How to Prevent "Damping-off" Mold on Seedlings

seedlings in pots

The Spruce/ Margot Cavin

Project Overview
  • Working Time: 30 mins - 2 hrs
  • Total Time: 1 - 8 wks
  • Skill Level: Beginner
  • Estimated Cost: $0 to $20

A very common problem when attempting to grow plants from seeds started indoors is the appearance of fuzzy white, gray, or black mold that causes the freshly sprouted seedlings to decay and collapse at the soil level. Collectively, the conditions caused by these fungi are known as "damping-off" disease, and it is a death sentence for 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.

What Is "Damping Off" Disease?

Damping-off is caused by several soil-borne fungi, including Pythium spp., Rhizoctonia solani and Fusarium spp. Seedlings with damping-off disease cannot be saved, but there are steps you can take to prevent the disease, mostly entailing good hygiene and control of moisture.

When to Prevent Damping-off Disease

Seedlings are most vulnerable to the fungi that cause damping-off disease just after they germinate and sprout, while they are still dependent on the early cotyledon leaves that appear before the true leaves sprout. Once several sets of true leaves appear, the seedlings outgrow their susceptibility to damping-off disease. But with slow-germinating, slow-growing seeds, you'll need to be watchful for as much as eight weeks before the risk of damping-off disease lessens.

Before Getting Started

Cool, wet conditions and stagnant air are the perfect conditions for damping-off disease to take hold of your seedling flats. Making sure your seed-starter trays are kept adequately warm (but not hot) is one of the best ways to avoid the disease. In addition to the techniques described below, damping-off fungus can be discouraged with these practices:

  • Never use garden soil to start seeds indoors, as garden soil almost always contains fungal spores likely to cause damping-off disease.
  • Use seed starter mix as a growing medium, not ordinary potting soil. Seed-starter mix contains a well-draining mixture of fine particles of organic material such as peat moss, as well as perlite, vermiculite, or other fine mineral particles. Seed starter mixes typically are more porous and thus better draining than ordinary potting mix, and seeds are less likely to be constantly soggy. Commercial seed starter mixes are also sterilized during packaging, so they will be free of living fungi.
  • Starter mixes containing peat moss have a natural acidity that may help hinder fungal disease. Expandable peat moss discs, which swell when saturated with water, can offer a great medium for starting seeds.

With the right starter mix, you stand a good chance of avoiding damping-off disease if you follow the tips outlined below.


Click Play to Learn How to Prevent White Fuzzy Mold on Seedlings

What You'll Need

Equipment / Tools

  • Small fan
  • Shallow tray for watering


  • Diluted bleach solution


Materials and tools to prevent mold on seedlings

The Spruce / Jayme Burrows

  1. Use 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.

    Scrub brush cleaning garden spade with bleach solution in sink

    The Spruce / Jayme Burrows

  2. Avoid Soggy Soil

    The presence of fungus is a usually sign that your potting medium is too wet. Overly wet soil 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.

    Seedlings soil checked for moisture in black tray

    The Spruce / Jayme Burrows

  3. Increase Airflow

    Another 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.

    Black tabletop fan blowing air towards seedlings in black tray

    The Spruce / Jayme Burrows

  4. Improve the Light

    Take a careful look at how much light the seedlings are getting. Newly emerged seedlings need 12 to 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.

    Seedling inside small black pot placed near bright window

    The Spruce / Jayme Burrows

  5. Manage the Temperature

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

    Thermometer showing temperature of 78 degrees over potted seedling

    The Spruce / Jayme Burrows

  6. 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.

    Seedling repotted in small clay pot with garden spade

    The Spruce / Jayme Burrows

  7. Water Properly

    Most seedling mold is a result of watering too much. Don't water 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. When you do water, 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.

    Moisture level checked in potted soil with seedling

    The Spruce / Jayme Burrows