I have a mildew crisis on indoor terra cotta pots!

mpursellOctober 17, 2007

I am newly moved to the Pacific Northwest with a load of various houseplants grown in my native Minnesota. Upon watering my plants (some new and acquired here) two weeks ago, for the first time I noticed a uniform white film of mildew on the exterior of all the terra cotta pots.

I have never had any such problems before, either with mildew of the soil or foliage. The plants remain perfectly healthy, but even after wiping the mold off the pots and allowing them to dry out, the mold is beginning to regrow.

Has anyone else experienced similar problems with mold/mildew on the exterior of terra cotta pots? Could it be a question of air humidity or overwatering, or is it something more sinister?

Any suggested remedies?

Thanks for your suggestions!

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jean001(z8aPortland, OR)

Your "uniform white film of mildew on the exterior of all the terra cotta pots" *isn't* mildew.

Instead, it's accumulated soluble "salts" from the potting mix, the water, and any fertilizers you have added.

If you don't like how it looks, the remedy is to scrub the exterior of the pot.

And yes, it will again accumulate -- not grow. All because it comes from the potting mix, water, and fertilizer. Not sinister at all. Not to worry.

    Bookmark   October 17, 2007 at 2:45PM
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mareas(OR Zone 8)

But if it was mildew, I would treat it with a toothbrush & hydrogen peroxide and, if possible, a sunbath.

    Bookmark   October 17, 2007 at 4:11PM
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jean001(z8aPortland, OR)

But it's *not* mildew. Mildew needs a live plant, not a dead pot.

    Bookmark   October 17, 2007 at 8:45PM
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reg_pnw7(WA 7, sunset 4)

Salt. Not mildew.

Now it's possible that lichens would grow on a clay pot outdoors I guess, and some could look like a white film, like the Corkia that grows on the alder trunks, or the British Soldiers that grows on rocks, which is a greenish white film. But not indoors, and not in two weeks.

Salt accumulation on clay pots is common and not particular to the PNW - you never saw that before in MN??

I never do anything about the salt on the pots.

    Bookmark   October 19, 2007 at 7:25PM
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Just wanted to let you know you are not the only one with this problem. I moved out here to OR from WI and am having the same problem with my terra cotta pots. It is definitely not salt buildup. It looks like the mold that sometimes grows on the surface of the potting soil. I wonder if since clay pots are porous the mold spores that sometimes form this white mold leeches through the clay pots every time the plant is watered and grows on the damp outer surface of the pot. The good news is that it hasnÂt spread to anything else and my plants as well as their roots are mold free and very healthy. If you really want to stop the mold from growing a sulfur based fungicide will kill mold but it can take several applications and the pots become so coated in the stuff they look really awful. (Believe me I tried this on some of the pots and ended up having to repot the plants it looked so bad.) I decide that if the mold problem gets worse I will use a very watered down bleach solution on the outside of the pots and see what happens.


    Bookmark   November 13, 2007 at 12:33AM
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buyorsell888(Zone 8 Portland OR)

My advice would be to reduce watering. Even though we are warmer here in winter than Minnesota we aren't as sunny and houseplants need less water.

They need a sunnier window too. If they grew in a north window in Minnesota, they need east here. Almost nothing can survive a north window here.

I always had a crust of minerals on my clay pots in Phoenix AZ but never here. Most water supplies here aren't high ph or heavy with minerals and salts. There are exceptions but soft water is the norm here.

    Bookmark   November 13, 2007 at 11:26AM
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I live in New York but when my indoor terra cotta pots get like that, I spray the outside with a white vinegar and water solution (maybe 50:50 give or take - I don't measure but keep it in a spray bottle). Then wipe. I had a problem with a few pots where it literally smelled like vomit. I sprayed the vinegar, let it sit but not dry, then wiped it away.

    Bookmark   September 20, 2011 at 9:53AM
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