The Walla Walla onion is considered a long day onion (long-day onions need about 14 to 15 hours of daylight to bulb-short-day onions need 10 hours of daylight). The Walla Walla onion is a medium to large, yellow fleshed, sweet onion. Mature bulbs vary in size from 5-10cm (2-6”). Start seeds indoors as early as you can just to ensure that you have the biggest transplants available for spring planting. Seeds can be planted either in the spring for fall harvesting or in August for over wintering. Walla Walla onions are hardy to -23 °C (-10°F). Spring planted Walla Walla sweet onion seeds result in bulbs that are not quite as mild as the overwintered ones and may not size up as well. They are not a long-term storage onion so enjoy them fresh.
Onions can be grown from seeds, sets, or transplants. If starting from seeds, start them indoors 8-10 weeks before the last expected frost date in your area. Transplants can also be purchased from a nursery or garden center. Onions prefer a sunny location and well-drained soil that is rich in organic matter. Soil pH should be between 6.0 and 7.5. If planting sets or transplants, space them 10-15cm (4-6”) apart in rows spaced 30cm (12”) apart. If planting seeds, sow them 1cm (½”) deep and 2.5-5cm (1-2”) apart in rows spaced 30cm (12”) apart.
Keep the soil moist but not waterlogged during the growing season. Onions prefer consistent watering throughout the season. Fertilize with a nitrogen-rich fertilizer every 3-4 weeks during the growing season. Onions are ready for harvest when the leaves turn yellow and start to fall over. Carefully lift the onions out of the soil and lay them out in a dry, well-ventilated area to cure for 1-2 weeks before storing.