March: Seed Starting and Damping-Off

Preparing for Spring

March is the month when many gardeners begin to think about starting vegetables and ornamental annuals from seed indoors.  While seed-starting can be an economical method for producing plants for home gardening, damping-off can put a damper (no pun intended) on this fun winter gardening activity.

Forms of Damping-Off

Damping-off can take on many forms:

  • rot of seeds prior to germination,
  • rot of seedlings before they reach the soil surface,
  • collapse of seedling stems after emergence followed by plants toppling onto the soil surface (the most classic form of the disease).

Gardeners often blame damping-off on poor seed quality, but disease-causing fungi (and other similar organisms) are the real cause of the problem.  Luckily, only seeds and young plants are susceptible to damping off; older plants are immune to the disease.

Lower stem collapse of Zinnia seedlings due to damping-off.
Lower stem collapse of Zinnia seedlings due to damping-off.

Prevention of Damping-Off

  • Use new or decontaminated growing containers, working surfaces and tools. Damping-off pathogens can survive on inert surfaces.  Rinse items like pots and potting stakes thoroughly to remove soil and then soak them for 20 to 30 minutes in 10% bleach.  Rinse items well after treatment to remove bleach residues.  This treatment works well to decontaminate clay and ceramic containers, but is less reliable for plastic items.  If you have had damping-off problems in the past, discard plastic containers and labels and start with new items.  Also use 10% bleach to decontaminate surfaces where you work with seeds and plants.  For metal tools, use a 30 second dip in 70% alcohol (e.g., rubbing alcohol) to decontaminate or alternatively use a spray disinfectant (spray until tools drip and then allow them to air dry).
  • Use pasteurized soil to start seeds. Pasteurized soils are steam treated which helps kill disease-causing organisms.  Never use garden soils as these typically contain low levels of damping-off organisms.
  • Use high quality, vigorous seed. High quality seed germinates rapidly and resulting plants quickly mature and become immune to damping off.
  • Plant seeds at the proper depth. Check seed packets for information on proper panting depth.  When seeds are planted too deeply, plants mature less quickly and remain susceptible to damping-off for a longer period of time.
  • Germinate seeds at high temperatures. High temperatures stimulate rapid germination and growth, resulting in a shorter period of time when plants can become infected.  Check out heated seed germination mats, which promote rapid plant growth.  They can help prevent damping-off.
  • DO NOT overwater. Damping-off pathogens are more active in wet soils.  Keep soils drier to reduce pathogen activity and limit infections.

For More Information

See the University of Wisconsin Garden Facts “Damping-Off” available at https://pddc.wisc.edu/fact-sheet-listing-all/.