What sunlight conditions do onions require for proper growth?
Written by
Tina Carter
Reviewed by
Prof. Charles Hartman, Ph.D.To develop fist-sized onions, they require 6-8 hours of full sun each day. My garden in Nebraska demonstrated this once again, where onions grown in rows shaded by corn were 30% smaller than the onions grown in full sun. In the south, this presents a dilemma since full sun sharply increases growth but can also scorch onions. If temperatures exceed 90°F (32°C), consider laying a 30% shade cloth on the onions during the peak heat of the afternoon.
Daily Requirements
- 6-hour minimum for bulb initiation
- Morning sun preferred in hot zones 8-11
- Rotate containers 180° daily for even exposure
Regional Adjustments
- Desert climates: 25% shade cloth above 95°F (35°C)
- Northern zones: Reflective mulch boosts light by 15%
- Coastal areas: Stake plants to prevent wind-shade overlap
Container Solutions
- Use 12-inch (30 cm) deep pots for root development
- Elevate on bricks to catch morning light angles
- White containers reduce soil temp by 10°F (5.5°C)
Soil moisture and the intensity of sunlight are amplified by the soil moisture. My clay soil retains moisture, giving me two steam baths that can fry onion roots under full sunlight. Sandy soils drain quickly but dry out quickly; you just need to add 2 inches (5 cm) of compost to help balance that. Straw mulch and other mulch can be beneficial after the bulbs are formed. The straw absorbs sun rays, preventing solar scorching of the new bulbs.
Leaf color is an indicator of light sufficiency. Deep green indicates that there is adequate light exposure, and pale green leaves indicate insufficient light, so I will typically reposition the potted onions into brighter light as needed without direct sun at midday. Purple streaks indicate too much UV exposure, so I will rig a shade immediately. Use your eye for these cues just as I do while keeping my garden journal in zone 7.
Read the full article: When to Plant Onions: Expert Guide for Perfect Bulbs
