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Thread: Unexpected source of copper poisoning

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    Lucky Greenhorn Lil Stinkpot's Avatar
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    Unexpected source of copper poisoning

    Over the past couple years I've had a bit of a mystery, involving a glazed ceramic pot. It has a pretty, dark, bronzy glaze; it's one of my favorite pots, actually.

    Two years ago I had a large clump of Sarracenia leucophylla in the pot. It's un-drained, but that just means extra attention with watering. But then, the next spring my leucs spontaneously died. It was very fast, within one day they wilted; within a week they were dead. I was stumped! I thought it was some kind of funky rot caused by keeping them in un-drained pots, so all the Sarrs got moved to regular pots.

    Skip ahead to last summer, nearly fall, when I had had enough of reading about others' un-drained pots succeeding, without me having my own, so I put my big pots back into use. I know they work; the pots don't kill the plants. My problem must have been overwatering, bad soil, or...or... something. Four pots were dug out: two plastic-lined aluminum, one clear-glazed terra cotta, and the bronze-glazed. During winter I changed the soil in all my Sarr pots, even the bog. This is important, because it means that what happened next was not caused by rotten soil.

    Skip once again to two days ago, late spring, when all the plants are springing forth with amazing vigor. That old mystery has shown up again, with my three S. "Judith hindle" spontaneously wilting. I thought, and thought, and thought... I'd seen this before, and never solved it. It only ever happened in the bronze pot. What about THIS pot was killing plants?

    I talked it out on the chat here, and voiced an idea that I was kicking around. It sounded ridiculous, but.... What if it's something in the glaze? I was thinking about leaded glaze poisoning people, and thought that maybe that pretty color was made with copper, a poison for plants. We agreed, us on the chat that night, that it was almost certainly coppered glaze that killed my plants.

    I've been thinking about it, and I wonder if the situation might have been exacerbated by the acidic peat soil. I think of the time when people thought tomatoes were poison, becaust they were eating them off of lead plates, and the acid was dissolving the lead. Who knows? All I know is that I'm not going to use that pot again for my precious bog plants.

    I may make an experiment of it later, by drilling out a drainage hole, and putting regular flowers in it, to see just how toxic the glaze is. I know it won't kill everything at a touch like Roundup does; it's often used as a fungicide on fruit trees. So I may yet be able to use this thing.



    And the "Judith hindle"? They are recovering quite nicely in a broad dish-garden type pot with fresh soil, sitting in a water tray. Whatever tissues wilted and wrinkled have since dried up and got crumpled off (still green, not brown!), and whatever was only wilted is slowly re-inflating. They will live! Unfortunately, the only flower was the first to go crispy. I was hoping to cross her with Dainas!
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    caesium's Avatar
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    I've worked with ceramics and glazes a lot in the past, and copper is a very common component of glaze, so avoiding it entirely can be somewhat difficult. That said, there's also the chance that there is no copper in the glaze. In general, copper is used in turquoise/greenish blue, red, and some metallic glazes. Without testing, I don't think you can know for sure that copper poisoning is the problem. Is the pot glazed on the inside as well? The bronzy finish can just as easily be done with iron oxide, which shouldn't damage plants. In general though, black, brown, white, yellow, or dark blue glazes don't contain copper (some contain other heavy metals like chromium or cobalt though). I know that some ceramicists recommend not using copper glazes on the interior of bowls if they are going to come in contact with acidic food since copper can leach into the food.
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    As with the above statement, if there where a substantial amount of copper leeching, you'd more than likely see some eveidence of this in copper oxidation spots within the soil or on the inside of the pot. As well, yes, copper can be harmful in high doses, but the amount that you might expect to leech or accumulate would serve more to dwarf plants, and only minimally at that. This is one of the reasons that many bonsai growers prefer copper wire to other wire that might even be easier to work with. But, it's mostly a myth with bonsai that copper contributes to dwarfing anyway. Last point... if you happen upon a wheat penny (old, higher copper content than today's pennies) you might want to try an experiment. Split an extra plant (same DNA). Place one half in the "offending pot" and another in a plastic pot, but place the wheat penny in the soil, making sure to use the same soil, same volume, and same basic shape and dimensions in each pot. This will leech far more metal into the soil, potentially, than any glaze would. My bet is that the copper penny pot will be just fine. It's also my guess that it's the fact that it's sealed. How deep is this pot? More than 2" and you're looking and some deep anoxic zones. You also have little to no movement of water and air through the soil. My guess is that it's the stagnation that is a problem, not the pot. What are you using in the base of the pot to allow the soil to drain? Now draining may not be an issue when you're in a huge pot, like 12" deep or so, but it definitely will be in shorter pots. This isn't all just from a CP perspective, this is really across the board. I've walking Iris (Neomarica, a tropical Iris) that are potted in sealed pots and they either have to be in tall pots, or pots with drainage filler. I've no idea on the recommended care for thses plants, but I've treated them as bog Irises and they've tripled in size every year, blooming every February, in a north window no less. Anyway, my guess is sealed system versus copper.

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    Lucky Greenhorn Lil Stinkpot's Avatar
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    Good point, it might be stagnation, but WHY just this one pot? I have two others that are just as deep, and nothing bad has yet to happen.


    I'd be interested to see the copper penny experiment. Your plants first.
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    LOL, yeah, but I'm jus' a startin' and've nary a plant to spare.

    Why one pot... there could be a number of reasons. Even slight porosity through the pot wall might allow for some exchange in other pots. As well, porosity can work the other way. Minute fungal or other spores/organisms/cysts could potentially continue to exist in minute fissures in pots for decades. A famous case would be Uromastyx sp. lizards infected by Mycobacterium marinum. HOW? Well the tank previously held marine organisms. And, even though the tank (glass, mind you, very low porosity there) was scrubbed and held over in a grage for years (I'm pretty sure it was 2 years, but maybe 3), the bacteria persisted. You should be able to actually dig up that story first hand, even still, on the internet, though I haven't tried in years. Anyway, it could certainly be something like that. I haven't seen much talk in the CP community about pot sterilization when reusing pots, but it is a decent practice. Admittedly I don't do this myself, but would if I'd a question about a $50 bonsai pot, for sure. A 4:1 water:bleach solution should work just fine. Just remember to dechlorinate and rinse well afterward. This is a little more difficult with clay, but should be no problem in a fully glazed pot. Worth a shot, no? The last thing I would offer is that two (or even twenty) instances of correlation do not prove causation. While definitely worthy of a second look, it's not necessarily the pot (or anything related to it) and may, indeed, be simple coincidence.

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    Lucky Greenhorn Lil Stinkpot's Avatar
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    It may be easier to not use the pot again.
    If you shake a rain stick, you get rain. I need a hamata stick.
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    Yeah, my thoughts too. If it wasn't a high quality bonsai pot, likely it's cheaper in the long run to toss the pot than to kill $20, $30, $40 in Sarrs.

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