My Adventures, and Misadventures, With Homemade Seed Starting Mix

A few weeks ago I wrote about some experimental seed starting mix that I made. I didn’t have any store-bought seed starting mix, which I normally use. So I made my own by mixing some of my homemade compost with pearlite.

I filled a tray and planted some cabbage, kale and other winter greens. I gave my parents some of the same mix and they planted some sweet alyssum and nasturtium. I waited and waited, but my seeds never germinated. A few of my mom’s nasturtiums sprouted, but I don’t think any of the sweet alyssum germinated. I’m judging it a failure.

What went wrong? My best guess is that the compost had some pathogens that harmed the seeds in some way. When you spread compost in your garden, perhaps it mixes enough with the good bacteria in the soil to negate any harmful pathogens that might be in it. But it was the primary ingredient in my seed starting mix.

Commercial seed starting mixes are sterilized. They don’t have any microbial life in them like garden soil. I suppose I could have heated my mix in the oven to sterilize it. Perhaps I would have had better luck.

Commercial seed mix doesn’t normally include any compost, either. Seeds don’t require any special nutrients to germinate. So compost isn’t really necessary. Instead, commercial seed starting mix usually contain peat moss or coconut coir with some perlite or vermiculite or both.

I’m not a big fan of peat moss. It has antimicrobial properties. Some folks think that’s a benefit for starting seeds. But when you eventually transplant into the garden, you’re adding peat moss to the soil. I think if you do that over and over every spring and fall, for years and years, you’re adding a lot of peat moss to the garden.

I have no idea how much that affects the microbial life in the garden. But I generally want to improve the microbial life in the garden. So I stay away from peat moss.

Coconut coir has some similar qualities. Like peat moss, it soaks up water like a sponge. It provides structure for roots to grow. But it’s not anti-microbial. So I prefer it to peat moss.

A few months ago, my sister-in-law gave us a bunch of garden supplies, including a big sack of peat moss. Despite my reservations, I can’t pass up free. I’m going to mix it with equal parts perlite and vermiculite. Hopefully, I’ll have some better luck this time.

If you don’t want to go through the trouble of making your own, you can always buy some. I don’t normally buy seed starting mix from the garden stores. They usually only sell it in smaller bags. I’d rather buy it in bulk. My favorite brand is Promix. Their BX General Purpose mix includes a biofungicide to ward off harmful pathogens. Lots of commercial nurseries use it.

You can find it online. A seven cu. ft. compressed block costs a little over $100.