I live in the northeast and am starting a bunch of mine today underneath lights. The following are a few tips gleaned from my own past blunders and successes to help you get started with yours.
Onions & Shallots: Depending on the type, onions are fairly flexible plants that will tolerate a certain amount of rule-breaking on your part. Bunching onions aka scallions tend to be tougher and can be direct sown outdoors in mid-Spring with some frost protection (a cold frame, bottle cloche, or cover).
Still, I usually sow a few indoors underneath lights along with my bulbing types so that I have a variety of plants available at slightly different times. Bulbing types require a longer growing season and must be started indoors early in cooler climates where the growing season can’t support them. The goal is to get seedlings into the ground sometime around or even before the last frost date. I try to start seeds around the beginning of Feb, but I find I can get away with as late as March in my region.
Don’t worry if you miss the early sowing period. Bunching onions are fast growers that can be harvested at any size. For that reason seeds can also be direct sown outdoors again just before the last frost date, and a final time in late summer in order to reap a fall crop.
If you’d rather forego seeds altogether, sets — small onions and shallots that are planted just like regular flowering bulbs — are another viable, albeit slightly more expensive option. I’ve had some luck with sets in the past, but will say that they tend to be more disease prone and are not as hardy and tough as seed-grown plants. There’s always a few duds in the bunch that don’t make it so it’s important to plant more than you hope to harvest.
Leeks: Like bulbing onions, leeks are a long season crop that should be started indoors early when growing from seed. Like onions you can purchase seedlings later in the season so options abound should you miss the late-winter seed-starting window. Mature leeks are tough, hardy plants. I have managed to over-winter mine in the past, which results in beautiful, pom-pom-like flowers (and eventually seed) in their second summer.
- I sow mine in trays, specifically recycled plastic clamshell packaging that I punch drainage holes into with a nail that I warm up over a tea light. Smaller pots with drainage holes (always with drainage holes!) are fine too when you’re doing a small amount of a particular variety.
- Onion seedlings tend to grow a bit leggy indoors. It’s okay to give them a haircut with scissors now and again. Add the tender, young clippings to your meals!