Allium fistulosum is a perennial herbaceous plant in the Amaryllidaceae family cultivated for its hollow green leaves and mild onion flavor. It is commonly known as the spring onion, Welsh onion, bunching onion, or Japanese long green onion. The genus name Allium is the classical Latin term for garlic and onions, while the species epithet fistulosum derives from Latin fistula, meaning “pipe” or “tube,” referring to the plant’s characteristic hollow leaves. Although the English name “Welsh onion” might suggest an origin in Wales, the word “Welsh” is an old English outdated botanical term from the German word "Welsch" meaning something of Romance or Celtic origin, or in other words, something foreign. The plant is actually believed to have originated in northern China and Siberia before spreading widely throughout East Asia and Europe. In Japan it is known as negi (ネギ), a staple aromatic vegetable in Japanese cooking.

The plant grows in clumps rather than forming large bulbs like the common onion (Allium cepa). Its cylindrical leaves emerge from a small basal swelling and can reach considerable length, making them well suited for repeated harvesting. Allium fistulosum thrives in temperate climates and is widely cultivated in China, Japan, Korea, and many other regions. It plays an important role both agriculturally and culturally, appearing frequently in traditional cuisines where its fresh, pungent aroma enhances soups, noodles, stir-fries, and grilled dishes. As a food ingredient, Allium fistulosum is valued for its bright, savory flavor and aromatic sulfur compounds. The leaves are commonly sliced raw as a garnish or cooked briefly to add depth to broths and sauces. In Japanese cuisine it appears in dishes such as ramen, soba, and yakitori, while in Chinese cooking it is often paired with ginger and garlic as a foundational aromatic combination.

The edible parts contain a range of organosulfur compounds that develop when plant tissues are cut or crushed. These compounds arise from enzymatic reactions involving sulfur-containing amino acid derivatives and contribute to the plant’s characteristic sharp aroma. Welsh onions provide dietary fiber, vitamin C, vitamin K, and small amounts of folate and potassium. Allium fistulosum contains various flavonoids such as quercetin derivatives, sulfur-containing molecules like allyl sulfides, and various phenolic compounds that have been studied for antioxidant and antimicrobial properties. The plant also contains compounds that may influence cardiovascular health and immune function, although most research has been conducted in laboratory or animal models.