How to Grow Onions from Seed (Beginner Start to Finish)

There are three ways to plant onions: from sets (tiny dormant bulbs), from already-sprouted starts, or from seeds you sow yourself. Sets and starts give you a head start, but they cost more and you are stuck with the one or two varieties a shop happens to carry that year.

Seeds win on choice and value. A packet costs a few cents and gives you any variety you want, from sweet Walla Wallas to long-keeping yellow storage onions. The plants you raise yourself also tend to be sturdier than bought starts. That is what these healthy seedlings grew into.

Onion seeds will technically sprout in garden dirt or straight compost, but you are after the best germination and the strongest seedlings, and a real seed-starting mix gets you there. The light, fluffy texture holds moisture without packing down around the shallow roots.

You can buy a bag or make your own by blending regular potting soil with about 25 percent coconut coir or peat moss and roughly 5 percent perlite, then sifting out the big chunks. That is the mix being crumbled here, with the white flecks of perlite mixed through.

Almost anything works as long as it is at least two inches tall and has drainage holes. A standard 1020 tray with cell inserts, like the one shown here, is the easy choice because it keeps each seedling tidy and drains well.

Onions are forgiving here. You can crowd several seeds into one cell and pull the little clump apart at planting time without much fuss. Reusable cell trays pay for themselves over a few seasons.

Fill each cell with the seed-starting mix and press it down gently to settle it, then top off and skim the surface level. You want firm contact for the seeds, not compacted soil.

Before sowing, water the soil from below by setting the tray in a shallow bath for a couple of hours. Bottom-watering wets the mix all the way through without blasting seeds around or crusting the surface.

Onion seeds go in shallow, about a quarter to a half inch deep. You can poke a small hole and drop a seed in, set the seed on the surface and push it down with a blunt tool, or scatter seeds and sprinkle a little more mix on top to bury them.

Pick whichever method feels natural. Here the seeds are being placed cell by cell into the moist mix. Multi-seeding a few per cell is fine since you will separate them later.

Pop a humidity dome over the tray to make a little greenhouse, then keep the soil between 68 and 78 degrees Fahrenheit. That is slightly cooler than tomatoes and peppers like, and it is the sweet spot for onions.

At that temperature the seeds sprout fast, usually within a week. You will see thin green loops push up out of the mix like the young sprouts shown here. Take the dome off and move the tray off the heat mat as soon as they break the surface.

This is the tip that trips up most beginners. Onions decide when to form a bulb based on how many hours of light they get. Give tiny seedlings 14 to 16 hours like you would tomatoes and you trick them into bulbing way too early, before the plant can support it.

Keep the light under 10 hours for short-day varieties and under 12 for long-day ones while they are this small. They do not need intense heat or light at this stage, so an inexpensive shop light on a timer is plenty.

Once the seedlings reach a few inches they flop over and get unruly. Trim the tops back to about three or four inches with clean scissors and they stand up straight and put energy into their roots instead.

You may also spot the onion loop, where the growing tip stays stuck in the soil. Do not yank it, the roots are too fragile and the whole plant will lift out. Just snip the loop and pull the top free. Keep the seedlings moist but never soggy, hold the schedule for the full two-plus months, and they are ready to plant out in spring.