harvested onions
Updated: June 10, 2021

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, tree, and multiplier (walking) onions are hardy perennials.
  • 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 1 inch deep.
    • 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.

Onion problems

Allium leafminer
Flower stalk forms-bolting
White rot

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.  

Harvesting onions

  • 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.