Gardening Plant diseases Sowing Seeds

When White Mold Appears in Seedling Tray

White mold in the soil of your seedling tray can be a cause of panic, but is actually harmless and even quite normal.

By Larry Hodgson

Question: I was horrified to discover white mold growing in the soil of two trays of seedlings. Am I going to lose them? Should I destroy them to prevent the mold from spreading to my other trays? This is the first time I’ve tried sowing seeds and I’m so disappointed!

Linda Asturias

Answer: I hope you haven’t done anything drastic, as white mold in seedling trays is just a minor problem and so easily fixed.

What Is It?

This kind of mold is a type of filamentous fungus. Such fungi are ever-present in our environment. Their spores float around in the air both indoors and out and germinate when they find a substrate that suits them. And spores may be present in the potting soil you buy. Or on the seeds you sow. Or carried on a potting tool. Or your fingers. There are certainly some in any garden soil you brought indoors. (But don’t use garden soil for seed sowing!) They’re pretty much everywhere. And when they find an environment they like, they start to develop.

This mold is a type of saprophytic fungus. It lives by decomposing particles of organic matter (wood, peat, leaf mold, coconut fiber, paper, etc.) found in the potting mix. It isn’t pathogenic or directly harmful your plants . . . nor is it harmful to people or pets. In fact, it actually helps your seedlings to a certain degree. That’s because, as it decomposes the organic matter, it frees up minerals your young plants can use for their growth.

Getting Rid of White Mold

OK, so white mold in seedling soil is normal and even beneficial to a certain degree, but you still probably don’t want it. For esthetic reasons, partly. However, some types form a network of mycelium so dense that it impedes air and water circulation. You won’t want that. So, it’s best to remove it.

Scraping soil around seedling in pot.
Scrape off the mold and top layer of soil and replace with fresh soil. Photo: shinylion, depositphotos

Simply take a small tool—fork, mini-rake, etc.—and carefully scrape off the upper surface of sowing mix. You can put the mix of soil and mold into your compost bin or even directly into the garden. Then replace with fresh soil.

Or if you were going to transplant your seedlings to their own little pots anyway, simply remove the mold as you transplant them.

Mold usually doesn’t grow back, but if it does, repeat the above.

Preventing White Mold

My real concern is not so much the mold, but the fact that it usually develops under overly damp conditions with little air movement. Also, conditions favorable to white mold are also suitable to damping off disease. And you certainly don’t want that!

So, you might want to water more carefully in the future, only when the top layer of sowing mix has dried out. Certainly, don’t let the seedlings soak in soggy soil.

Also, once your seedlings have germinated, expose them to moving air. For example, remove the plastic dome that covers them as soon as germination occurs.

Or you might want to turn on a small fan in the same room. Don’t direct it at the seedlings, unless from across the room. That could dry them out! Just make sure the air is “buoyant.”

You could also sow your seeds in an inert material like perlite or vermiculite. They offer nothing of interest to fungi spores! But that means you’ll necessarily have to transplant your seedlings to a more organic mix (peat or coco fiber based soils, for example) as the seedlings develop, and that can mean an extra step.

Another possibility: apply ground sphagnum moss or powdered cinnamon over the mix as you finish sowing. Both have antifungal properties. As does chamomile tea, which you could water into the growing mix after you sow.

Mold on Pots

White mold on peat pots.
When you find mold on biodegradable pots, just ignore it. Photo: Coraldaisycreates,

Sometimes you find such molds—and not just white ones: a whole smorgasbord of colors—forming outside biodegradable pots, such as those made of pressed peat, coco fiber, cow pats or newspaper.

In such cases, it really isn’t worth doing anything at all. Just ignore the presence of the mold, seeing it as a positive sign. That of Mother Nature at work breaking down the fibers into usable minerals!  

Finding white mold in your seedlings can be quite a shock the first time you deal with it. However, most gardeners quickly come to see it as nothing particularly worrisome.

Garden writer and blogger, author of 65 gardening books, lecturer and communicator, the Laidback Gardener, Larry Hodgson, passed away in October 2022. Known for his great generosity, his thoroughness and his sense of humor, he reached several generations of amateur and professional gardeners over his 40-year career. Thanks to his son, Mathieu Hodgson, and a team of contributors, will continue its mission of demystifying gardening and making it more accessible to all.

4 comments on “When White Mold Appears in Seedling Tray

  1. Lucy Ellinson

    Very helpful thankyou!

  2. Laurence

    Thank you! This was really helpful!

  3. Good to know. Have never experienced mold on the surface but I always water seedlings from below so the top surface of the soil never gets super wet.

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