How to Prevent Damping Off

A zinnia seedling, felled by damping off. Photo: Suzanne DeJohn

Starting seeds? Be aware of overly cool, wet conditions, which can easily take down your batch of seedlings.
When the stems of young plants wither at the soil level and then topple over, they have probably been killed by a very common disease known as "damping off". The culprit is actually pythium, a common fungus. The spores of this fungus require cool, wet soil in order to infect plant tissues. Pythium can also infect seeds as they are germinating, causing seedlings to rot before they make it to the soil surface.

How To Prevent Damping Off

• Use a well-drained growing medium when starting seeds indoors or in a greenhouse. Allow the growing medium to become relatively dry between waterings.
• Avoid sowing seeds too thickly.
• Most seedlings require at least 15 hours of light. Provide supplemental lighting to maintain healthy, vigorous growth.
• Sprinkle a layer of fine vermiculite or sand over the soil surface of seed starting containers to keep seedling stems dry. Water seedling containers from the bottom rather than overhead.
• Operate a low-speed electric fan to keep air circulating gently around young plants and across the soil surface.
• Before reusing seedling pots or trays, wash them in a 10 percent bleach solution.
• If you've had past problems with damping-off, start seeds in a sterile seed-starting mixture.
• In spring, hold off on outdoor planting until the soil is relatively dry and warm. Seeds will germinate and develop more quickly in warm soil.