Dear Aggie: I usually grow onions from sets, but I’ve heard that you can grow them from seed. What are the benefits of seeds – and are they easy to grow that way?
A: Yes, onions are easy to grow from seed. Like you, I used to grow onions from sets, but have switched entirely to starting onions from seeds and transplants. I find that seed-started onions get bigger and store better than those grown from sets.
Onions are biennial plants. In the first year, they grow from seed into a bulb. After experiencing the cold of winter, the bulb begins growing again in spring to produce a flower spike during the summer. When you start with sets, your onions are already in the second year of their growing cycle. Energy that would otherwise go into the bulb is directed into flowering. As the bulbs are at the end of their growing cycle, they are more prone to sprouting and rot in storage.
In commercial agriculture, onions are typically direct-seeded into fields. Because onions compete poorly with weeds, particularly at the seedling stage, commercial growers often use selective herbicides to keep weeds under control. Less commonly (due to labor costs), commercial growers will start seeds in cold frames, greenhouses, or beds and then transplant them into the field.
In the home garden, direct-seed onions into a well-drained bed that is free of weed seeds. Raised beds are ideal. Onions are good for intercropping with other garden plants, especially early-maturing spring greens. Sow seeds when the soil reaches 50 F. Plant seed ¼ inch deep, ½ inch apart, in rows 12 to 18 inches apart. Thin to 4-inch spacings for large bulbs, 2-inch spacings for smaller bulbs but higher yields, or 1-inch spacings for scallions.
Direct-seeding in a home-garden often means lots of hand-weeding. This can be difficult because onion seedlings are tiny and difficult to see. Furthermore, direct-seeding in the garden may not allow enough time for long-season varieties to mature. For that reason, I limit direct-seeding to scallions (otherwise known as spring onions, which are harvested before the bulb forms). For my main season onion crop, I plant transplants instead.
Start transplants inside under grow lights or in a greenhouse about 8 to 10 weeks before the last frost date. Plant 4 or 5 seeds in each cell, or seed into flats ¼ inch deep and ½ inch apart. If tops grow too tall and begin to droop, trim back to about 3 inches tall with scissors. After hardening off, transplant 2 to 4 weeks before the last frost date. Space 4 inches apart for large bulbs or 2 inches apart for smaller bulbs.
If you forget to plant onion seeds in the spring, you can purchase bundles of transplants from commercial growers by mail order. Some local nurseries offer bundles of onions as well. Another option is to grow pearl onions – which can also be used as sets the following spring. Sow seeds thickly in a block in midsummer. About 2 months after planting, roll down the tops, forcing the plants to form small bulbs. After the tops dry, clip them off, leaving about ½ inch of stem. Cure and store in a cool, dry place as you would any other onion for storage.
Finally, the most important thing to do when selecting onion seeds and transplants is to select the correct variety for our latitude and daylength. Here in Northern New York, select only long-day varieties.
Written by Michael Shane Nuckols, agriculture and natural resources leader