For many of us, starting our own seedlings is one of the many highlights of gardening. It’s a great way to kick off the garden season well before we can play in the dirt outside. But if you start seedlings long enough, eventually, you’ll have a brush with damping off.
There’s nothing quite as frustrating as losing all your hard work before the growing season has even started. But as frustrating as it is, damping off is preventable.
But Everything Started Out So Well!
If you’ve yet to have a run-in with damping off, consider yourself lucky. Many of us know all too well what the beginnings of this headache look like.
You’re patiently (or not so patiently) waiting for your seedlings to emerge, but it’s taking a bit longer. When they finally start poking through the soil, there are fewer than you expected, and they’re pitiful-looking things.
They look yellowish instead of the bright green you would expect. The stems look almost watery; they’re quite thin, and it looks like someone pinched them where they come out of the soil. It’s not long before your poor little seedlings turn brown and collapse into a mushy mess.
Usually, the soil is especially damp, and mold or fuzz grows on the surface. You might also notice a funky smell. Once you spot it, you’ve got to move fast, as it spreads quickly, and there’s no saving infected seedlings.
Damping Off – So, What Is It?
Aside from an annoying start to your growing season, damping off is a fungal disease (although it can also happen without fungi present) that affects germinating seeds and new seedlings. Seedlings are most susceptible when they’re still tiny, before their first true leaves have developed.
There are a couple of ways it manifests:
- Pre-emergence damping off: The pathogen attacks seeds before they emerge from the soil, leading to poor germination rates.
- Post-emergence damping off: The disease affects young seedlings after they have emerged, causing their stems to become thin, water-soaked, and weak, ultimately leading to collapse.
It’s not uncommon to see both pre and post-emergent damping off at the same time.
This stuff is bad news in greenhouses, large seed trays, raised beds or any place where the environment is just right for the pathogens to grow. And what are those pathogens?
What Causes Damping Off?
Damping off is caused by several soilborne pathogens, including fungi and oomycetes. (Oomycetes are fungus-like microorganisms that are found naturally in water or soil and can attack roots and stems.)
If you were to round up the Damping Off Most Wanted Pathogens, these four would be at the top of the list as the most common culprits.
- Pythium spp.– A water mold that thrives in overly wet and compacted soils.
- Fusarium spp. – A fungus that causes stem and root rot, leading to seedling death.
- Rhizoctonia solani – A soilborne fungus that produces dry, brown lesions on stems.
- Phytophthora spp. – Another type of water mold that can contribute to seedling death.
If your seed starting environment has the following conditions, then it is primed for the growth and spread of these pathogens. The more of these environmental factors that come into play, the higher your chances of losing seedlings to damping off. (And the faster it can spread to other seedlings.)
- Excess moisture – Overwatering or poor drainage creates the perfect environment for fungal growth.
- Poor air circulation – Stagnant air fosters fungal spores.
- Contaminated soil or equipment – Reusing old or unsterilized potting mix or dirty seedling trays and tools spreads pathogens.
- Crowded seedlings – Dense planting limits airflow and increases humidity, making seedlings more susceptible.
- Cool soil temperatures – Cold, wet soil slows seedling growth and makes them more vulnerable to infection.
- High humidity – Encourages fungal growth and spore spread.
How to Prevent Damping Off
Unfortunately, once infected, there is no way to cure or save seedlings. If you have infected seedlings, you need to remove them and dispose of them properly. Don’t compost them. You’ll also want to dispose of or sterilize any equipment, such as trays.
Fortunately, damping off is easy enough to prevent, and knowing the environmental factors that lead to it gives us plenty of clues about how.
1. Use Sterile Growing Medium
Skip the potting mix and use a seed-starting mix. These are always sold dry and sterilized, meaning they are less likely to contain fungal spores. You can even make your own with high-quality ingredients.
2. Sanitize Containers and Tools
I know it’s tempting to grab your tools and pots from last year and get started, but I can tell you from experience it’s a lesson you will only need to learn once before you make it a point to sterilize everything. It’s better to be safe than start all over again.
Before planting, sterilize seed trays, pots, and tools with a solution of one part bleach to nine parts water. This helps kill any lingering pathogens from previous plantings. Dip everything in the bleach water solution and then set it aside to dry. Don’t dry your equipment off. It needs to remain damp on the surface for a few minutes to be effective. Let it air dry completely.
3. Practice Good Watering
Water seedlings lightly and only when necessary. Use a misting spray or bottom-watering technique to keep the soil evenly moist but not soggy. (This also keeps seeds in place so they don’t get moved around from aggressive watering.) Only use pots or trays with drainage holes, and make sure they don’t become clogged.
4. Provide Good Air Circulation
Use a small fan to keep air moving around seedlings, which helps to reduce humidity and dry out excess moisture on plant surfaces. Proper spacing between seedlings also improves airflow. (If you really want to ensure success, check out my personal seed-starting setup, which includes a hack for adding a temperature-regulated fan to your basic mini-greenhouse.) Remember, moving air also stimulates strong stem growth in seedlings.
5. Maintain Optimal Temperatures
Soil temperatures are important for germination. I highly recommend using a seedling heat mat, which does more than ensure good germination rates. They also help maintain consistent warmth and prevent excessive moisture retention. (This is the brand I swear by and use every year without fail.)
6. Use Natural Fungicides
Mist seeds and soil with natural fungicides like chamomile tea, cinnamon, or hydrogen peroxide which can help suppress fungal growth. Remember, most of these pathogens are normally occurring in the soil and only become an issue under the right environmental conditions. Suppressing them with a fungicide is a good preventative.
7. Avoid Crowding Seedlings
Thin out seedlings as they grow to improve air circulation. This also reduces competition for nutrients and light, allowing the remaining seedlings to grow stronger and more resilient.
There is no sugar-coating it – damping off is devastating for us seed-starting enthusiasts. Luckily, it’s largely preventable. Yes, it means a bit more work, practicing good gardening hygiene, and paying close attention to the conditions in which we start our green babies. But in the end, it means we can prevent this seed-starting menace from rearing its ugly head. And that means getting your growing season off to a great start with strong, healthy seedlings.
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