Seed Starting Mix -- Trust but Verify

[feldon30]

Well I'm officially behind on getting ready for my gardening season here in the Piedmont of South Carolina. After 2 years co-gardening at a friend's house down the road, we've gone our separate ways and I am now starting from scratch in my own smaller yard. I've built three 4x16 beds with 2' walkway all around. I'm a tomato fiend and if I could, I'd grow all 6,000 varieties of tomato just to compare them all, but I'll have to settle with a dozen or so per year.

My go-to seed starting mix is Premier Pro-Mix BX. Fortunately I've found a place (A.B. Poe's Farmer's Exchange) that sells it and at a reasonable price to boot. After starting seeds last year, I had some leftover mix which had been moistened, so I returned it to the 5 gallon bucket I'd been storing the unspoiled mix in and put the lid on very loosely. The bucket had been left undisturbed since that time. I opened the lid last week and I didn't notice any unpleasant smell, so I began my usual seed starting process.

After two days, I was surprised to note that nothing had sprouted. Worse, the moistened soil was now putting out the unattractive odor of a damp basement. After two more days, a few very scraggly looking seedlings appeared. After a week, the smell was almost unbearable and the seedlings weren't much better.

Sadly I've dumped out everything and am starting over today. I'll be bleaching out the flat tray (and that 5 gallon bucket) for future use. Lesson learned: Don't try to be frugal with seed starting mix.

My documented seed starting process is already exhaustive, but I'll be adding some more cautionary notes:

• When in doubt about seed starting mix, add a bleach solution or moisten with boiling water to kill any undesirable microbes.
• Don't try to save moistened seed starting mix for the next year.

[jal_ut]

Have you tried the little peat pellets?

[applestar]

Oooh, something to think about! The BX formulation contains mycorrhizae so insufficient ventilation in the sides and bottom of the bucket, or temperature during storage, etc. may have resulted in die-off and subsequent upsurge of undesirable fungi and bacteria. I suppose you could end up with "spoiled" product if the store you are getting them from sold you unsuitably stored old inventory, too.

Thanks for the heads up.

...I wonder if the ProMix website or customer service has any storage tips for their product...?
...I read somewhere that myco shelf life could be anywhere from 6 months refrigerated to one year -- not sure if this would apply to potting mix since the potting mix itself could potentially provide the substrate for the myco to sustain themselves, but I would imagine there is a max/min optimum temperature range.

[imafan26]

Actually if the seed starting mix was never wet it would not have soured. Starting mix that is wet needs to be used in a few weeks. Orchid mixes that are sold ready to use have been soaked and mixed so they need to be used within 6 months or I have to soak them again in bleach. When you add water to the mix, you start growing fungi and bacteria that feed on the organic matter and it starts to break down. If it is in a sealed container it will be more anaerobic and more acidic. In a container exposed to air it could be more alkaline or acidic depending on how wet the container is. When it smells bad it is usually more acidic.

[feldon30]

Thanks for the comments and suggestions.

I think imafan has a diagnosis. Once the soil is wet, it seems to start a countdown clock. After that, the mix must be sterilized with boiling water or bleach solution. I'm sure the promix itself was fine.

I have not tried peat pellets. I actively discourage the use of peat *pots* due to the peat wicking moisture AWAY from roots and being an attractive medium for mold and mildew, as well as the pots not breaking down when buried, thus constraining roots. But the pellets are probably fine once you cut the webbing off.

[imafan26]

Some people have good results with those pellets but I never have. The seedlings always dampen off. I just don't know how to manage the water with them. So, I go with what works for me which is to make a starter mix and use a 4 inch communty pot. It takes up too much of my space to plant in individual cells and I have too much die back in very small pots like the six packs. I have never tried planting in cell trays so I don't know how that would work. Transplanting is not a problem for me so having 8 or 50 plus seedlings in a 4 inch community pot saves me space until I have to plant out and I have a lower mortality rate that way. I move the trays off the nursery bench a couple of weeks after I transplant anyway. Even they would take up too much of my 8 ft x 4 ft nursery bench if I let them hang out longer.

[GardenThrive]

Moisture, and darkness are a breeding ground for mold, mildew, and bacteria. So once it's wet and stored there goes the original sterilization of the seed starting mix. I had to find out the hard way too. Lost a lot of tomato seedlings and time a few years back. Now any left over moist mix I just dump into my compost bin. I just considered it a "learning experience".

[Rue Barbie]

You can also solarize your 'tainted' mix. Make sure it's quite moist and put it in a large shallow metal tray and cover with glass, maybe 1 to 2 inches deep, and put it in the full sun. Don't use any plastic because that will warp and give off fumes. The next day stir it with a tool of some sort. It can get quite hot (which is what you want). I forget to what temp. There will be lots of condensation, and the mix will dry out because of that. I usually leave it out for a few days since I don't have a specific menu for doing it. But it kills any seeds - if you are reusing mix - and most if not all bad guys.

[imafan26]

Actually it will keep a couple of weeks just fine in an open container exposed to air and if it is just damp not soggy. If you use it within that time it is fine. Other wise it would have to be dumped in the garden or spread out in the sun on a tarp to dry it out again. I only mix a five gallon bucket at a time and I make 3 or 4 batches when I am planting or transplanting. I try to finish everything I have mixed before I call it a day. But I have kept left over mix for a few days and sometimes a couple of weeks and it was fine out in the open bucket on the lanai. I keep my mix on the dry side. If my mix is really wet, I will either add more ingredients to make it drier or I will use it up and rinse out the bucket for the next time.

[feldon30]

So in response to imafan26, I start tomato and pepper seeds in sterilized 72 cell trays and then pot up to 4" pots, backfilling with more Pro-mix. I typically plant two seeds per cell and I've generally had good results even with separating them. I rarely lose a plant.

[jal_ut]

I have been getting those 9 oz clear plastic drinking cups and a bag of potting mix. Fill the cup with potting mix and plant two seeds. Whatever comes up can grow in these cups till time to plant out. No fussing with potting up. I don't thin these. If two come up, two can grow. Oh, do punch a couple of holes in the bottom of these cups for planting.