Here in Zone 7, January is the time for starting onion seeds indoors: two months before the last expected frost. I got mine planted a little over a week ago, and the shoots are already green and unbending towards the sun. (When an onion seed first germinates, its stem is folded over with both ends in the ground, but slowly one of those ends reaches toward the light, making it into a tall, skinny spear of a leaf.)
It’s very easy to start a lot of onion seeds indoors, because the plants will get moved outside before they’re very big and so don’t need much soil. Also, the plants are very easy to separate, so you can grow them in a flat instead of individual cups. I use the flats that clementines come in at the grocery store.
I plant just one of these flats each spring. I sow about 120 seeds and expect to get about 60 plants. That will fill 20 feet of row in the garden. (I plant onions 4″ apart in rows 2′ apart.) 60 onions is about all my family of four can use between harvest in late July and when the onions start rotting in storage around December.
To fill a clementine flat, I use almost a gallon of potting mix. I don’t use commercial potting mix. It’s too expensive and doesn’t contain all the microorganisms that are important for healthy plants. Instead, I mix my own, using about 5/8 gallon garden soil, 3/8 gallon compost, and 1/4 cup all-purpose organic fertilizer. (See my recipe for mix-your-own organic fertilizer.) I mix it all together in a 2-gallon bucket.
Once I’ve filled the flat with my potting mix, I scatter my 120 or so seeds on top. Then I gently mix them into the top 1/4″ of soil with my finger or a pencil. I don’t water, since the soil is already moist. I just cover with plastic or aluminum foil and set in a warmish place until the first seeds begin to germinate, usually in five or six days. Then I remove the cover, move the flat to a sunny south-facing window, and begin to water each morning (with room-temperature water).
That’s all there is to it until planting time arrives at the end of February!
Oh, and in case you’re wondering, the variety I grow is Australian Brown. It’s an heirloom variety of bulbing onion that did very well for me last year. I had 56 plants in 19′ of row, and the harvest weighed in at 23 lbs. I didn’t have to do anything special to store them, just put them in a well-ventilated spot in the coldest room of the house (maybe 60°F), and they kept fine through December. I could probably keep them a lot longer if I made a little extra effort, but by the time these start rotting, I can just use green perennial onions for cooking–or wild garlic tops from the yard!