How do I prevent mildew on top of soil?

livvyjamsOctober 24, 2009

Hi there!

I hope someone can help me in this matter.

This summer, I brought my Ming Aralia, an indoor plant, outside. It leaned a lot but it seemed to tolerate it otherwise. Being outside, it needed more water. When I brought it back inside once fall arrived, a week later, it developed mildew just on the top of the soil.

So I repotted the Ming in a brand new pot, changed the soil, and watered it once more. Because it's inside, I placed it back on its normal watering schedule (about once per week or so). After a couple of weeks, mildew began to develop on the top of the soil again.

So I removed the mildewed soil, and didn't water the plant (this was a few days ago).

And now, mildew is developing again.

Can anyone tell me why, and what can I do to prevent this from happening?

The Ming is in the same spot indoors that it used to occupy before, and it was fine for years. I'm not sure what's happening.

Thanks in advance to those who can help :-)

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tapla (mid-MI z5b-6a)

Odds are, it's probably a result of your keeping the soil too wet - partially your fault & partially the fault of a water-retentive soil. You can lighten the hand on the watering can, or switch to a faster (draining) soil. You might also consider abandoning the use of any organic fertilizers you might be using. I doubt what you're seeing is actually 'mildew', btw.


    Bookmark   October 24, 2009 at 5:57PM
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tapla (mid-MI z5b-6a)

Oh - I forgot to mention - increasing air movement might be helpful for reducing the symptom you describe, but probably wouldn't help with the underlying cause.


    Bookmark   October 24, 2009 at 5:59PM
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Hi Al,

Thanks for the advice. If it's not mildew (and by all appearances, it is), what else could it be?

    Bookmark   October 24, 2009 at 6:04PM
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Mold.....from decomposing organic matter :-)

    Bookmark   October 24, 2009 at 8:17PM
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But where's the decomposing organic matter coming from? And how can I help my poor Ming? Do you think the Ming has a mold problem? Is the Ming decomposing?

It's weird, because the Ming is actually fine. It's not losing leaves or branches. Its colour is great.

It's such a puzzle...

    Bookmark   October 24, 2009 at 9:13PM
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livvyjams, might it be mycorrhizae?

    Bookmark   October 25, 2009 at 7:04AM
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Your potting soil is probably loaded with organic matter, it came that way. Do not water until the top of the soil is dry down to the first joint of your finger pushed into the soil. Most commercial potting soils are far too water retentive, to appeal to those who do not water regularly. Al

    Bookmark   October 25, 2009 at 9:50AM
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All commercial potting soils contain organic matter - some more than others - and it does breakdown and decompose over time. That's just how that stuff works. The 'mold' (probably more accurately referred to as mycelia) is just the visible portion of the fungal organisms at work. While it may be unsightly, it is not considered harmful in and of itself but does indicate some concerns about your potting mix. Too moisture retentive, as others have said. And you may be aggravating the problem by over-watering. Rather than adhering to a watering schedule, water only when the potting soil becomes dry (stick you finger in to check).

And I'd consider a better, less retentive, faster draining soil :-) Lots of threads on this forum will point you to a better mix.

    Bookmark   October 25, 2009 at 10:02AM
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Thanks for the tips everyone! I'm going to buy some better soil right now :-)

    Bookmark   October 25, 2009 at 11:50AM
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tapla (mid-MI z5b-6a)

Why not learn to make your own soil at half the price and probably of much superior quality? It's kind of a catch 22 situation. You almost need to have enough knowledge about how to make your own soil, to pick a good soil; so why not just make it? ..... and a good soil isn't the easiest thing to find, btw.

Take care.


    Bookmark   October 25, 2009 at 12:18PM
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No kidding! Most places that sold soil up till now are fresh out. A seasonal thing. But I really have to get some more soil. Turns out it's not just the Ming. Some other plants that were repotted using the same soil are suffering from the same thing. D'oh!

    Bookmark   October 25, 2009 at 1:14PM
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It's highly unlikely you'll find a pre-mixed, bagged potting medium that is inorganic and more porous. You almost HAVE to mix your own to get the desired results.

It's really quite easy, though, once you locate the ingredients. I mix small batches all the time for my potted indoor plants, and since I've begun to make my own, I've not experienced any fungal problems, at all.

My mix consists of small pine bark pieces, granite chips, and perlite, along with a couple of other small additions. My plants love it, and they all have healthy roots because of it... and healthy plants begin at the roots.

There's lots of fabulous information on mediums right here in the forum. Read the threads on Container Soils - Water Retention and Movement... everything you need to know about growing healthy potted plants is in there!

    Bookmark   October 26, 2009 at 8:53AM
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You wouldn't by chance happen to have "fungas knats", do you?

They go hand and hand with the conditions you described.

    Bookmark   October 26, 2009 at 1:36PM
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Meyermike: I think I do. I've had little black flies since bringing the plants in. Maybe that's part of the problem.

Since using new soil, I've eliminated the problem in most plants, but my vincas are now experiencing the problem. I honestly don't know what to do.

    Bookmark   November 6, 2009 at 5:35PM
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