Many gardeners have experienced buying onion sets or seedlings from the local garden store, growing nice tops, but never seeing a nice, large onion bulb. Why is this?
Bulbing onions are day-length sensitive. Their bulbs begin growing only after the number of daylight hours has surpassed a minimum quantity. Onions may be "long day", "intermediate day", "short day" or "day length neutral". "Long-day" onions, commonly grown in the Northern United States, produce bulbs only after there are 14 or more hours of daylight. "Intermediate day" varieties, require only 12–13 hours of daylight to begin forming bulbs. "Short-day" onions, which are commonly grown in the Southern United States and other mild-winter areas, are planted in the Fall and form bulbs in the Spring. They form bulbs when daylight hours reach 11–12 hours and are increasing. Day length neutral varieties can be planted in a wider range of latitudes.
It's also important to plant onions at the right time. The size of the bulb is determined by the amount of leaves that the plant has grown when it begins to form a bulb. It's important that it have enough time to grow before day lengths signal it to start forming a bulb. The ideal planting date will be different for different varieties and different areas. In cold winter areas, onion sets and seedlings can be planted outdoors about 4-6 weeks before the last frost. In Southern latitudes, short day onions are planted in the Fall for harvest in the Spring.
Garden centers and large home improvement stores don't necessarily carry the varieties of onions that do well in the local area. The onions may only be labeled "Yellow Onions", making it impossible to know what variety they are. Some nurseries do sell named varieties of seeds or seedlings or will call their suppliers to find out what the variety is. However, you may need to order from a seed catalog to know what variety you're planting to ensure it is suitable for your area. Seeds are best started in pots indoors 6 or more weeks before planting. They can be planted very close together as they don't mind their roots being disturbed when they're transplanted. Aim for 1/4 inch spacing, but don't worry if some are closer together. When planting, the root balls can be put in a bowl of water and swished around to remove the dirt so that the roots can be disentangled without breaking them.
Short day varieties:
Intermediate day varieties:
Long day varieties:
Day neutral varieties:
Please share your experiences growing onions. What varieties have you tried and which did well in your area? What are the best planting dates in your area?