Answer: Double bulbing is caused by a combination of genetics and stress, explains John Navazio, vegetable breeder at Garden City Seeds in Victor, Montana. Older varieties tend to be more likely to double bulb than modern varieties, since this trait is now selected out during the breeding process, he adds. Plant a variety such as Buffalo that's adaptable to Montana and less likely to form double bulbs, he advises. If an onion seedling is stressed during the first two months of its life, it will be more likely to double bulb, adds Navazio. To avoid stressing your bulbs, grow them in raised beds or in light, sandy-loam soil. Amend the soil with 15 pounds of compost per 10-foot row before planting and keep moisture steady, recommends Navazio.
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