Botanical Name: Allium cepa
Days to Maturity: 110–130 days
Native: Exists only in cultivation
Hardiness: Biennial grown as an annual
Variety Information: Large, 3 ½"–6", up to 2 lbs., pale yellow, globe-shaped bulbs with coppery wrapper; 12"–18" tops. 'Yellow Sweet Spanish Utah' is a bulbing, yellow, long-day onion with a moderate storage period.
Type: Long-day onion (Learn more)
When to Sow Outside: 4 to 6 weeks before your average last frost date, or as soon as soil can be worked; when soil temperature is at least 45°F.
When to Start Inside: RECOMMENDED. 10 to 12 weeks before your average last frost date. Transplant outdoors 4 to 6 weeks before your average last frost date. The earlier the start, the bigger the bulb. Ideal soil temperature is 60°–85°F.
Days to Emerge: 7–15 days
Seed Depth: ¼"
Seed Spacing: A group of 2 seeds every 6"–8"
Row Spacing: 12"–16"
Thinning: When 2" tall, thin to 1 every 6"–8"
Harvesting: When onion tops have fallen over and turned yellow or brown, they are ready for harvest. Harvest in the morning, lifting onions with a garden fork. Dry them in the garden in the sun for 2 to 3 days, lightly covering the bulbs with straw, or the tops of other onions to prevent sunscald. Cure onions for 3 to 7 days in a dry area with good air circulation. Once dry, cut the roots to 1/4", and the greens to 1" to create a seal, preventing decay.