Master gardener in training: What is ‘damping off,’ and how can I spare my seedlings from it?

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There are few more discouraging garden problems than the botanical version of crib death, generically known as damping off: rotting seeds and shoots below soil or stems and seedlings above ground. It can be caused by a number of fungi, some of which are ubiquitous in soils and can live indefinitely — however the main culprit is poor gardening: overcrowding, overfeeding and over-watering.

In my L.A. at Home series on the UC Extension’s Master Gardener program, I’ve mentioned but not fully explained the term ‘damping off.’ It’s easy to write off seeds that don’t germinate to a bad batch rather than bad gardening, but at other times damping off is unavoidably apparent as seemingly viable seedlings suddenly lose all their vigor, wilting on hot days, dropping leaves prematurely, and, in worst cases, withering up and dying. This most commonly occurs in the first month. You’ll have to uproot the plant to determine if damping off is the problem and not salty soil, excess fertilizing, or insects.

Depending on the fungal culprit, the tap root may be slimy, wet, show lesions, or the lateral roots may be dark brown and dry (black root rot). A blackened sick-looking taproot suggests Pythium root rot especially if the soil is wet due to poor watering or poor drainage. The root will look rotted but won’t smell. You may also see white fungal threads encircling the root.

Damping off is a worldwide problem, especially in greenhouses with a mono-culture where fungi easily spread through contaminated tools. There are fungicides but they’re not recommended for the home gardener. Prevention is the preferred route.

You can begin by sterilizing your potting mix so the seeds don’t catch anything in the cradle. You can do this in the oven or the microwave and it can mean a bit of an earthy smell in the kitchen, but you’ll wind up with potting soil relatively free of pathogens. There are various recipes online but all involve baking the soil in a pan for at least 30 minutes at 140 degrees F (or in the microwave, at full power, for seven minutes). You can also place soil in a clear plastic bag in direct sunlight and let it steam until it hits the 140 minimum. Damping off is also more prevalent in soils with a high pH. rate, above 6.5. In Los Angeles, our soils are alkaline and our municipal water is also alkaline — both of which tip us toward a fungi-friendly environment. You can lower the pH of your tap water by adding a tablespoon of vinegar to a gallon of water but don’t do this without checking the alkalinity of both your soil and water.

For new seedlings showing signs of damping off try misting with chamomile tea (two cups boiling water and ¼ cup of chamomile blossoms). Sprinkling powdered cinnamon on a non-soil seedling medium is also said to be effective.

In general, don’t start plants in cold, wet or compacted soil. Plant seeds as shallow as possible. Improve the drainage by adding mulch, avoiding green compost, or planting in raised beds. Improve air circulation by thinning your seedlings as they emerge. If planting in containers, do NOT use your garden soil but rather a good quality potting soil that drains quickly. Discard all sick or infected plants and trimmings and sterilize all containers and tools before using them.

-- Jeff Spurrier

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