Days to Maturity: Approximately 100-110 days from transplant to harvest (often 105 days typical; intermediate-day type suited to Utah's latitude for reliable bulbing).
Sun Needs: Full sun with at least 6-8 hours of direct sunlight daily—essential for large bulb size, maximum sweetness, and proper development in Utah's sunny climate.
Season Type: Cool-season crop with bulbing triggered by day length; transplant in early spring (late March to mid-April in the Salt Lake City area) when soil is workable and temperatures are consistently above 40°F.
Plant Habit & Support: Upright, single-stemmed plants grow 18-24 inches tall; no support needed—space 4-6 inches apart in rows 12-18 inches apart for optimal bulb growth and airflow.
Other Notes: Sweet onions like White Sweet Spanish are more susceptible to storage issues than pungent types; watch for thrips, onion maggots, or downy mildew—organic mulch (like straw or compost) helps retain moisture and suppress weeds; harvest when tops yellow and fall over for best quality and flavor (sweet Spanish types store 2-4 months in cool, dry conditions with proper curing).
Best Planting Method: Transplant when soil is workable and dry enough to avoid compaction in early spring. Space plants 4-6 inches apart in rows or beds; plant at the same depth as in the pot or slightly deeper for strong roots. Amend soil with compost for fertility and excellent drainage—raised beds are ideal in heavier or alkaline Utah soils to prevent rot and improve performance. Organic mulch (such as straw, compost, or grass clippings) around plants retains moisture, moderates soil temperature, and suppresses weeds.
Watering Needs: Provide consistent, even moisture (about 1 inch per week) especially during bulb formation—drip irrigation is best in dry Utah conditions to keep foliage dry and reduce disease risk. Avoid overwatering or letting soil dry out completely, as both can cause splitting or smaller bulbs; mulch helps maintain steady moisture during warmer periods.
Fertilization Needs: Incorporate compost or balanced organic fertilizer (e.g., 10-10-10) at planting for nutrient-rich soil. Onions are moderate feeders—apply a light side-dressing with a balanced or higher-phosphorus formula midway through growth if needed, but avoid excess nitrogen late in the season to prevent soft tops and encourage solid bulb development. Harvest when about half the tops have fallen over—gently pull or dig, cure in a dry, shaded spot for 1-2 weeks (until outer skins are papery), then store in a cool, dry place for 2-4 months of good keeping quality. It's great for containers (with spacing adjustments for smaller bulbs).