You’ve planted your seeds in their containers.

You’ve watered them carefully and kept them warm.

You’ve watched as they germinate and start to grow.

And then, all of a sudden, one by one they just lean over and die. 

Most gardeners have been there at one time or another. And most gardeners will have plenty of advice on how to stop it from happening again. There is, however, a growing school of thought that is breaking from conventional wisdom on at least one important aspect of how to prevent what’s known among gardeners as damping off.

What is damping off?

Damping off — a horticultural disease caused by numerous fungi and other pathogens — can move through an entire tray of seedlings in a matter of days. It is probably the single biggest problem for gardeners who start their seedlings indoors.

If you’re not sure what to look for, here’s how the Texas Plant Disease Diagnostic Lab at Texas A&M describes the phenomenon:

Typical symptoms occur soon after plant germinates. This can occur very quickly and appear to start at the base of the seedling. It may begin with a water-soaked appearance before progressing to becoming a constricted darken region. While seedling tops may still be green, they tend to flop over due to the loss of structural integrity. ... Sometimes these fungi can attack the germinating seedling before they emerge resulting in a what appear to be poor germination rate. But upon closer inspection, you might find rotten tiny seedling on the surface of the soil.
Preventing damping off: The basics

Once started, damping off can be very hard to treat. That’s why most gardening websites and articles focus on prevention, rather than cure. Here are some of the measures that are recommended by almost everyone to keep damping off from infecting your seedlings:

  • Ensure adequate air circulation: According to John Fendley (aka Farmer John), founder of the Sustainable Seed Co., providing adequate air circulation is the single most important aspect of raising healthy seedlings. Removing or opening up the cover on a cold frame, or opening the vents on your greenhouse, will allow air to circulate and prevent pathogens from building up on seedlings. Adding a fan can also help to improve air circulation — with the added benefit that a light breeze will also cause seedlings to grow stockier, sturdier stems.
  • Don’t water too frequently: Before seeds germinate, the growing medium needs to remain moist. Once seedlings have appeared, however, you should allow them to dry out before watering again. Watering from below can also help prevent plant stems and leaves from getting wet, thus also decreasing the likelihood of fungal or mold infections. In addition to exercising moderation when it comes to watering, Farmer John also cautions growers to ensure that all containers have adequate drainage to allow excess moisture to escape.
  • Maintain correct temperatures: Allowing plants to become either too hot or too cold can increase vulnerability to damping off. Protect seedlings from frost with a cold frame or greenhouse — but be sure to provide ventilation during the day so temperatures don’t get too high. Providing bottom heat to seedlings with a heating mat can speed up germination. However, once plants have germinated, plants like tomatoes should be removed immediately from the heat mat to avoid them becoming leggy. Peppers, on the other hand, are said to benefit from bottom heat for at least two weeks after germination has occurred. Check temperature recommendations for each seed you are starting, and use a soil thermostat to control your heating mat and avoid overheating.
Is sterilizing soil counterproductive?

So far, so uncontroversial.

But read most conventional gardening resources and they will tell you that you should also sterilize your seedling soil in order to eliminate any pathogens it might contain. Many gardeners, in fact, will literally bake their soils and seedling mixes in the oven to remove microorganisms and create a supposedly “safe” environment for their soon-to-be-born young plants.

There is, however, a growing school of thought that suggests this might in fact be counterproductive.

Probiotics for plants

Troy Beuchel, horticultural specialist for Premier Tech Horticulture — makers of Pro-Mix Ultimate Organic Seed Starter Mix explains why his company actually adds fungi and other microorganisms to its seedling mediums, and why they urge gardeners to not sterilize their soils:

“Damping off pathogens are typically not coming from the growing medium. They are a lot like the common cold — they are everywhere in our environment. Sterilizing the growing medium is not good because it kills any natural microorganisms that come from the peat/compost. These natural microorganisms use root exudates as food. Damping off pathogens also use these exudates as a food source. If the natural microorganisms are present, they use up the food coming from the plants roots which slows down the rapid development of plant pathogen populations. If the growing medium is sterilized, the pathogens still enter the growing medium. Since all the natural microorganisms have been killed, there is nothing in the growing medium to keep plant pathogen populations from quickly establishing and overwhelming plants.”

Healthy soil means healthy plants

This view is backed up by Justin Kirby of Fox Farm Fertilizer, another maker of organic soils and soil amendments which formulates its Light Warrior seedling starter mix using a diverse range of inputs including mycorrhizal fungi, beneficial microbes, humic acid and earthworm castings:

“As with sick people, killing all bacteria both beneficial and detrimental is not always the best approach. A great practice for germination of healthy plants without damping off is to load the soil with beneficial bacteria and fungi like mycorrhizae, and bacillus subtilis. This most closely resembles how things happen in the wild. Without healthy living soil, we can’t truly expect healthy living plants.”


How live soils boost a seedling’s immune system

Alison Jack, a researcher at Cornell University, has demonstrated how one particular water mold known as Pythium aphanidermatum, a common culprit in damping off, is inhibited by the presence of microorganisms commonly found in worm compost. Jing Jin of the Cornell Daily Sun explains more:

“The microbes present in compost are the key to suppression. These microbes colonize the seed surface within eight hours of being planted in vermicompost. The microbes chemically modify the seed as it germinates so that signaling between the seed and the motile zoospores of P. aphanidermatum is interrupted, preventing the pathogen from accessing the plant.”

Jack explains in more detail how the disease-suppressing qualities of vermicompost work in the video below:

The hidden world of soil

With scientific knowledge increasing about the vast diversity of species under our feet, it’s perhaps no wonder that many gardeners are finding benefits in promoting living soils.

Just as rice farmers have increased yields through nurturing soil biodiversity, so too many growers are now finding that a sensible approach to disease suppression is more about nurturing beneficial microorganisms rather than adopting a kill-everything-that-moves approach of creating lifeless, sterile growing environments. The more we learn about the complex relationships in our soils, the better we’ll be able to fine-tune our strategies for fighting damping off and other diseases.

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