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How to Get Rid of the Mold Growing in My Seedling Flat

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Mold in a seedling flat probably isn’t harming the seedlings, but it’s a sign that the growing conditions aren’t quite right. White or brownish molds that appear on potting media are saprophytic fungi, which live on organic matter. These molds don’t attack the living tissue of seedlings, but they can make it difficult for seeds to germinate. You can sometimes remove the mold by agitating the surface of the potting soil.

What Causes Mold in Seedling Flats?

Mold in a seedling flat is a sign that the potting soil is too wet. The seedlings may be growing healthily, but they’re at risk of developing a condition called damping-off, which causes their stems to collapse and the seedlings to die. Another cause of mold is placing the seedling flat in direct sunlight or an excessively warm spot. The mold forms a crust on potting soil that water doesn’t easily penetrate and that blocks germinating seeds.

How to Treat Mold in a Seedling Flat

Removing the mold from a seedling flat risks harming the seedlings, but you can discourage it from increasing. Allow the surface of the potting soil to dry before watering again and reduce the amount of water you give the seedlings so that the potting soil is damp, not soggy. If the seedling flat is in a place where air doesn’t circulate, move it to an airy spot. But don’t place the seedlings in direct sunlight or in an area that experiences high temperatures.

How to Prevent Mold From Growing on Potting Soil

Prevention is the best cure when it comes to mold on potting soil. Removing the mold once it’s appeared is tricky and there’s a risk of damaging the seedlings’ stems. To discourage the growth of mold, use a potting soil that doesn’t contain compost or manure. Equal parts vermiculite and peat moss create a mix that holds moisture evenly and doesn’t form a moldy crust in sunny conditions. Compost and manure supply developing seedlings with plant nutrients, however, so in their place add a liquid fertilizer that’s suitable for seedlings after the seedlings have developed two sets of true leaves. Dilute and apply the fertilizer according to the manufacturer’s directions.

How to Sterilize Seedling Flats

Sterilizing seedling flats before using them helps prevent the spread of mold spores and dangerous plant diseases. Add 1 cup of household bleach to 9 cups of water to create a 10 percent bleach solution. Soak the seedling flats in the solution for 30 minutes, rinse them in clean water and then allow them to air-dry before adding potting soil.

How to Get Rid of the Mold Growing in My Seedling Flat. Many gardeners worry that the appearance of fuzzy white mold on the soil of their seedling flats means certain death to the plants. While it is a signal to change their environment, the white mold itself does not pose a danger to your seedlings. The white mold …

How to Prevent White Fuzzy Mold on Seedlings

The Spruce/ Margot Cavin

Fuzzy white mold on seedlings is a visual warning that your plants are in danger. Fungi, like Rhizoctonia spp. and Fusarium spp., along with water mold Pythium spp. can severely damage your seedlings, causing damping off–a death sentence for your baby plants. Plants experiencing damping off may look like the seedling was “pinched” at the soil line, with the stem becoming water soaked and thin. The cotyledons (the first leaves to appear) turn soft, mushy, and may appear grayish brown. Young leaves wilt and turn brown. Roots are stunted or absent, and fluffy white cobweb-like growth may appear on the infected plant. Sadly, once seedlings are infected, they need to be destroyed, because it’s impossible to recover from damping off.

The good news? With some easy steps, you can avoid moldy seedlings and raise healthy plants.

Avoid Wet Soil

Fungus is a sign that your soil is too wet. Soil that is overly wet can cause the delicate roots of your seedlings to rot, which will eventually result in plant death. While it’s important that your seed starting mix does not dry out and damage the young roots of your seedlings, it’s equally important that it’s not soggy. Check the mix in the containers daily to determine watering needs. If the soil feels moist, no need to water. Make sure to use trays or containers with drainage holes in the bottom to avoid soggy soil.

Increase Airflow and Light

A second simple way to prevent mold from growing is to increase the airflow around your seedlings. Install a fan near your seedlings, and run it for at least a few hours a day. If your seedlings are growing in a covered tray, prop the top open or remove it for a bit to increase airflow. Not only does this help prohibit fungal growth, but the air movement encourages strong stems, resulting in sturdier seedlings.

Take a careful look at how much light the seedlings are getting. Newly emerged seedlings need between 12-16 hours of good, strong, indirect light per day to grow well. That also helps the water to dissipate and not sit stagnantly. Avoid direct sunlight, because covered trays will get too hot and may damage the seedlings.

Start with Clean Tools

Before you begin filling your trays or pots with seed starting mix, sterilize them with a diluted bleach solution to kill off any lurking pathogens. Use a solution of 10 percent bleach and 90 percent water, and scrub the trays, pots, and any tools like shovels or plant tags to create a clean, healthy home for your seedlings to reside. Rinse with clean water after scrubbing.

Manage Temperature

Few seedlings will flourish in chilly or hot surroundings. The best photosynthesis occurs when the temperature stays between 77 and 82 F. If you are using a heating mat under your seedlings, turn it down or off to avoid overheating your plants.

Thin or Repot Seedlings

Most gardeners plant several seeds in the same seedling pot or tray. If you’re lucky, all of the seeds will sprout. Once they start to really take root, though, they will start to crowd one another. Crowding reduces adequate airflow and can lead to fungus.   To avoid this problem, simply thin out your plants by pinching out a few seedlings from each of your pots. Don’t wait too long to repot your seedlings. Once they sport two sets of “real” leaves, it’s time to pot-up the seedlings into their new home. (The first leaves that appear on the seedling are the cotyledon, or “seed” leaves. Wait for two sets of actual leaves before transplanting.)

Watering

Most seedling mold is a result of watering too much. Don’t water your plants-to-be unless the seedlings need it. It is easy to get into a routine of just giving them a quick watering every day or so “just to be sure,” but this can sometimes do more harm than good. Check the soil’s moisture with your finger, and only water if the soil is dry. However, be careful not to allow the seedlings to completely dry out.  

Rethink how much water you add when you do water your seedlings. You may need to cut down on the amount of water you put on each seedling—another way to reduce white fuzzy mold for healthier seedlings. Make sure the water drains well, and don’t allow seedlings to stand in water.

Finally, if at all possible, consider a system that lets you water from the bottom of the seedling container. Add water to a solid bottom tray and allow the insert with the seedlings to soak up the moisture for an hour, then pour off the excess water. By avoiding water on the delicate stems and new leaves, you can help keep your plants healthy.

By following some easy tips, your seedlings should avoid the dreaded white mold and other diseases, growing into healthy, happy plants!

Fuzzy white mold won't kill your seedlings, but it is a sign that something's wrong. Learn how to prevent mold with these simple tips.