Onions vary considerably in several ways, prompting gardeners to make selections carefully. There are differences in color, pungency or sweetness, bulb size and shape, and suitability for storage. Bulbing onions are sensitive to the hours of daylight they receive and are grown successfully only where they are best adapted—specific long-day or short-day varieties in northern or southern latitudes and intermediate-day types are best in areas in between as in Sonoma County.
Green onions, also known as scallions or bunching onions, do not respond to day length and may be planted March-October for a continuous, staggered harvest.
- Bulbing onions are typically white, yellow, or red. White types tend to be mild and do not store for long periods. Yellow ones are either sweet or pungent and best for cooking. Reds are very crisp, often eaten raw, but do not store well.
- Highest quality bulbs are produced when early growth stages occur during cool weather.
- Classification as long-day, short-day, or intermediate indicates the number of daylight hours required to make a bulb. Seed gives the greatest choice of variety.
- Intermediate-day types are best suited to Sonoma County’s mild weather and hours of daylight.
- Sets are onions that were planted from seed last year and are best for producing a quick crop of green onions rather than mature bulbs since they are prone to bolting. Sets smaller than ¾ in. are most likely to succeed.
- Sow seed close together in warm soil. Thin to use very young sprouts or transplant 2 in. apart in rows 12 in. apart. Shorten ¼-½ of root length to encourage denser rooting when transplanting.
- Harvest at ¼-½ in. in diameter, about 60-80 days from seeding to maturity.
- Provide ample nitrogen fertilizer in amended, loose, weed-free soil with regular watering.
- Spread mulch over soil surface to maintain moisture over shallow roots.
- Remove flower stalks in mature Japanese or Welch bunching onions if you want plants to form clumps that will continue growth and slowly expand. Continue to harvest individual green onions from clump edges.
Growing and Harvesting Bulbing Onions
- Prepare well-drained, friable, fertile soil by mixing in 3 in. of compost or well-rotted manure at least a month before planting.
- Consult seed packets for details on required day length, optimal planting time, and days to harvest. Individual varieties have very specific requirements.
- Seed early varieties indoors November-January to transplant after soil warms above 50 degrees for late spring or summer harvest. Seed or transplant late varieties January-March for late summer or fall harvest. Germination takes 2 weeks at 50 degrees, 3-5 days at 75 degrees.
- Purchase starts in spring from a nursery, alternatively, in 6 packs or bundles of 25-50 seedlings. Slightly trim roots and tops before transplanting to encourage strong growth.
- To plant, insert seedlings into a narrow hole in moist soil; tamp firmly around stems.
- Space plants about 6 in. apart for large bulbs, closer if you thin early shoots for green onions. Allow 20 in. between rows.
- Keep beds weed free, being careful not to disturb roots when cultivating. Mulch with straw, compost, or other organic material as soon as shoots are established.
- Provide uniform moisture to produce fully formed bulbs. Feed lightly with an organic fertilizer every three to four weeks. Excess fertilizer can cause double or split bulbs.
- Withhold irrigation when leaves turn yellow and bulbs are slightly raised out of the soil. Bend over leaves and allow them to wither.
- Lift bulbs out of the ground with a garden fork to use fresh or leave bulbs in unwatered ground to dry for up to 2 weeks.
- Cure onions in a single layer in a dry, shaded location with good air circulation. Brush off soil after 1-2 weeks and cut stems 1-2 in. above bulbs.
- Store in a cool, dry area on flat trays or hang in mesh bags away from potatoes, apples, and bananas that release ripening ethylene gas and can cause onions to decay.
- Sonoma County Master Gardener Vegetable Planting Summary.