I have been trying to mix my own seed starting medium. I know I need to sterilize the mix materials, particularly compost. I tried doing that in the oven at 240 degrees with water added to a closed container for about an hour. When I planted my celery seedlings in the medium, only a few germinated before a green algal growth formed on the top of the soil. Anyone have any better ideas to sterilize a seed starting medium after mixing? I seem to remember an OG article on it sometime in the past, but can't find it.

you do not need to sterilize the seed starting mix, actually. It won't stay sterilized, after all, and by sterilizing it, you remove any of the protective beneficial fungi that would otherwise combat any pathogenic fungi and larvae in the soil. I stopped using sterile soil four years ago and haven't had a single incident of damp off, since.

At my house we've been washing the containers in a mix of bleach and water. Is that not necessary either? We've been doing it for several years but I think I remember losing more seedlings before we did this.

Thanks, brennewoman for your insight. How then do I stop the green algal growth which appears to be on a race to shortcircuit seed germination in the medium? The celery seed is fresh new seed, but most of it didn't germinate before the top of the cell packs became a green mat. Its clearly coming from the soil, so I doubt sterilizing the container will help. Any beneficials that can be encouraged or added into the mix??

If you have algae growing on your medium you are keeping it too wet and it may be coming from the water you are using instead of the potting medium.
The media to start seeds needs to be moist but not wet.

Like the last post mentioned, you probably have the medium much too wet. Also, air movement can help to stop the algae and moss from growing on top of the soil...a small fan helps. You can also water with chamomile tea, sprinkle cinnamon and/or cloves/milled peat moss on top of the soil, or use a garden inoculant for the soil, which competes quite effectively for space, getting rid of most problems, for me. How loose and fluffy is your seed starting medium? If it is a bit dense, you could need to lighten it up with perlite, coir, peat or vermiculite, so that air flow in the soil itself helps to keep an overabundance of fungi from growing

About the pots, I don't sterilize them, either. If they housed a plant with a disease, I use them for plants that are not susceptible to that disease. I do wash them off with soapy water at the end of the season, but that's about it.

mdubois28,

Welcome to the forums!

If you are creating the perfect conditions for algae, you will get it. Do you have a heated area for seed germination, using plenty of bright light and water? Are you using a clear cover creating a nice greenhouse for your germination? Sounds like the perfect conditions for algae production to me. As others are saying, it sounds like you may be watering too much, so don't water as often, or with as much water. You only need the top inch of growing medium to be moist during germination. Or water from the bottom, but don't let the water stand in the tray for too long. When the starts or cells are moist, pour the excess water out. I have a little problem with a white fuzzy mold growing on my tp rolls with my onion seeds germinating. I used some ground cinnamon right on the mold, and then sprinkled corn meal all over. This combination should help keep things under control until I can remove the clear cover and turn a fan onto them to improve air circulation and reduce humidity. The fan will also toughen the seedlings up for going outside later. Just out of curiousity, are you using well water, spring water or city water? I do use water from a local spring, and I also see a bit more "extra" growth than I used to. Let us know how things go!

In "Rodale's Illustrated Envyclopedia of Organic Gardening" it is recommended that the soil in your seed starting mix be pasteurized. Their method is this:
"Place moist, not wet, soil in a shallow tray to a depth of about 4 in. Cover with foil. Put in a preheated oven at 180 degrees F for 30 mins. Remove promptly, uncover and let cool. This process does tend to impart a rather earthy aroma to the kitchen."
I do the above in those disposable roasting pans you can get at the grocery store. (Although I reuse mine.) I've never noticed an aroma of any kind, so don't worry about that issue. Some sources recommend using a soiless seed-starting mix, but Eliott Coleman's seed starting mix does contain soil, and I use his recipe.

Thank you wkdsdenn! I knew I had seen that in an OG related publication but couldn't find it in my OG back issues . . . I'll see if that helps with green alge formation . . and I will plan to reduce the amount I wet the medium after seeding too. . thanks to all. - Matt in Maine