The Q&A Archives: Growing Onions

Question: I started my onions from seed last September and planted them the first week in November. They are growing well, but are starting to bolt. I've been cutting back the flower. I have 2 questions:
1) Last year, even though I cut off the flower stem, the large hollow stalk kept growing. How do I prevent this?
2) When do you suggest I stop watering the onions? When they start to yellow and I bend over the stalks, or just before harvest?

Answer: Flowering of onions can be caused by several things but the most common reason is temperature fluctuation. An onion is classed as a biennial which means it normally takes 2 years to go from seed to seed. Temperature is the controlling or triggering factor in this process. If an onion plant is exposed to alternating cold and warm temperatures resulting in the onion plant going dormant, resuming growth, going dormant and then resuming growth again, the onion bulbs prematurely flower or bolt. The onion is deceived into believing it has completed two growth cycles or years of growth in its biennial life cycle so it finalizes the cycle by blooming. Flowering can be controlled by planting the right variety at the right time.

Flowering causes a decrease in bulb size as well as a central flower stalk which enhances decay during storage. Once the onion plant has bolted, or sent up a flower stalk, there is nothing you can do to eliminate this problem. The onion bulbs will be edible but smaller. Use these onions as soon as possible because the green flower stalk which emerges through the center of the bulb will make storage almost impossible.

The best way to insure success is to either plant the onion seed from October 1 until November 15 or plant transplants from January through February.

The size of the onion bulb is dependent upon the number and size of the green leaves or tops at the time of bulb maturity. For each leaf there will be a ring of onion; the larger the leaf, the larger the ring will be. The onion will first form a top and then, depending on the onion variety and length of daylight, start to form the bulb. Onions are characterized by day length; "long-day" onion varieties will quit forming tops and begin to form bulbs when the daylength reaches 14 to 16 hours while "short-day" onions will start making bulbs much earlier in the year when there are only 10 to 12 hours of daylight. A general rule of them is that "long-day" onions do better in northern states (north of 36th parallel) while "short-day" onions do better in states south of that line.

Onions are fully mature when their tops have fallen over. After pulling from the ground allow the onion to dry, clip the roots and cut the tops back to one inch. The key to preserving onions and to prevent bruising is to keep them cool, dry and separated. As a general rule, the sweeter the onion, the higher the water content, and therefore the less shelf life. A more pungent onion will store longer so eat the sweet varieties first and save the more pungent onions for storage.

The closer to harvest the more water the onion will require, but, once the tops yellow, you can stop watering.

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