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|Onion stalks with bulblets, or miniature onions grown at the top of the stalk.|
Tree onions, topsetting onions, walking onions, or Egyptian onions, Allium ×proliferum, are similar to common onions (A. cepa), but with a cluster of bulblets where a normal onion would have flowers. Genomic evidence has conclusively shown that they are a hybrid of the common onion and the Welsh onion (A. fistulosum). However, some sources may still treat the tree onion as A. cepa var. proliferum or A. cepa Proliferum Group. Tree onion bulblets will sprout and grow while still on the original stalk, which may bend down under the weight of the new growth and take root some distance from the parent plant, giving rise to the name "walking onion". It has been postulated that the name "Egyptian onion" is derived from tree onions being brought to Europe from the Indian subcontinent by the Romani people.
The phenomenon of forming bulblets instead of flowers is also seen in garlic and other alliums, which sometimes may also be referred to as top onions or tree onions. The bulblets are usually marble-sized, between 0.5 cm to 3 cm in diameter.
Many tree onions are very strong flavoured, although some cultivars are relatively mild and sweet. The underground bulbs are particularly tough-skinned and pungent, and can be quite elongate, like leeks, or in some types may form bulbs up to 5 cm across. Young plants may be used as scallions in the spring, and the bulblets may be used in cooking similarly to regular onions, or preserved by pickling.
- Friesen, N. & M. Klaas (1998). "Origin of some vegetatively propagated Allium crops studied with RAPD and GISH.". Genetic Resources and Crop Evolution 45 (6): 511–523. doi:10.1023/A:1008647700251.
- Ruttle, Jack. "Confessions of an Onion Addict". National Gardening Association. Retrieved 17 February 2011.
- Chandoha, Walter. "Egyptian Onions are the Easiest" (PDF). Cornell University Cooperative Extension. Retrieved 26 April 2011.