Sealing Concrete Raised Vegetable Beds

dogpeach(Arizona 9-10)July 3, 2014

I bought a house last year that has 5 large raised gardening beds made of concrete block. The exteriors are painted. Living in Arizona, there is irrigation to each bed. The water leeching out of the beds causes the paint to flake and peel. We have been doing an extensive remodel on the house and last November I hired a company to come out, remove the soil, coat the insides of the beds with a waterproof coating, refill the beds with half of the previous soil and bagged organic soil I purchased at a nursery. They left the bottoms uncoated to allow for drainage. The beds are sitting on top of soil. My daughter became very ill at her University. I left town for 5 days to take care of her and when returned, the work had been done. The company left their containers of sealant near my garbage bins and I was alarmed when I found them. Roofing tar. I immediately researched the ingredients, reading MSDS sheets and as much info as I could find online. There were 3 ingredients, only 2 were okay, one was not and was actually banned for use in CA as cancer toxic. I called the company, had them come out and went over it in detail, just as I did when I hired them. They said they always use this item when waterproofing beds. I gave them copies of the MSDS I pulled and asked them if they would eat vegetables grown in planters where this was used. They felt that if the tar was dry, it couldn't leech into the soil and eventually into the plant roots. I don't care. I got my money back and now am revisiting this again. Arizona's climate is very harsh in the summer and plastics become very brittle in the heat and would not due to line these beds. I am hoping someone can suggest something I can use to recoat over the tar to make my beds safe.

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zeuspaul(9b SoCal)

It may be difficult finding a coating that will hold up over tar.

Perhaps you could attach cement board using metal fasteners and thinset. Then line the bed with ceramic or porcelain tile.

    Bookmark   July 6, 2014 at 8:47PM
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calistoga_al ca 15 usda 9

It sounds like your 'raised beds' have a concrete bottom, and are not open to your garden soil. If that is so, they are not raised beds, but are concrete containers. Why are you so sure that tar will leach into your plants vascular system and make any fruit produced, inedible? It may be difficult to find a contractor with similar thoughts, but you will need to make this clear, before hiring him. Al

    Bookmark   July 7, 2014 at 10:05AM
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dogpeach(Arizona 9-10)

Thanks zeuspaul. Never thought of a hard surface to reseal. I wonder if I could skip the cement board? I have laid a ton of tile before and thin set and tile should adhere fine. But the cost may be to prohibitive. Something to think about.

    Bookmark   July 10, 2014 at 1:07AM
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dogpeach(Arizona 9-10)

Hi calistoga. Thanks. My raised beds are definitely on soil. We asked the previous owners (who built them) and we double checked by digging into several of them several feet below the lowest block. I am just afraid that when the tar is wet, along with our Arizona heat it might break down over time. Just don't want to take that risk. I am having a hard time finding anyone who knows what to do. I contacted several local gardening clubs, botanical gardens and a local gardening magazine for info and referrals on who to do this. Nothing yet - keeping my fingers crossed!

    Bookmark   July 10, 2014 at 1:12AM
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zeuspaul(9b SoCal)

You wouldn't need cement board if the surface is somewhat even and if the thinset sticks. I would spot grind the surface in a few places to help adhesion if there is no exposed concrete.

You can get tile for a buck or less per sq ft. With thinset and grout you may be at about $1.50 per sq ft if you do it yourself.

There is a good chance you would be fine doing nothing but you would not have peace of mind. My wife would not eat anything grown in tar but she worries about everything.

If thinset sticks you could probably skip the tile and just stucco. I use Rapid Set stucco mix available from Home Depot sometimes as a sealing coat. It is not a portland cement based product. It has almost no shrinkage, a short work time and is very waterproof (resistant). I doubt it would react with the tar coat as most sealing paints would.


    Bookmark   July 10, 2014 at 2:53AM
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Aside from "I fear it" ... have you any hard data on the effects of tar as a waterproofer? It's been used since biblical times (the "pitch" referred to is usually bitumen from natural tar seeps)

Tiling will be a waste of money, because the tar is a shear layer. Your tiles will fall off.

Sheet metal roofing cut to fit inside the planters will work, until it rusts out and exposes the tar layer. Buried in a well-watered planter it will last a couple of years.

    Bookmark   July 10, 2014 at 3:19AM
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