Back in November, when I posted my Great Catalog List, I sang the praises of buying seeds. In the comments, Lisa said she would like to grow more from seed, but has trouble with things damping off, and asked if I could give some help.
So, today's Sciency Answer is: Everything I Know About Damping off.
What is it?
Damping-off is a generic term for a number of disease (fungi and other bad stuff) that attack young seedlings. The seeds sprout, they look all happy, and then they start wilting and collapsing. It is pretty devastating, but quite preventable. You'll usually have problems with damping off if you have a LOT of fungal spores in your seed starting area for some reason, if your seedlings are weak or stressed, or if you keep the soil to wet. By following a few guidelines, you can keep your seedlings happy, the fungi unhappy, and your seed starting trouble-free.
Use sterile, high-quality potting media
I use Baccto, a sterile peat-based media, for my seedlings. There is nothing special about that brand, it is just what my favorite garden center carries. I know peat is controversial environmentally but it is also the best -- at least in my experience, and in the experience of most growers I have talked to. Feel free to disagree with me in the comments. I use a variety of other media for bigger pots and containers, mixing in my own compost and good garden soil along with peat alternatives like coir. But seedlings need the best, and since you only need a tiny container to get seedlings started, a little high-quality potting media goes a long way. Starting with sterile media means you aren't putting seeds into a giant population of pathogens waiting to consume them. I've read recommendations to top your media with a very thin layer of coarse grit, the reasoning being it will dry out quickly, making it difficult for fungi and friends to grow on the surface. I've never tried it, nor have I ever seen it used commercially, but it makes sense, and might be worth a try.
Use sterile containersEither buy new ones for each batch of seedlings, or give them a soak in a bleach-water solution and rinse them thoroughly. Old, unsterilized containers can be loaded with potential disease.
Keep the air moving
This is critical. In perfectly still air, a layer of nearly 100% humidity hangs right over the moist soil, making ideal conditions for all the nasties to grow and attack your seedlings. A fan very gently keeping the air circulating breaks up that super-high humidity layer and will make a dramatic reduction in your disease problems. HAF (Hortizontal Air Flow) fans are ALWAYS used by commercial growers and should be by home seed starters as well.
Give them enough light, sun light if possible
If seedlings don't have enough light, they become long, leggy, and extra susceptable to disease. Many commercial available seed-starting light systems simply don't have enough bulbs to produce healthy seedlings. I use two big florescent shop lights side-by-side, for a total of four florescent bulbs over my seedlings, keeping them vigorous and healthy enough to fend off pathogens. Sunlight is extra beneficial, because the UV rays provide a sterilizing effect, knocking back the disease organsism at the soil surface. In cold climates like mine, sunlight usually isn't an option because I'm starting most of my seeds in the later winter when it is way too cold for my seedlings outdoors. However, when I do get a rare warm, sunny afternoon, I always rush my seedlings outside into a sheltered, lightly shaded spot. One year, I neglected to have a fan running in my seed starting room, and a tray of tomato seedlings started collapsing left and right, but a few hours of sterilizing sun stopped the disease in its tracks.
Keep 'em warm, with bottom heat if possible
If your house (or wherever you are starting your seeds) is on the cool side, giving your seedlings a bit of extra heat will help them germinate and grow quickly, getting them past that vulnerable just-germinated stage faster. Some seed companies, like Johnny's, will list the best temperatures for germination of each plant. If I can't find specific information for a plant, I aim for the upper 70s F (~25 C). The extra heat is best if provided from below with a seedling heat mat. Bottom heat encourages the roots to grow rapidly without causing the stems to grow long and floppy, producing healthier seedlings -- and healthy, happy seedlings resist disease the best.
Don't keep them to wet
Seedlings have tiny root systems, and can't be let to dry out too much, but keeping them constantly soggy is asking for trouble. I never let the soil dry out until the seeds germinate, but once they're up and have some roots, I try to let the very surface of the soil get a little dry before watering again. It can be a bit of a balancing act, but if you are doing everything else right, you'll have some room for error on the watering.
In an emergency...
Sometimes, despite your best efforts, your seedling start damping off. As soon as you see a seedling wilting despite the soil being moist, you know you've got a problem. First, get the diseased seedling, the soil and other seedling around them out of there as quickly as possible. One they're infected, you can't save them, and they'll only spread disease to your other seedlings. Then you need to keep your other seedlings from dying as well. I like treat them with hydrogen peroxide, watering them with roughly 1 part hydrogen peroxide from the drug store to 9 parts water. Hydrogen peroxide acts directly to sterilize the soil, and it also is used by plants to signal their natural defense systems to get up and running, so watering with it acts like a warning: "Hey! Damping off is coming!"
What about commercial fungicides?
I don't spray any sort of pesticides (synthetic or organic) in my garden. If a plant can't cope, I pull it out, and find something better to grow. But I think fungicides make some sense for seeds and seedlings. When you spray a whole plant in the garden you have to use large quantities of the chemical and there is a high potential for it to drift, leach into ground water, and disrupt natural ecosystems. But treating a flat of seedlings under lights in my basement requires a tiny quantity of chemical with virtually no chance of it getting out into nature. That being said, I've never used fungicides on my seedlings -- the methods outlined above work fine for me, and I haven't seen the need. If you do, just be sure to do your homework to find the safest, most effective one possible, remembering that organics can be toxic and dangerous too!
Starting healthy seedlings isn't hard, provided you take the time upfront to create a seed starting area that promotes growth by seedlings not fungi. Now go buy some awesome seeds!
Have a question? Get a sciency answer! Just e-mail me: engeizuki at gmail dot com or ask it in the comments.