Unlike other plants that are grown in the spring, onions are planted in the late fall and winter. Frost does not harm the plants, but they are susceptible to other problems when they are not planted or harvested properly.
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Onion seeds, sets and transplants are typically planted three to four weeks before the last frost in Northern, colder climates. In the South and warmer climates, onions are planted in mid- to late fall. Depending on the variety, onions mature in 55 to 300 days.
The onion bulbs are ready for harvest when the tops begin to lean over. This can take up to 300 days for some varieties, such as Walla Walla. Knock the tops down to prevent the bulbs from continuing to grow. Immediately harvest bulbs that prematurely produce seeds. Proper harvesting will help prevent bulb rot and ensure ripe onions.
Although frost will not hurt onion plants, soil diseases, thrips and onion maggots do pose a problem for onions. Prevent these issues by moving onions to a new location in the garden each year.