Onions are classified into 3 distinct day-length categories: Short Day, Intermediate Day and Long Day.
Short Day (SD)Best suited for southern growing regions of the US atlatitudes between 25-35°.Require 12 to 13 hours of day length to initiate bulb development. Short Day onions are most often planted in the Fall and harvested in the Spring, often referred to as “over-wintered” types.
Intermediate Day(ID)Best suited for latitude range between 32-42°. Require 13 to 14 hours of day length to initiate bulb development.
Long Day (LD) Best for northern growing regions at latitudes between 37-42°.
Require 15 hours or more of day length to initiate bulb development.
Direct Seeding: Seed Short Day types in October, Intermediate Day types in late winter/early spring based on location, and Long Day typesanytime during the Spring months. Soil temperature for germination is 50-75°.
Transplant: Short Day types in November and December. Intermediate and Long Day types in late winter to early spring. In northern areas, transplants of Intermediate types offer earlier maturity compared to Long Day types.Tops can be clipped prior to transplanting for more rigorous root growth.
Spacing: Sow seed at ¼” depth, ½"-1” apart in rows that are 18-30" apart. Thin to 3-5" apart to achieve adequate size bulbs, or set transplants at 4-5" apart.
Growth needs: Grow best in well-drained, sandy soil with high organic matter. Make sure to choose the correct variety for your growing region. Onions need a lot of water, about 1” per week for optimal bulb growth. Avoid overhead watering and watering at night to limit the exposure to bacterial and fungal pathogens. Keeping a good balance of nutrients will help fight off disease while providing their needs to make adequate size and quality.
Harvest: Onion foliage will fall over when ready for harvest as necks soften. Pull the onions out of the soil and let cure in the sun in the field or greenhouse for a few days and up to 1 week.
Short Day types are harvested in the spring.Intermediate Day types are harvested in the summer.Long Day types have a wide range of maturity from 90 to 135 days. Harvest in summer and fall.
Storage: Make sure that onions are fully cured and dried before storage. Clip tops and store in onion bags at 35° F at 65% relative humidity. Short Day types can store 2-3months. Intermediate Day types can store up to 4 months. Long Day types have excellent storage ability of 5 months or more.