7 Steps to Prevent White Mold on Seedlings

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White mold on seedlings or their soil indicates a problem in the growing environment. Mold thrives in warm, moist environments. Solve the problem by adjusting environmental conditions like temperature, light, humidity, and airflow.

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Unfortunately, white mold is a common issue when starting seeds. Most seedlings need conditions that are both warm and moist, and those conditions are also perfect for the development of mold. However, seedlings are likely to die off if white mold is present.

What Kind of Mold is White Mold?

White mold is not a particular variety of mold; instead, this term is used to refer to any of the many types that are white or very light in color. For many gardeners, it is difficult to identify what type of mold is attacking the seedlings, since they often look similar to one another. Among the most common types of fungus that attack seedlings are powdery mildew, Fusarium, Rhizoctonia, and saprophytic mold.

7 Steps to Prevent White Mold on Seedlings

Some types of white mold grow on the seedlings themselves, but the more common types grow on potting soil and feed on decaying organic matter. So, while these do not directly damage the seedlings, they indicate a problem in the environment that is harmful to plants.

Ultimately, most gardeners won’t need to identify the specific type of mold that is on their seedlings, since prevention and treatment is the same regardless.

Is White Mold Bad for Seedlings?

Seedlings can survive a bit of white mold, especially if it is removed promptly. However, if no changes are made to the seedlings’ environment once mold is detected, the seedlings are likely to die off soon.

7 Steps to Prevent White Mold on Seedlings

Mold on seedlings leads to a condition called damping off. Symptoms of damping off include visible white, fluffy mold on the seedling; soft, thin stems; and discolored and soft leaves. Under the surface of the soil, the roots are smaller than expected and weak.

7 Steps to Prevent White Mold on Seedlings

Once the seedlings show signs of damping off, they are unlikely to recover. It is best to start over with fresh seeds and try to avoid the same issue occurring again in the future by adjusting environment conditions.

7 Steps to Prevent White Mold on Seedlings
Removing White Mold

If you see white mold growing on the surface of the soil around your seedlings but there is no damping off, the mold can be removed by simply scraping it away with a sterilized garden knife. This will not get rid of all of the mold, and it will come back quickly if this is the only step you take. However, this should help eradicate the mold when combined with the following steps.

7 Steps to Prevent White Mold on Seedlings
Preventing White Mold

Preventing white mold from taking hold in the first place is preferable to trying to remove it later — it is both more effective and easier. Ideally, all of the seven steps below will be used in combination to avoid white mold affecting your seedlings.

1. Sterilize Your Tools

Firist, it is important to avoid introducing mold to the seedlings’ environment. This may seem obvious, but many people overlook the important step of sterilizing containers and tools before planting seeds.

7 Steps to Prevent White Mold on Seedlings

Thoroughly sterilize tools and equipment to kill off any mold spores that may be present. There are several good options, so choose whichever one you have on hand:

7 Steps to Prevent White Mold on Seedlings
2. Use the Proper Medium

Mold develops on potting soil that is too wet, which is why it’s important to choose a proper medium. It should be lightweight to allow enough air to circulate and water to drain freely. Avoid using a potting medium that contains compost or manure when starting seeds. It is best to use a specific seed-starting mix that is lightweight to allow adequate drainage.

7 Steps to Prevent White Mold on Seedlings

A good seed-starting mix might contain vermiculite, coco coir or peat moss, and perlite or pumice. These ingredients will ensure plenty of air circulation and drainage, but this mix does not contain any organic matter to provide nutrients to the seedlings. After they have developed true leaves, seedlings can be fertilized with a liquid fertilizer. Fertilizer provides them with the nutrients that would have otherwise come from the compost or manure in a regular potting mix.

7 Steps to Prevent White Mold on Seedlings
3. Avoid Overly Moist Soil

While it’s important that seedlings not dry out, it is also equally vital that they not be overwatered. Mold is a clear sign that the potting medium is too wet, which can quickly lead to your seedlings rotting.

7 Steps to Prevent White Mold on Seedlings

Check your seedlings at least every couple of days to be sure that the soil retains the right balance of moisture. Be sure to plant seedlings only in containers that have drainage holes. Seedling trays are the ideal container for starting seeds, but other options can also be adapted to the task.

7 Steps to Prevent White Mold on Seedlings
4. Improve Airflow Around Seedlings

Many seedlings are grown inside closed containers that allow for very little airflow, but this kind of environment makes it more likely that white mold will be able to develop. Proper airflow keeps the environment from getting overly moist by promoting evaporation, and also prevents mold spores from being able to establish themselves. Once seeds have sprouted, it is a good idea to add some ventilation for at least part of the day.

7 Steps to Prevent White Mold on Seedlings

If the seedlings are growing in an enclosed tray with a lid, open it part-way or remove it for several hours a day to allow air to move around the seedlings. In a larger space, a fan can be used to increase airflow.

7 Steps to Prevent White Mold on Seedlings
5. Maintain the Proper Temperature

Many people use a heating mat to encourage seeds to germinate, but it is best to turn it down or off once they have sprouted. Try to keep the temperature within the recommended range for the type of seeds you are growing, since higher temperatures encourage mold growth and can also overheat new seedlings.

7 Steps to Prevent White Mold on Seedlings
6. Provide Proper Light Levels

Light is one of the most important factors in guaranteeing a suitable environment for seedlings to flourish while discouraging white mold growth. Seedlings generally need up to 16 hours of bright, indirect sunlight per day to stimulate growth and speed up evaporation.

7 Steps to Prevent White Mold on Seedlings

If the season or location are not sufficient to provide the correct light levels to your seedlings, consider supplementing with a grow light that will encourage seedlings to develop properly.

7 Steps to Prevent White Mold on Seedlings
7. Don’t Overcrowd Seedlings

Many gardeners plant multiple seeds close together, in order to increase the odds of some viable seeds sprouting. However, if many or all of the seeds end up being successful, you can end up with too many seedlings in a small space that will crowd one another.

7 Steps to Prevent White Mold on Seedlings

Too many seedlings growing close together can lead to fungus, as there is not enough airflow around the developing roots. This can be remedied by pinching out the smallest seedlings to allow the stronger ones to have more suitable conditions. You may also want to repot seedlings into their own individual containers once they have several sets of leaves. This will allow them proper drainage and aeration, and avoid the issue of overcrowding.

7 Steps to Prevent White Mold on Seedlings
Conclusion

White mold on seedlings can be alarming, but it can often be treated by changing the environmental conditions. Mold grows best in humid locations without much air circulation, so be sure to allow the seedlings a chance to dry out a bit and don’t let them sit in a damp location. With a few simple tweaks to your seedling set-up, you can avoid issues with white mold in the future.