Onions are a cool-season biennial that prefers full sun and fertile, well-drained soils. For more information on onions, click here.
VARIETIES: Evergreen White Bunching is a good, mild green onion. Utah Sweet Spanish, Fiesta, Wall-Walla and Sweet Sandwich are all excellent bulb onions. Many of these varieties come as seeds, sets or transplants.
Soil Preparation: Before planting, incorporate 4 to 6 inches of well-composted organic matter in the top 6 inches of soil. Fertilizer should be worked into the soil prior to seeding at 4 to 6 cups per 100 square feet. For more information on soil preparation, click here.
- After seeds emerge, thin to 3 to 4 inches apart in rows 8 to 16 inches apart.
- Green onions can be left at closer spacing and thinned as they are picked.
- High-density planting will reduce bulb size unless adequate water and nutrients are supplied.
- Onion seeds can be planted in mid to late August for overwintering.
Irrigation: Onions require regular watering during growth for best production. Moisten the soil thoroughly to a depth of 18 inches every seven days. Water needs are critical since rooting depth in onions is shallow. Drought stress during growth will decrease yield, reduce bulb size and affect flavor. Excess water as the crop matures delays curing and may cause storage problems. For more information on watering, click here.
Fertilization In addition to the fertilizer used at planting, onions need additional nitrogen fertilizer to produce optimum yields. Place a ½ lb side dressing of nitrogen fertilizer per every 100 square feet in late May and June. Do not fertilize after mid-July as extra nutrients stimulate late season growth and reduces storage potential of dry onions. For more information on fertilizing, click here.
Harvest: Onions vary in their maturity times. Green onions may be harvested as early as 50 days after seeding. Bulb onions generally require 100 to 120 days to mature. Leave bulb onions in the ground until the top falls over. Once this occurs, lift the bulbs from the soil but leave them in the garden to dry for two or three weeks. The tops will help protect the bulbs from the sun. When fully cured, the onion skin should be papery and the roots dry. Onions store best in cold, dry conditions.
COMMON PESTS AND DISEASES: Thrips, onion maggots and rot are common pests to onions. These problems can managed with insecticides, crop rotations and proper watering. For more information on controlling pests and diseases, click here.