Welsh onion

Welsh onion is a long-lived herb forming dense clumps, usually 0.3–0.7 m (up to about 1 m) tall. The underground part is an elongated bulb whose base is only slightly swollen and never forms a large bulb like the onion; the roots are fibrous and white. The leaves are hollow and tubular (fistulose), 20–50 cm long, dark green with a thin waxy bloom, and become ivory-white at the base; the leaves arise together from the base to form a false stem. The inflorescence is a rounded umbel 3–7 cm across at the tip of a long stalk, made up of many small pale yellow to white flowers; it rarely sets fertile seed and often bears small bulbils in the flower head instead. The plant is propagated mainly vegetatively by dividing clumps, and can be harvested year-round.

According to Kew's Plants of the World Online, Allium fistulosum has no clearly defined wild population and is treated as a cultigen originating in China, with its recorded native area in North-Central and South-Central China; it is a bulbous geophyte of the temperate zone. Many sources hold that it was domesticated very early around Lake Baikal (Siberia), the Altai Mountains and western or north-western China. From East Asia it spread to Japan and Korea and became a familiar crop across Asia, Europe, the Americas and beyond. In Vietnam Welsh onion is grown all year in the lowlands and around cities, from small pots to specialised fields, and is one of the best-selling culinary herbs.

The aroma and activity of Welsh onion come mainly from its organosulfur compounds. When cells are cut or crushed, the enzyme alliinase converts odourless precursors such as alliin (S-allyl-L-cysteine sulfoxide) and isoalliin into highly reactive thiosulfinates (for example 1-propenyl thiosulfinate), allicin, propanethial S-oxide (the lachrymatory factor) and sulfides such as diallyl disulfide; cycloalliin is a stable, non-pungent sulfur form. In addition, the leaves are a notable source of flavonols, especially quercetin and kaempferol (with a relatively high quercetin content compared with many vegetables), together with fructan polysaccharides, adenosine, carotenoids (provitamin A), vitamin C, vitamin K and the minerals potassium, calcium and sulfur. This combination underpins the antibacterial, antioxidant, anti-inflammatory and antiplatelet effects reported in research.

In the kitchen, Welsh onion is the most common herb and seasoning of Vietnamese cooking: sprinkled over soups, noodle soups, pho and rice porridge; stir-fried with eggs, meat and seafood; rolled into bánh xèo and spring rolls; and turned into crisp fried shallots and aromatic scallion oil to finish dishes. In East Asia it is a base seasoning — Japan's negi (for ramen and nabemono), Korea's daepa (for soups and savoury pancakes) and an important onion of China. In traditional medicine it has been used to relieve colds, induce sweating, dispel gas, stimulate digestion and as a mild antiseptic, and the bulb and young leaves are sometimes applied externally. In modern research, a review of Welsh onion seed physiology and breeding (Plants, 2022) summarised the species' characteristics, while pharmacological studies of the genus Allium highlight the antibacterial, antioxidant, anti-inflammatory, anti-obesity and liver/neuroprotective effects of its extracts and sulfur compounds; a Springer review (2025) on the therapeutic versatility of Allium records this potential, with quercetin and the thiosulfinates regarded as two key active groups.

Used as a culinary herb in ordinary amounts, Welsh onion is considered safe and is a familiar food worldwide. However, very large amounts can irritate the gut (bloating, indigestion, heartburn), and because its sulfur compounds may inhibit platelet aggregation, people taking anticoagulants (warfarin, aspirin…) or preparing for surgery should be cautious with concentrated high doses. Some people are allergic to Allium species; notably, onions and other Alliums are toxic to dogs and cats (thiosulfates causing haemolytic anaemia), so they must never be fed to pets. On conservation, Allium fistulosum is grown worldwide and has no clear wild population; in the sources consulted the species has no clear global risk assessment on the IUCN Red List and is not listed in the CITES appendices. Some naturalised populations (for example in Norway) are ranked in national red lists, but overall the species is not globally threatened thanks to widespread cultivation.