It’s a sight that every seed starter dreads: a seemingly healthy seedling, perhaps even the first to sprout, suddenly slumped over the next week with a wizened stem.
You may have even blamed lousy seed germination for a meager crop of seedlings when in fact, microscopic plant pathogens were at work below the surface. Collectively, these pathogens cause a condition called damping off.
What Is Damping Off?
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 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.
While damping off can strike seeds and seedlings started outdoors, it most often affects indoor seedlings due to high humidity, poor ventilation, and thickly seeded trays.
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 just pinched it off.
Causes and Prevention
Plant pathogens exist everywhere in the environment, but they thrive under certain conditions caused by:
- Reusing dirty containers
- Sowing a seed too deep
- Wet leaves
- Too much humidity
- Other environmental stress, like low light or cool conditions
To minimize the chances of damping off:
- Start with clean pots, plant markers, and tools that have been washed with hot water and soap
- Use clean potting soil that was not infested with disease last season
- Make sure your potting soil is light and fast-draining (here’s how to make your own)
- 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 surface is less likely to harbor fungi
- Thin your seedlings as needed and provide good air circulation around them
- Water your pots from the bottom up to avoid wetting the stems and leaves
- Keep the soil evenly moist but not water-logged
What about home remedies? 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. (All of these remedies are known for their antifungal properties.) But I see 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. The reasoning? 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 fungi and bacteria — simulate the environment your plant will eventually move into. Rather than starting your seeds in a sterile potting medium, use a clean potting medium (free of disease) and drench it with compost tea as your seedling grows. The compost tea will gradually build the microbial populations in the soil and strengthen the seedlings’ immune systems, in much the same way humans need bacteria to boost our own health.
Truth is, most cases of damping off result from overwatering and poor ventilation. Watch for signs of both as you start your seeds, and you’ll have a greater chance of raising strong, healthy seedlings.