Onions from Seed
Starting your onions from seed is a great way to try interesting varieties you'll never find as sets at most hardware stores. The important thing when buying onion seed is to know whether you need "long day" or "short day" onions. Basically, short day onions grow better in the south (zone 7-10) and long day onions are better suited for northern climates (zone 6-frigid). I'm talking about onions that make bulbs, not green onions/scallions. Those guys are much less picky.
Bulbing onions grow relatively slowly, so I plant seeds indoors early - about 11 weeks before my average last frost.
How do I calculate that? I use the handy Skippy's Vegetable Planting Calendar App! Yes, I built it and am so proud I can't stop talking about it.
In my experience, onions don't need a ton of personal space during this phase. As long as I'm careful teasing them apart and space them nicely in the garden, things turn out just fine.
I've used shallow take-out trays before but this time I'm going to try my new "20 row flats". Of course, I didn't think to buy the "shallow germination trays" that coordinate, so I'm doing a bit of surgery to make them compatible with my standard 1020 seedling trays. Not a pretty solution but it will work for now.
Fill the tray with damp sterile seed starting mix. I drew a little trench down the center. That is where I aimed my seeds. It does not need to be perfect. You can see that my seeds are not all exactly spaced.
Then I gently tucked any stray seeds into the trench and misted them with the spray bottle. This makes sure that the seeds have good soil contact without splashing them all over. Then I set the onion tray into the 1020 tray and let it soak up water from below.
A layer of plastic baggie creates a bit of greenhouse effect, keeping the soil from drying out too fast. I remove the plastic after most of the seeds have sprouted.
This tray is under my more powerful T5 lights so I can have them at a greater distance. When growing under T8's (standard shop fluorescents) you need to have the lights just a few inches away from the plants/seeds.