It’s a sight that every seed starter dreads: a seemingly healthy seedling, perhaps even the first to sprout, suddenly slumped over the next day with a wizened stem.
Collectively, these pathogens cause a condition called damping off disease.
What causes damping off disease?
There’s never any warning when damping off might occur. The disease can take hold of a seed before it’s even sprouted, or a seedling before it’s formed its first true leaves.
Caused by several species of seed-borne and soil-borne fungi including Pythium, Rhizoctonia, Fusarium, and Phytophthora, damping off disease can move through an entire tray of seedlings in a matter of days and once they’re infected, they’re near impossible to treat.
The plants that do survive the infection are often stunted and afflicted with “wire stem” symptoms: twisty, constricted stems that result in abnormal growth and smaller yields.
Damping off disease occurs in all types of seedlings, from tomatoes and peppers to leafy greens and root vegetables. One variety is not more susceptible than another, and disease-resistant strains will not prevent it from occurring.
While damping off can strike seeds and seedlings started outdoors, it most often affects indoor seedlings due to high humidity, poor ventilation, and overcrowded seed trays.
What does damping off look like?
Damping off is a fungal infection that’s generally characterized by the absence (or rotting) of roots, and thin, thread-like stems where the seedlings are infected.
But, it can also wreak havoc in seeds below the soil in two different ways.
In pre-emergence damping off, fungi infect the seed as it germinates.
The infection progresses swiftly and the seed decays before a stem ever emerges. This is sometimes the cause for thin and patchy stands of seedlings where unviable seeds tend to take the blame.
In post-emergence damping off, fungi infect the stem near the soil surface.
The stem takes on a discolored, water-soaked appearance from the bottom up, weakens and withers and eventually collapses, unable to support itself. It often looks like someone—or something—just pinched it off.
How do you prevent damping off disease?
Plant pathogens exist everywhere inside and outside, but they thrive under certain conditions that are typically found in poorly ventilated greenhouses or indoor seed starting environments.
As with most seed starting problems, it’s easier to stop damping off from happening in the first place, than try to fix it if it does. Once a stem shrivels at the base, there’s nothing you can do to save that seedling.
The good news is, damping off disease is easily preventable and you don’t need any chemicals to control it. The key is to give your seeds a clean, healthy start and keep moisture levels in check.
Common causes of damping off disease
1. Reusing dirty containers.
You don’t have to wash your pots if you didn’t have any problems with them last season, but it’s best to discard pots that previously held diseased plants.
To ensure healthy seedlings and minimize the chances of fungi spreading, start with fresh pots and plant markers, and tools that have been properly cleaned.
2. Using infected soil or heavy garden soil to start seeds.
When starting seeds indoors, always use clean seed starting mix that was not infested with disease last season. Make sure your seed starting mix is light and fast-draining (mixing in some perlite can help with drainage).
Resist the temptation to simply dump the soil from your yard into a pot to start your seeds. Garden soil is too heavy for seed starting pots and trays, and it often brings on other problems (like dormant weed seeds sprouting and competing with your seedlings).
2. Sowing a seed too deep.
Seed packets usually have instructions for sowing seeds, and it’s important to pay attention to seeds that need light or darkness to germinate.
As a general rule of thumb, seeds should be planted as deep as their size (measured by thickness or length).
For example, a pea seed should be planted about 1/4 to 1/2 inch deep, while a pumpkin seed should be planted about 1/2 to 1 inch deep.
For tiny seeds like basil, mustard, or carrots, sow your seeds on top of the soil, then add a layer of vermiculite or very fine granite (like chick grit) to cover. The drier, grittier surface is less likely to harbor fungi that cause disease.
Seedlings need proper airflow and spacing to not only strengthen their stems (which is part of the process of hardening off) but also to promote good root development and reduce the chances of disease.
After your seeds germinate and the seedlings grow their first true leaves, remember to thin the seedlings as needed to provide good air circulation around them.
Seedlings don’t have very deep roots, so they do better with frequent but shallow watering where the roots are concentrated.
Because of this, it’s easy to water too much, especially if your indoor seed starting mix or outdoor garden soil is on the denser side. Try to keep the soil evenly moist but not waterlogged, and make sure the water drains freely out the bottom of your pots.
At the same time, don’t let the soil dry out between watering because peat-based soil mixes are very difficult to rewet once dry, leading you to water more than you have to.
5. Wet leaves
Do you constantly find moisture on the leaves after watering? It happens easily when you water with a spray bottle, squirt bottle, or watering can from overhead, even if you try to be careful.
Soil splash is a common cause of fungal disease in plants, especially with seedlings that don’t have the benefit of mulch to protect them.
To solve this problem, try watering your pots from the bottom up to avoid wetting the stems and leaves.
6. Too much humidity
Seedlings started in greenhouses with poor air circulation tend to suffer from damping off if humidity levels are too high.
The same goes for seedlings started under plastic humidity domes (which essentially act as mini greenhouses). These domes work well to trap heat and provide the warm environment that most seeds prefer for germination, but once they sprout, the domes should be removed.
In either situation, running a low-speed electric fan near young plants can gently circulate the air around them while strengthening their stems at the same time.
7. Other environmental stress, like low light or cool conditions
Damping off is sometimes exacerbated by environmental stress such as too little light (common when germinating seeds in a window, even if it’s a south-facing window), cold temperatures, early pest damage, or excess nitrogen.
When seedlings are stressed, their immune defenses are down, leaving them susceptible to pathogens they otherwise might have been able to fight against.
Can home remedies save your seedlings?
Since the key to treating damping off is preventing the infection in the first place, it might make sense to preempt the problem by applying chamomile tea, clove tea, or a sprinkle of cinnamon to your soil. After all, that’s what the Internet often tells you when you Google “how to treat damping off.”
All of these home remedies are known for their antifungal properties, but they’re 50/50 on whether or not they actually work.
Personally, I approach seed starting as a process of natural selection. Allow your seedlings to develop naturally and the strongest ones will adapt to their environment.
Should you sterilize your soil?
Various sources advise gardeners to sterilize their soil by baking it in an oven… literally spreading it out on a sheet pan and baking at low heat to rid the medium of microorganisms.
When you start with a clean, blank slate, no nasty pathogens are threatening to claim your poor defenseless plants.
But by sterilizing your soil this way, you’re also removing the good microorganisms that plants depend on in the circle of life, rendering them even more defenseless.
Without populations of good microbes to balance the bad, you’re inadvertently lessening your seedlings’ chances of survival in the real world.
“Living soils”—those inoculated with bacteria and fungi—simulate the environment your plant will eventually move into.
Rather than starting your seeds in a sterile potting medium, use a clean potting medium and drench it with compost tea as your seedling grows.
And truth is, most cases of damping off result from overwatering and low ventilation.
Neither of these problems can be solved with fungicides.
Watch for signs of excess moisture or poor airflow as you start your seeds, and you’ll have a greater chance of raising strong, healthy seedlings.
This post updated from an article that originally appeared on March 10, 2015.