Grow Guide Conditions:
Onions are a hardy crop adapted to a wide range of growing conditions. Day length is important in variety selection of bulbing onions. Growers north of 38 degrees should plant long day varieties as early as possible to achieve full size prior to maturity. Growers south of 38 degrees should plant short day or intermediate day varieties as these need less day length to reach full size. See our onion chart for more information about latitude ranges and planting dates. Onions are an acid sensitive crop and grow best on fertile, well drained soils with a pH of 6–6.8. Shallow roots require light, frequent irrigations when plants are young. Stop watering when bulbs have reached full size and the tops drop.
Grow Guide Seed:
Begin direct seeding spring crops 4-6 weeks before the last frost; fall crops 6-12 weeks before the first fall frost. Germination occurs when soil temperatures are 50–85°F. Direct seed in a 2 inch wide band, ¼ inch deep, ½ inch between seeds, allowing 12-18 inches between rows. Thin to one plant every 3-5 inches for bulbing types, 1-2 inches apart for scallions. To start indoors, sow spring plantings indoors 7-9 weeks before the last frost for transplanting 3 weeks after last frost. Scatter seeds across an open flat or 4 inch pot, ¼ inch deep, ½ inch between seeds. Harden off for two weeks prior to transplanting. When seedlings are 10-12 weeks old, tease apart crowded plants and place one plant every 3-5 inches in a 3-4 inch deep trench, filling soil back in to the original soil level. Keep consistently watered and well weeded.
Grow Guide Pests:
Botrytis Leaf Blight, Gray Mold, Downy Mildew, and Purple Blotch are fungal diseases of onions that can be challenging to control. Practice 3–4 year crop rotations for all onion family crops (onions, shallots, leeks, garlic) and thoroughly compost or turn under all crop debris. Cover with floating row cover in areas with high onion maggot or thrips pressure. Use insectary plantings to shelter beneficials to help control pests as they become troublesome.
Grow Guide Harvest:
Onions for use in the green stage are harvested as soon as they attain edible size. For onions that are to be stored, harvest when tops are between 80–100% broken down. Pull onions and field dry in windrows if possible, or bring into covered area to dry. Cure for 3–4 weeks ensuring that neck tissue is dry before topping and storing. Store in the refrigerator or a cool, dry storage room above freezing. Warm onions gradually when removing from storage to avoid sweating and minimize deterioration.