How to Germinate Sweet Spanish Onions Inside

Fill a seed-starting flat up to 1/2 inch from the top with moist, sterile seed-starting mix.

Sprinkle the sweet Spanish onion seeds over the soil's surface and cover them with a 1/4-inch layer of the mix. Lightly tamp the soil surface, and moisten it with a water-filled spray bottle so the seeds stay put and don't wash away.

Move the tray to a warm area that's about 64 to 77 degrees Fahrenheit. Check the medium in the tray daily, and if needed, spray it with water. Keep the soil moist -- not wet -- throughout the germination period. Expect the seeds to germinate within seven to 10 days.

Place the tray in a sunny, south-facing window as soon as the seeds sprout. Periodically rotate the tray so the seedlings grow upright and don't have to reach for the light.

Trim the seedlings with scissors if they get too tall and might fall over, shortening them to 4 inches. You might have to trim the young plants multiple times.

Harden off the seedlings for two weeks before transplanting them. Place the tray in a sheltered area outside for two hours a day. Gradually extend the duration that the seedlings stays outside so that at the end of the two weeks they're acclimated and can stay outside overnight.