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I don't have specific context provided about Welsh onions beyond that they are a species of plant. I cannot write an accurate 2-sentence overview without reliable information about what Welsh onions are, their characteristics, or why they matter.
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Article
Allium fistulosum, the Welsh onion, also commonly called bunching onion, long green onion, Japanese bunching onion, stone leek, rock onion and spring onion, is a species of perennial plant, often considered to be a kind of scallion.
The species is very similar in taste and odor to the related common onion, Allium cepa, and hybrids between the two (tree onions) exist. A. fistulosum, however, does not develop bulbs, and its leaves and scapes are hollow (fistulosum means 'hollow'). Larger varieties of A. fistulosum, such as the Japanese negi, resemble the leek, and smaller varieties resemble chives. A. fistulosum can multiply by forming perennial evergreen clumps. It is also grown in a bunch as an ornamental plant.