Green onions are a hardy, low-maintenance crop that mainly needs plenty of sun, and rich, well-draining soil. Make sure to amend your garden bed with an inch or two of compost before planting, or use a high-quality potting mix if you’re growing in a container.
You can plant green onions indoors 8-10 weeks before your last frost to get a jump on the season, or direct-seed them outdoors throughout the summer and into early fall. They’ll do best in cool temperatures, but a little afternoon shade can help them take some summer heat. Most varieties are frost-tolerant, but none will tolerate long periods of below-freezing temperatures, so some type of season extension, such as row cover or a cold frame, is a must for most gardeners who want to harvest green onions into winter.
The easiest way to plant green onions is in clumps of several plants together. Make your planting holes about 1/4 inch deep, and drop about 6-8 seeds in each. Space your clumps about 6 inches apart. If you’re starting your seeds indoors in a plug tray, plant several seeds per plug, and transplant the whole clump together once the seedlings have filled up the space. You can transplant green onions outside about 2-4 weeks before your last frost. If you’re direct-seeding, make sure temperatures are at least in the 50s F, to ensure good germination.
Bunching onions can be harvested at basically any stage, but most gardeners wait until they are at least as thick as a pencil. Pull up whole plants, or clip the greens for cut-and-come-again production.
Bunching onions (Allium fistulosum) are biennial, meaning they flower in their second year after overwintering. If you live in an area where the ground freezes, you will need to dig up the plants you want to save seed from and bring them inside somewhere with temperatures between 40 and 50 degrees F, then plant them back out once the ground thaws. If the ground doesn’t freeze where you live, just cover your bunching onions with row cover for the winter. They will send up flower stalks as the weather warms up in spring. Onions are insect-pollinated, so your bunching onions need to be separated from other Allium fistulosum varieties by at least 800 feet in order to save pure seed. If you can’t be sure they are isolated by distance, you can place bags over the blossoms to prevent insects from reaching them, and hand pollinate by moving pollen from one flower to another with a paintbrush. Once the flower heads have turned brown, and black seeds are visible within, clip the heads and shake the seeds out into a container. Make sure the seeds are completely dry before storing in an airtight container in a dark, dry location. Onion seeds typically don’t remain viable more than 1-2 years, so use your seeds soon!