University of Maryland Extension


onions ready to be harvested

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 better.


Plant onions in early spring as soon as you can cultivate your garden. 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. Rows are spaced 12-24 inches apart. You can also plant onions in a block with each plant 4-8 inches apart.

  • 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.
  • 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.  


  • Fertilizing - Heavy feeder. Fertilize transplants after planting. Sidedress (apply fertilizer next to plants in the row) 1 to 2 weeks after bulb enlargement begins with a balanced fertilizer.
  • 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. Shallow cultivation is necessary; do not hill up soil on onions, as this encourages stem rot. Maintain a weed-free bed and apply an organic mulch to maintain soil moisture.  

Common Problems


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 in a well-ventilated attic or porch out of direct sun for 1 to 2 weeks. Tops may be left on or cut off; if cut, leave at least 1 inch of the top when 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. They can also be frozen by washing, chopping to desired size and blanching for 3 minutes then put into ice water or sauté in a small amount of oil and bring to room temperature before freezing. Drain (if you have blanched or to minimize oil) and spread on trays and put into a freezer. Once frozen, transfer the onions to a freezer bag.


A source of Vitamin C and fiber.

Preparation & Use

Store onions in a cool, dark, well ventilated place to prevent mold. Peel and bake whole or cut into pieces and use raw in salads or in cooked dishes.

Back to top

Maintained by the IET Department of the College of Agriculture and Natural Resources. © 2020. Web Accessibility

University programs, activities, and facilities are available to all without regard to race, color, sex, gender identity or expression, sexual orientation, marital status, age, national origin, political affiliation, physical or mental disability, religion, protected veteran status, genetic information, personal appearance, or any other legally protected class. If you need a reasonable accommodation to participate in any event or activity, please contact your local University of Maryland Extension Office.