Mold & pebble trays

hj89November 15, 2010


I'm wondering if anyone can help me solve a mold issue that I'm having that I think is related to pebble trays.

I have three different plants on pebble trays (a goldfish plant, begonia, and prayer plant) that have started growing white, fuzzy mold on the outside of their terra cotta pots. It's definitely mold, opposed to a mineral buildup. The reason I think that this is related to the pebble trays is because all of my other plants are a)not on pebble trays, and b)not moldy.

So the question is: is the mold due to too much humidity, perhaps too much water actually touching the bottoms of the pots, or is it due to something that is being leached up from the actual pebbles (which I believe are granite chips, though not positive)? I tried placing the pots in saucers on top of the pebbles, which did not help.

I've never had a mold problem on the outside of a pot before, so any insight would be helpful.

Thank you!

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pirate_girl(Zone7 NYC)

I use a lot of pebble trays & have no such problem.

I do use pebble trays under plastic pots, tend not to under clay pots, that's a bit confusing.

If the plant needs a pebble tray, I'd personally pot it in plastic not clay. Clay (unglazed terra cotta) will absorb water through its porous clay walls, not the best idea for a pebble tray.

Also, 2 out of 3 of those plants (at least) want at least medium if not high humidity, so I'd grow them in plastic, not clay, plastic retains the moisure a bit better.

    Bookmark   November 15, 2010 at 5:01PM
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tapla (mid-MI z5b-6a)

Most importantly, when you're using a pebble tray, the pot (regardless of the material it is made from) should be isolated/separated from the water in the pebble tray. Terra cotta is permeable and acts as a wick. If you set a dry terra cotta pot in a bowl of water, you can almost see the water level fall as it is absorbed into the pot. Dissolved nutrients pass through the pot, or are wicked upward from the pebble tray, which also probably contains nutrient-rich effluent that has passed through the soil. The result is a nutrient-rich moist exterior pot surface that mold and other fungi find a hospitable place to grow. The fact is though, that it really doesn't hurt the plant.

Plastic retains water better, but in most cases, that is not a good thing. Also, if you think about it, terra cotta allows gas/vapor exchange, and thus evaporation through the container walls. This actually RAISES humidity in the immediate area of the plant, so for raising humidity around the plant, terra cotta is a superior choice.

From the plant's perspective, or when the goal is optimizing growth and vitality, terra cotta will in almost all cases be a superior choice to plastic, ceramic, or other nonpermeable materials. It's only when you start factoring in aesthetics, grower convenience, expense ..... as opposed to growth and vitality, that other materials gain a foothold.


    Bookmark   November 15, 2010 at 9:44PM
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Thank you for the replies.

It's true -- I've noticed that the plants themselves seem totally unaffected.

So I should just make sure that the pots are raised enough so that the water does not touch them. I suppose I need to invest in some more pebbles or perhaps some eggcrate.

How about an effective way to get rid of the mold? I'm hoping that I don't need to repot the plants entirely. Would wiping the sides down with vinegar work?

Thanks -- you are all truly a wealth of knowledge.

    Bookmark   November 16, 2010 at 8:36AM
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tapla (mid-MI z5b-6a)

You can set the pot on some sort of material that will elevate it at least slightly above the water in the tray - soda bottle caps, e.g.

I'm thinking that I would moisten the outside of the pot(s) and sprinkle with an anti-fungal preparation you might use as a remedy for athletes foot. Vinegar would be effective for the short term because it would lower pH dramatically and kill the fungi/mold. I think the effects would probably be short-lived, as the base deposits on the outside of the pot quickly neutralize the vinegar's acidity.


    Bookmark   November 16, 2010 at 9:53AM
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