Walla Walla onions, named for the Washington State area where they have been grown for over 200 years, are known for their large size and sweet taste. Legend holds that a lone French soldier found a sweet seed in Corsica and brought it to Washington in the 1800s. Farmers noted the onion's hardiness and began to grow it extensively. Over time, the Walla Walla onion was born. The schedule for planting Walla Walla onion seeds may vary by region.
In Walla Walla, which is largely in USDA hardiness zone 7, farmers plant onion seeds in the fall. The timing of the planting allows seeds to hibernate underground through the winter before they grow in spring in preparation for the upcoming summer harvest. When planted in the fall, Walla Walla onions will be ready for harvest beginning in mid-June.
Like most varieties of onions, Walla Wallas prefer well-drained soil, though they can be grown in poor or heavy soils that have been amended to improve conditions. Improve soil by adding 1 to 2 inches of organic material to the soil, then working it into the ground to 8 inches. Clear the ground of pebbles, roots and other debris. Each seed should be planted 1 inch into the ground, approximately 4 inches apart. Plant large crops of onions in rows, spaced 10 inches apart. Water frequently to keep soil moist, and weed often to prevent unwanted plants that will steal nutrients from your onions.
Grow Walla Walla onions indoors by planting seeds in 4-inch-by-6-inch pots in late February or early March. Bury seeds 1/4 inch deep into the soil. Germination takes two to three weeks. Seedlings may be transplanted outdoors in April or May. Onions do not have deep root systems, and they may be grown successfully in pots prior to outdoor transplanting.
The tops of Walla Walla onion plants will grow long enough to flop over onto the ground in summer. Stop watering the onions and break the bulbs off the plant. Lay them on the ground so they may cure for about a week. Store bulbs in a cool (but not freezing) location inside a dark box or bag. Do not store Walla Walla onions with apricots, apples, bananas, tomatoes or any other kind of fruit, because they will begin growing again around the ethylene gas fruit produces.