on old growth wood. Second growth or plantation farmed redwood is maybe slightly better than comparable white lumber but it seems to be pretty marginal. I recently completed a project using old growth redwood and it is basically an entirely different wood. Avoid the treated options and so on, but I would say don't feel compelled to pay more for redwood on the basis of durability, at least. It certainly tends to look better and weathers to a nicer surface but I'm not really sure anyone cares for raised beds.
posted by feloniousmonk at 5:11 PM on April 18, 2020 [1 favorite]

If you DO use straw bales, watch out for two things:
- Make sure the material is not treated with any herbicides; we tried this years ago and nothing grew - we found out later that the straw was likely treated with herbicides. Untreated straw in WA is quite rare.
- Be sure you get straw, not hay - hay will have weed seeds and lead to lots of No Fun.
posted by dbmcd at 5:53 PM on April 18, 2020 [2 favorites]