Answer: Your onions are perfectly safe to eat. You can harvest now before they bolt (send up a flowering stalk). If onions are allowed to bolt, they will redirect all of their energy into producing a flower instead of maintaining a bulb.
A day or two before harvesting, bend the foliage down to ground level to stop additional leaf growth. When harvesting, lay the onions out in the sun to help them dry, or lay them out on racks in a protected area if the weather is rainy. When the leaves are completely yellow you can cut them off, leaving an inch or two of stem attached to the bulb. Walla Walla's don't keep too long, so plan to enjoy them within a few weeks of harvest.
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