The germination of onions is slow at 6 to 7 C, the optimum germination temperature range is 10 to 35 C, and the maximum temperature is 40 C. The onion crop is adapted to a growing season with air temperatures at 13 to 24 C. Low temperatures early in the season are desirable with higher temperatures after bulb formation. The onion is tolerant of frost but seedlings are generally only tolerant down to -1 C.
Seedlings will be ready for transplanting in about 40 days when 10-15 cm. 7-10 days before transplanting gradually reduce watering and remove the shade. Transfer onion transplants into the garden when they reach pencil size in diameter (about ¼ inch). Transplants are pulled and bundled in groups of 50-80 plants and tied with a rubber band. Bunching onions may be planted in early August and overwintered.
Bunching onions also may be planted in the spring for harvest in late July and August. The plantbed soil should have a pH range of 6.0-6.5 for optimum growth.
It is possible tunnel houses could be used to extend the season on bunching onions. Large Spanish onions are usually grown from transplants as they take a long time to mature.
It is critically important that seedbeds be irrigated regularly to develop a good plant stand. Applying 1/10 inch of water several times a day may ensure consistent soil moisture. Plants are ready for harvest in about 8-10 weeks.
Onion cultivation
