Answer: You are on the borderline for growing onions like Vidalias. Onions can be categorized as "long-day" or "short day", depending on the amount of daylight that stimulates them to form bulbs. If you plant short-day cultivars too far north, the plants will rush to form small bulbs. These bulbs won't grow as large as they do in areas further south where summer day lengths are shorter. I don't know of anything you can do to stimulate the plant to prouduce larger bulbs; just be sure you are providing them with good growing conditions--well-drained soil, adequate fertility and water. I wonder if you planted them behind a taller-growing vegetable that shaded them a bit in the afternoon, whether that might stimulate them to form slightly larger bulbs? I don't know if it would or not, but it could be worth a try.
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