Walla Walla onions (Allium cepa) are so sweet and juicy that some people bite right into them, eating them like apples, and they are a great treat to grow in your own backyard. Originally from Spain, they were first planted in the U.S. near Walla Walla, Washington in the early 1900s. One of the most popular sweet onions, it is usually ready to harvest 125 days after planting. This cultivar prefers the long days of northern climates, but it can be grown throughout the U.S. with proper care. While most onions are grown strictly for culinary use, the Walla Walla produces a large, rounded cluster of white blooms at the end of a long stem.
Walla Walla onions are normally planted from sets, or small bulbs, in early spring. Plant them in loose, well-drained soil in an area that receives full sun. The onions prefer mildly acidic soil with a pH range of 6.1 to 6.5. Place the sets just beneath the surface, with the top of the bulbs still visible. Space them 6 inches apart in rows that are 8 inches apart. In USDA zones 5 to 10, they can also be planted in fall, overwintered in the ground and harvested the following summer.
Slow, deep watering helps to promote healthy roots. Give the plants about an inch of water per week. You can also add a thin layer of mulch to reduce water evaporation, but keep mulch at least an inch away from stems. You should keep the area weed-free to prevent competition with the onion’s shallow roots. Apply a balanced fertilizer, such as 10-10-10, in early summer for better growth. The hollow stems of the plant can be harvested for use as green onions when they are 8 inches high. The entire plant can be harvested at any time, but the bulbs underground will have reached full size when the tops die back. The mature onions sometimes grow as large as 4 inches in diameter. After digging up the onions, let them dry in full sun for about four days. Then move them to screens or racks in a dark, dry location. Walla Walla onions don’t keep well; use them within a month.
The most common problems faced by Walla Walla onions are fungal spots, blights and blotches. They also suffer from root, stem, crown and collar rots. Proper spacing between plants helps prevent fungal infections. Avoid overhead watering, and water in the morning to give the foliage time to dry before nightfall. Use fungicides if cultural controls aren’t sufficient. Despite the onion’s sweet deliciousness, deer and rabbits usually leave it alone, so pest control should not be a major issue.