Planting onion facts
- Hardiness: Hardy (can withstand heavy spring frosts). Bulb and green onions are biennials (two years required to complete life cycle) treated as annuals. Egyptian onions, multiplier onions, and shallots are hardy perennials (planting information below in next section).
- Planting: Plant onions in early spring as soon as you can cultivate your garden. Use sets, transplants, or seeds in spring for bulb onions and for green or bunching onions. Planting too early, with exposure to cold temperatures, can cause seed stalk development.
- Sets: Plant onion sets in the spring for early onions, and in the fall for perennial or multiplier types of onions. Sets are planted with the pointed end up and covered with 1 inch of soil.
- Transplants: Either buy bunches of onion transplants or start your own by sowing seeds indoors about 8 weeks prior to planting. Tops that become too long can be snipped. This method gives the best results for most Maryland gardeners.
- Direct seed - sow seed ¼ inch to a ½ inch deep and cover lightly with fine soil. Keep the tops of your planted rows moist until you see plants emerge. Plant seeds thickly, then thin, using thinnings as green onions.
Full sun requires at least 6 hours direct light/day; prefers 8 - 10 hours/day. Green onions will grow with 5 hours of sun per day.
- Days to maturity: 85 - 120 (mature bulbs).
- Spacing: Standard 1" - 8" in-rows x 12" - 24" between row; wide row and block planting 4" x 4" equidistant spacing. Plant close, then thin, using thinnings as green onions. Regardless of how thickly they are sown or planted, onions should be thinned to a spacing of 2 inches apart in the row if you intend to harvest green onions, and 4 inches apart if you intend to harvest moderate size bulbs, and 8 inches apart for large bulbs.
- Fertilizer needs: High requirement for nutrients, either from soil organic matter or fertilizers. Apply fertilizer before planting, use starter solution for transplants and side-dress 1 to 2 weeks after bulb enlargement begins. Refer to Fertilizing Vegetables for details.
- Approximate yield: 20 to 25 lbs. per 10-foot row.
Flower stalk forms-bolting
Growing and care of onions
- Onions are often grouped according to taste (mild and strong-flavored), color (white, yellow, and red), and use (storage or freshly eaten). Globe varieties tend to keep longer in storage.
- Onion cultivars also have different requirements as to the number of hours of daylight required to make a bulb. If the seed catalog lists the variety as long day, it sets bulbs when it receives 15 - 16 hours of daylight and is adapted to Northern summers. Short-day varieties set bulbs with about 12 hours of daylight and are used in the deep South for winter production. There are also "intermediate" cultivars. Mid-Atlantic gardeners can experiment with all groups, although long day and intermediate types will probably perform best.
- Do not hill up the soil on onions, as this encourages stem rot. Bulbs emerge above the soil as they enlarge.
- Watering - Ensure ample moisture, especially after bulbs begin enlarging. Onion bulb size is directly proportional to the amount of water applied to the onions during the growing season.
- Weeding - Onions compete poorly with weeds because of shallow root systems. Hand-pulling weeds or shallow cultivation is necessary; do not hill up the soil on onions. Maintain a weed-free bed and apply an organic mulch to maintain soil moisture.
Growing perennial onions (Egyptian onions, multiplier onions, shallots)
- Shallots and multiplier onions (aka potato onions) are perennial types of onions- Allium cepa var. aggregatum- that are typically grown as annuals in Maryland.
- Perennial onions produce a cluster of small bulbs and allow you to extend the growing and harvesting season as they can be planted in spring or fall.
- Plant needs and growing techniques are very similar to those of large bulbing onions. Plant growth is temperature-dependent and bulbs enlarge as day length increases.
- There are multiple planting approaches to try. You can buy shallot bulbs with 2-6 cloves depending on the variety. Cloves planted in late March-April may be less likely to bolt but those planted in late September through mid-October may produce larger bulbs in spring and their greens can be harvested in late fall once they are well-established.
- Individual bulbs of multiplier onions can be planted in September/October or in late March-April. After planting (pointed side up), cover shallot cloves and multiplier onion bulbs with 1-2 in. of soil (fall) or ½ in. (spring). It’s also fine to direct sow shallot seed in the garden or indoors to produce transplants that are then planted in the garden.
- Both types are generally thinned to a 4-6 in. spacing for maximum growth, but spacing requirements may vary by cultivar.
- Protect fall-planted perennial onions from weeds and extreme cold with a mulch of shredded tree leaves or straw.
- Gently dig or pull shallots and multiplier onions when their tops start to dieback and fall over.
- You can divide and store some to re-plant in the fall.
- The plants go dormant in summer and can get overrun by weeds if not mulched well.
Egyptian walking onion (aka tree onion) is another type of perennial onion- Allium cepa var. proliferum- that self-propagates. It has tall stems with bulbils at the top that flop over allowing the bulbils (tiny bulbs that form where leaves connect to stems) to contact soil, grow roots, and start new plants. Egyptian walking onion is grown primarily for its greens. Egyptian walking onions are planted 9 in. apart and can be left in the ground throughout the summer as long as you manage weeds around the plants.
- Harvest green onions when tops are 6 inches tall.
- Bulb onions should be harvested when about two-thirds of the dried tops have fallen over. Careful handling to avoid bruising helps prevent storage rots.
- Onions may be pulled and left in the field for several days to dry, then cured, with tops intact, in a well-ventilated attic or porch out of the direct sun for 1 to 2 weeks.
- Cut off tops about 1 inch above the bulb prior to storing.
- Thorough curing will increase storage life.
Storage and preservation
- Onions can be kept under very cool (32 degrees F), dry (65%-70% RH) conditions for up to 6 to 7 months.
- Onions can be stored in ventilated bags or boxes for 2 months or more in a cool basement or garage. Warm temperatures induce sprouting and sub-freezing temperatures will injure bulbs.