Thursday, November 29, 2012

Raised bed remodel

I’ve known that the day would come when the landscape timbers that make the raised beds would need some attention.  The first two beds were created in 2008.  I built three more beds in 2009, then two more in 2010.  All of them were built by stacking three rows of landscape timbers and nailing them together with pole barn nails.  Early in the week I decided to have a look at the two oldest beds. 
I knew that the bottom row of timbers would have some rot.  My hope was that I could just lift the beds straight out of the soil and flip them over for a few more years of service.  They were in worse shape than I expected.  I ended up removing all the timbers.  I hope that some of the garlic that I planted in the front bed survives this.  Most of the foliage in the picture is green manure mix that I tried this year. 

The problem with these landscape timbers is they are AC2 treated, so I can’t let them just rot in the ground, they have to come out.  When I built the beds I looked for untreated landscape timbers and could not find any.  If the landscape timbers had no treatment I would just leave them rot into the ground and add another row on top when the bed sunk in enough.  At the time I was working and I wanted a quick and fast way to build raised beds.  Landscape timbers fit the bill. 

What is AC2?  The treatment has two components: copper oxide and quaternary ammonium chlorides, or quats, in about a 2:1 ratio.  The copper is ground into very fine particles so it can penetrate the wood fibers, and it acts as a fungicide.  The quats have antimicrobial properties, often used in restaurants as disinfectants.  Quats are similar to detergents in that they have a “greasy” ion, the quaternary ammonium piece, but with a positive charge (there’s more info on wikipedia if you are inclined).   Neither copper or quats are particularly toxic to people in low doses.  And AC2 is absolutely better than the old treatment of chromated copper arsenic.   

Before I built the beds I did some research on AC2.  There were some leaching studies that found that some copper leached from the surface of the wood after it first made contact with the ground.  After a few days the leaching stopped and treatment below the surface of the wood did not migrate out.  I found another study of plant uptake of copper.  Bottom line, there has to be a LOT of copper in the soil for the plant to accumulate enough copper to cause a concern.  The worst vegetable was bok choi, which can accumulate metals in the stem.  Fruiting vegetables did not accumulate nearly as much copper.  I can’t find that study but emphasize again that the amount of copper in the soil has to be exceedingly high to present a problem.  The quats are not taken up by the plant and will eventually decompose in soil.
Addendum:  I took a soil sample from the beds in 2009 and had it tested.  Copper tested at 1.4 ppm, well within normal limits.  The samples were taken from the edge and center of the beds and mixed into one sample.  I have to conclude that almost no leaching took place.

So how much copper can leach into the soil?  That depends on how much copper is in the wood.  Pressure treated wood rated for ground contact must have a retention of 0.4 pounds of treatment per cubic foot of wood.  But the landscape timbers are not pressure treated, they are deemed “treated to refusal.”   Pressure treated wood is kiln dried then put into a vacuum chamber where the treatment solution is introduced.  This forces the treatment solution into the fibers of the wood. 

There’s not much info on wood “treated to refusal” but here’s my understanding.  The wood is air-dried then soaked in a treatment solution until no further treatment is taken up.  My guess is the companies soak the landscape timbers for a time period, say a day, then remove them (time is money after all).  Most likely the amount of treatment in the landscape timber is a small fraction of the amount in pressure treated timbers.

That brings up the inevitable question “Why bother?”  If there’s not enough treatment in the wood to keep it from rotting in the ground, but the wood still has to be removed from the ground before it rots to avoid a buildup of copper in the soil, what’s the point?  Sure the treatment may keep the timber from rotting the first year, but it will still decompose.  The fact that I can’t even find untreated landscape timbers and will now have to landfill the timbers that I removed is certainly cause for griping.

So the landscape timbers came out.  This weekend is going to be nice and I’m going to make a new bed edge with yellow pine 2x8’s, untreated of course.  I won’t bury them, I’m going to drive 2x4 stakes (these will be pressure treated) into each corner and screw the 2x8’s to the stakes so the bottom of the board is just above the outside ground.  When the boards are rotted beyond usefulness I can unscrew them from the stakes and toss what's left into the woods where they will rot away harmlessly.  

Next fall I’ll have to replace the timbers in the next set of beds.  Maybe I can find some osage orange or black locust wood for those beds.  These trees grow around here and have very good resistance to rot.   Right now I'm researching ways to expand the garden with containers into the area north of the beds.  I can't add any more beds near the tree because it's roots will find the bed and compete.  Removing the tree is not an option.  It's a hop hornbeam and a very fine little tree.

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