How To Prevent Damping Off So That Your Seedlings Survive

Every gardener’s worst nightmare is damping off. It’s a quiet affliction that often appears out of nowhere: one minute your seedlings will be fine, and the next they’ll be on the ground. And at that point, there’s no saving them.

You know how they say that an ounce of prevention is worth a pound of cure? That’s never been more true than with damping off. As long as you start your seedlings off the right way, you won’t have to worry about this devastating disease. 

Damping off is a soil-borne fungal disease that primarily affects seedlings, and it is difficult, if not impossible, to reverse. Fortunately, damping off is easily preventable by using sanitized seed trays and sterile potting soil. If you are careful not to overwater seedlings and if you monitor humidity, temperature, and light levels closely, the fungus will never develop and your seedlings will grow strong and healthy.

So what exactly causes damping off, and how do you protect seedlings from it? Keep reading to find out!

Seedling damping off, explained 

What causes damping off?

Damping off is a soil-borne disease that can affect all types of plants, mainly seedlings because they are the most vulnerable. Damping off is often caused by overly wet soil and stagnant air. Cool temperatures and low light conditions can also contribute to this disease.

Damping off can be caused by fungus spores that are present in the soil and on unsanitized seed trays and plant pots. A blog post by the University of Minnesota identifies Pythium, Rhizoctonia, and Fusarium as the fungi strains responsible for damping off.

Since these fungi live below the surface, the pathogens may infect and kill seeds before they even germinate. The fungus attacks the sprouting root, causing it to rot before the seedling has a chance to emerge from the soil. Damping off might be to blame if none of your seeds sprout, or if germination is noticeably uneven and patchy. (Although you’ll also want to check the date of your seeds, because seeds that are several years old may be slow to germinate, or may not even sprout at all.)

Even after sprouting, seedlings aren’t immune to damping off. The fungus may still infect seedling roots and work its way up to the stem. Infected seedling stems will show signs of discoloration (yellow or brown) and may be noticeably thin. At the final stage of damping off, seedlings will fall over. At this point, the seedlings can’t be saved. 

How does damping off spread?

Since damping off is caused by soilborne pathogens, the disease is most commonly introduced to healthy plants from contact with infected plants or from contaminated soil or other growing media. 

Unfortunately, fungi and damping off can also be spread by wind, water, and even by insects—this is why it’s a bad idea to reuse potting soil or water for irrigation between seedlings. It’s okay to recycle potting soil and water once you have mature plants, as established plants are less susceptible to damping off, but this is a hard no when it comes to seedlings. 

Damping off can spread quickly from plant to plant and from one area of the greenhouse to another if the affected soil and plants are not removed. As hard as it is, as soon as you see the first signs of damping off, you have to toss that seed tray and try again, with new soil and new seeds. The fungus can be spread through infected seedlings and soil, as well as infected pots and trays.

Damping off is more common among seedlings that are planted too closely together. While it’s more than okay to multi-seed small seeds, once the seedlings reach an inch or so tall it’s time to thin them. 

To thin seedlings, grab a clean pair of garden shears or scissors. Identify the strongest two or three seedlings in each cell, and cut the rest out. If you’re up for a challenge, you can pop the entire cell out of the tray and tease the seedlings apart, then pot up the extra seedlings in another container. This method does stress seedling roots, which is why it’s often better just to cut out the extra seedlings. 

Symptoms of damping off

There are a few telltale signs of damping off: 

• Germination across the seed tray may be patchy and uneven—or seeds may not germinate at all. 
• Seedling stems and leaves, especially the cotyledons (the first set of leaves) may be discolored, most often brown or yellow in hue.

• Seedling stems and leaves may be mushy and soft to the touch. 

• Seedling stems may be thinner than normal, and may look weak and break easily.
• Seedlings may show signs of wilting and die before they can fully emerge from the ground. 
• Seedling roots may be shallow and small, or nearly nonexistent.
• White, fuzzy, threadlike mold may develop near or touching the seedlings. 

How to prevent damping off

The best way to prevent damping off is to use best practices to sow seeds. The fungi that cause damping off just won’t survive in the environments where most seeds thrive. So, if you use a sterile seed-starting mix, give seedlings the appropriate amounts of water and light, and monitor humidity levels and temperature, you won’t have to worry about damping off at all. 

Seed depth

One mistake growers often make is sowing seeds too deep. Make sure that you sow seeds at the recommended depth for germination—twice as deep as the seed is wide. Seeds that are sown too deep are more likely to never sprout and eventually rot. 

Water

Overwatering seedlings is the number-one cause of damping off. Far more detrimental than underwatering, overwatering can cause a myriad of issues, the most dangerous of which is damping off. The pathogens that cause damping off flourish in cool, damp soil. For this reason, it’s not generally recommended to water seedlings in the evening, when daytime temperatures drop.

Protect seedlings from damping off by watering them at the appropriate time (mid-morning is ideal) and by watering only when necessary. 

To figure out if seedlings need to be watered, scratch the surface of the soil. If the soil is dry to the touch, you can water. If the soil is still moist, wait another day or two. It’s also good practice to lift the seed tray or pot to check the bottom of the container. If the soil is noticeably wet, don’t water. If the soil has dried out, proceed with watering.

The best watering method to prevent damping off in seedlings and encourage healthy root development is bottom watering. Read this blog post for more information on bottom watering.

Watering from the bottom also prevents backsplash, which is another potential cause of damping off. Keeping seedling leaves dry is a great way to monitor appropriate moisture levels. 

Overwatering causes more issues than just encouraging damping off. Watering seedlings too much trains the plants to keep their roots at the surface of the soil, rather than encouraging the plants to grow robust roots that fill out the container. Shallow roots will result in a weak plant that won’t transplant well and won’t grow to its full potential.

Temperature

Damping off fungi thrive in cooler temperatures—an article published by PennState Extension suggests that low soil temperatures (below 20°C or 68°F) before germination and high soil temperatures (above 25°C or 77°F) after emergence are the major windows for damping off to take hold. 

You can avoid this issue by using a heat mat and thermostat to regulate temperature. A heat mat will ensure soil temperatures are warm enough for germination, and once the seedlings sprout, simply remove the tray from the heat mat to lower the soil temperature and dissuade the growth of dangerous fungi. 

Humidity 

An article published by the University of Massachusetts at Amherst recommends that the ideal humidity level for germinating seedlings is around 95%. While that is a lot of air moisture, as the seedlings grow they will need less and less humidity.

When your seedlings have grown big enough to not need as high humidity, it’s a good idea to vent the greenhouse or seed-starting room to allow some of the pent-up moisture to dissipate. You can also run a dehumidifier, but most growers find that an oscillating or box fan will do the trick. The fans encourage better airflow, which will lessen the chance of damping off taking hold. Plus, the light breeze encourages seedlings to grow stronger stems. 

The fungi that cause damping off love moist areas—that’s why just before and after germination is the most dangerous time for damping off. Fortunately, light helps keep damping off at bay.

Light 

Seedlings need at least 12 hours of direct light per day, and upwards of 14–16 hours is even better. Since most growers start seedlings indoors, you won’t see that kind of light in early spring without the addition of a grow light. 

Seedlings can get by with less light, but fungi thrive in low-light conditions. Adding a grow light to your setup will help prevent damping off, and will also result in healthier seedlings with stronger stems. Seedlings that have to stretch toward indirect light (like from a windowsill) will have spindly stems and will be more susceptible to damping off. 

Growing medium 

It’s been said before, but bears repeating: don’t reuse potting soil to start seeds. Always start seeds in a sterile potting soil mix specifically labeled for seed-starting. Don’t use soil from your yard or garden to start seeds either, as garden soil is certain to contain pathogens that seeds aren’t equipped to handle yet. 

In summary 

Damping off is a fungal disease that can quickly spread among seedlings, resulting in wilting, discoloration, and death of the seedlings. To prevent damping off, use sterile, well-draining soil, water appropriately (not too much or too little), provide enough light and ventilation, and sanitize seed trays after every use. If you detect damping off, cull the affected trays to stop the spread of the disease. With proper prevention and disease management techniques, you can help give your seedlings the best chance of surviving and thriving.