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Grow & Care - Onion

Fast Facts

Light Requirements Full Sun
Temperature Adaptations Hardy: Plant as soon as soil dries out in the spring
Acidity (pH) Tolerance 6.8 to 6.0 pH
Salinity (Ec) Tolerance 1.2
How Pollinated Insects
Growth Habits Biennial grown as an annual

Additional Information

Fertilizer Requirements

Add 2 lb. of 16-16-8 (or similar) fertilizer per 100 sq. ft. at planting. Four to five weeks after planting, add 1/2 cup of ammonium sulfate (21-0-0) per 10 ft. of row. Repeat this application 4 or 5 weeks later. Water immediately after fertilizing.

Water Utilization

Onions use about 1" of moisture per week. Use a mulch around onions to conserve moisture in the soil.

Plant Development and Care

Day length determines when onions will develop bulbs. They are classified as long day or short day. Long day varieties, recommended for Northern Mountain West growing conditions, begin bulb formation when days reach 14 to 16 hours with temperatures above 65?F. For large bulbs, plant as early as weather and soil conditions allow.

Seed: all onion types may be started from seed, but for long-term storage, seeds are preferred. Seeds may be sown indoors 4 to 8 weeks before transplanting into the garden after all threat of frost is past. Sow seed 1/4 inch deep in a light, soilless mixture. Grow in a cool location with as much sunlight as possible.

To start outdoors, sow seed as early as ground can be worked. Plant one to three seeds per inch 1/4 inch deep. When seedlings are 2" to 3" inches high, thin them to 2" apart. Thin again when 6" inches tall to a 4" spacing.

Sets: onion sets are small, dormant buds, ready to be planted outdoors. Plant about 2" to 3" deep and 1/2" apart. Large sets mature quickly and may be used as green onions four to six weeks after planting. Plant weekly to extend the green onion harvest. Sets are not usually sold by variety name, but as "fresh onions"or "transplants." Onions grown from sets will produce seed heads. Pluck them off to push growth into bulbs, not seed. They are not generally suitable for storage onions.

Transplants: before planting, work a high phosphorus fertilizer such as 10-20-10 into the soil 2 to 3 inches below the depth where the onions will be planted. Set transplants out to 3" to 4" apart and bury about 1" deep. Do not plant transplants when temperature will likely fall below 20? F.

Onions grow best in loose, well-drained soil. Onions are well adapted to raised bed gardening.

Cultivate carefully to avoid damaging tender roots. Keep well weeded.

Allow sufficient space between plants for bulbs to develop. Remove garden debris which can harbor insects that transmit disease. Check plants every few days for damage and destroy infected plants.

Germination Time (Days)

136 days at 32 F
31 days at 41 F
13 days at 50 F
7 days at 59 F
5 days at 68 F
4 days at 77 F
4 days at 86 F or higher

Seed Germination and Temperature Range

40 F to 60 F

Common Fertilizer Deficiencies

Excessive nitrogen fertilizer delays maturity



There are 13 rings and 13 leaves on a perfectly grown onion--never more.