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HDPE liner for raised bed idea  RSS feed

 
Mike Arr
Posts: 19
Location: Southeast PA, zone 6b
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I just built this spiffy raised bed for a small kitchen garden. I am toying with other more permie planting areas elsewhere, but this is right next to our deck and house so must look nice to my wife. I used non-treated dimensional pine so I could match the lines and angles of our deck. I have now spent many hours researching what I can do to place a barrier between the wood and soil so that:
   1. I can stain/paint it to match the deck (it may be toxic gic); and,
   2. I don't have to rebuild it in 2-3 years

Apparently, every potential solution will poison you or make it rot faster, so most people just plan to rebuild it ever couple of years. I was considering sheets of food grade plastic, but apparently that just holds moisture closer to the wood and accelerates rotting. This reminded me of a product I used years ago called Delta-FL. It is a dimpled plastic subfloor that creates an air-tight, 5/16" (8mm) vapor barrier between your finished floor and basement concrete. The air gap allows the vapor to equalize without condensing. Would this not also work between the soil and boards of the raised bed, allowing the wood exterior to be painted and extend its life?

Here is the brochure: https://www.spycor.com/v/vspfiles/templates/1/assets/DELTA-FL.pdf
Here is the MSDS: https://www.spycor.com/v/vspfiles/templates/1/assets/MSDS-DELTA-FL.pdf

The stuff worked great for my basement project and I still have a half roll of it left that I have been tripping over it for 5 years. It's strong. The MSDS tells me it's made of HDPE, which I believe is one of the more stable plastics used for food grade containers. I understand that HDPE is recommended over PVC for aquaponics applications because it does not leach.

This seems like a slam dunk to me. What might I be missing? Maybe not all HDPE is created equal? Maybe there are additional pigments or something that may not be good for you? The brochure says it's "non-toxic, non-polluting", but does not call it "food grade" for obvious reasons. They now have it at the big box stores but you can get it cheaper directly from the company at $0.60/sq ft.

Many thanks, Mike
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James Freyr
Posts: 214
Location: Middle Tennessee
11
books cat chicken food preservation toxin-ectomy
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Nice raised beds man! Those look good. Have you considered linseed oil to slow the decay of the wood? Certain boiled linseed oils can contain metals that may leech into your soil, but there is a brand called Allback that has an organic linseed oil for treating wood. That or tung oil may be an option. I believe your thoughts on plastic sheeting holding moisture to be right and that would accelerate the rot. If when they do start to rot (all woods will rot eventually of course) and you choose to rebuild them, perhaps consider a wood that is slow to rot like cedar or cypress. Yes they will cost a lot more than standard pine. I garden in raised beds built from western red cedar, with the oldest being 7 years old now and they appear to be holding up well so far, and I did not treat them with anything. Good luck!
 
Mike Arr
Posts: 19
Location: Southeast PA, zone 6b
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Thanks James. I did see that recommended as a solution, but now that you mentioned it I am looking into it further. There is a brand on Amazon called Sunnyside Pure Raw Linseed Oil that is a bit cheaper. https://www.amazon.com/SUNNYSIDE-CORPORATION-873G1-1-Gallon-Linseed/dp/B000BZYYJC

I still plan to use the Delta-FL dimpled membrane, but if I can find the organic raw linseed oil locally, I think I'll try to put a coat on the interior. The exterior will be stained/painted to match the deck.

Using cedar or thicker pine is the long game, but I am not going to get that done this year. I thought about just doubling cost/thickness with another layer of 2x10s, but that would be a lot of cuts and screws, and I think the water would still work its way in between the boards and cause the same problems as having a flat membrane tight against them.
 
James Freyr
Posts: 214
Location: Middle Tennessee
11
books cat chicken food preservation toxin-ectomy
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I think between the linseed oil on the interior and a coat of paint on the exterior you will get many years of good use out of those beds.
 
Mike Arr
Posts: 19
Location: Southeast PA, zone 6b
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I was able to find raw linseed oil locally, so now have a much shinier bed. It's thick, but lays on nicely with a 4" brush. I think most people cut the first coat with turpentine, but I put it on straight.  It smells like high school art class - not at all like the chemical stains I am used to. The brand (http://www.rockmiracle.com/productdetail.asp?cid=&pid=53) does not claim to be organic, but the only ingredient is flax seed oil. I'm not too worried since I still plan to install the Delta-FL HDPE dimpled membrane between the boards and soil. I hope that this + drainage rocks under the boards + linseed oil coating = a few extra years before rebuilding.

I used about 1/3 of the gallon which was around $36. I think it will be $12 well spent.
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James Freyr
Posts: 214
Location: Middle Tennessee
11
books cat chicken food preservation toxin-ectomy
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That looks nice man. I believe you've definitely added some years to the life expectancy of your beds.
 
Eric Hanson
Posts: 34
Location: Southern Illinois
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Mike,

Those raised beds look absolutely beautiful.  The only thing I don't like about them is I did not build them myself (I am slightly jealous).  Linseed oil will definitely work--for a while, but eventually that too will break down.  As an alternative, have you considered spray or paint on truck bed liner?  You would definitely want this to dry thoroughly before planting, but you can get truckliner paint that will practically armor the inside of your bed, and maybe even paint the tip edge for extra waterproofing.  Once the bedliner is dried, it will be impervious to just about anything you throw at it.  I would definitely leave the outside as oiled wood as it is absolutely beautiful.  Good job!

Eric
 
Kyle Neath
Posts: 47
Location: High Sierras, CA 6400'
10
dog hugelkultur trees woodworking
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Those look great! If you are not concerned with all plastics with regard to food production, HDPE will be a great choice. I always find it difficult to give advice on stuff like this as your level of comfort with petroleum / corporations / icky-stuffs always comes into play. We didn't know BPA was bad until we did, and there's always a chance that modern plastics have a similar unknown problem lurking in time. But if you're comfortable using food grade plastic containers, HDPE will do fine.

That being said, from your goals I think wood with a simple finish like you have would also work fine for a very long time. A raised bed is a lot different than a fence post or structural member, and that context is pretty important.

1. Your wood will have 5/6 surfaces exposed to air and 1/6 surface exposed to water. This is way different than something like a fence post where the ratios are reversed. Wood breathes a tremendous amount and this will slow wood rot considerably.

2. Raised beds usually have great drainage, which means the wood won't sit waterlogged.

3. Since it's non-structural, repairs are fairly trivial.

I think people tend to underestimate the lifetime of wood. 2-3 years seems extremely conservative, even for untreated pine. Untreated cedar or redwood that's exposed to air in a non-tropical environment can last for decades.
 
Mike Arr
Posts: 19
Location: Southeast PA, zone 6b
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Quick update for those that could find this useful. The raw linseed oil coating was an interesting/helpful detour, but my main focus is the HDPE liner. Nobody screamed don’t do it, so I did it. I suspect the abundance of plastic in the attached photo may trigger the gag reflex of some higher eco-level permies, and I sincerely apologize for that. Eric - I'm not sure about the truck liner stuff - no idea what's in it which scares me. Kyle - I take your points, particularly regarding toxicities that are currently unknown (or being hidden). I am probably over thinking it and over doing it. I build it in about a day and then say to myself that I don't want to spend another day fixing it in 5 years, so I proceed to spend 2 more days now researching, adding linseed oil, adding a liner, etc. Yes, it makes no sense and is a personal defect, but it's now a sunk cost and at least some growies are going in this weekend! It will be interesting to see how it holds up.

As to the process, the liner went on quickly. It cuts easily with tin snips, and the dimples make it easy keep things lined up and square without too much measuring. I fastened it to the boards with screws 1-2 inches from the top. There are no screws and no holes below that, so no way for water to get to the boards unless it comes from the bottom by way of capillary action, which I assume will happen to some degree.  Assuming you are cool with HDPE, I think this liner would work well for any rectangular-ish (or cylindrical-ish) structure where you want to keep water away from untreated boards. I had it laying around, but you would spend around $30-40 to add this to a 12x4’ garden bed, depending on the size roll you bought and waste.

Today I made two more additions that I’m not quite certain about: 1. Waterproof tape at the top to seal the screws and keep water/soil from working behind the liner; 2. Landscape fabric for a little physical absorption barrier between the liner/tape and soil/food/garden tools. On 1 – part of me thinks I should let that open to allow air to circulate. On 2 – the landscape fabric has an oily sheen to it that looks like toxic gick - have most permies abandoned this stuff? Both additions are easily reversible, so any thoughts on these two would be welcome.

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