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Allium is the genus of herbaceous perennial plants commonly known as flowering onions. In addition to supermarket onions, other commercially significant members of this group include garlic and scallions. Allium have edible, bulbous roots and leaves, characterized by a very pungent smell.
Although species exist in many areas, most are found in the northern hemisphere. Allium bulbs vary in size from tiny chives to large leeks, and mature plants likewise vary in height. They typically grow from rhizomes, or short root structures, but some species can grow bulbs from vertical tuber roots or horizontal runner roots called stolons. Most grow to flowering maturity from bulbs that survive year to year, courtesy of their unique structure of layered membranes of fibrous flesh with high water content.
Most allium have from one to 12 leaves, usually long, either flat or tubular. They can be straight or curled. Rarely, the leaves will have a stalk, and they are always attached directly to the root bulb. Often, leaves may die off when flowers appear.
Atop a leafless stalk, or scape, multiple flowers form into an umbel, or ribbed spherical umbrella shape. Six rigid petals in two symmetrical whorl patterns surround one female style and six stamen stalks with reproductive pollen. Some species, such as garlic, produce bulbils, or tiny bulbs within the umbel, sometimes replacing the entire flowerhead. While most plants propagate from flowers which have fruited to a small black seed, some species reproduce with offshoots from the mother bulb.
Allium are cultivated throughout the world as different crops, but different types favor varied soils and climates. Leeks prefer well-drained, mineral-rich soil; some lilies are entirely hydroponic. Indeed, they are common plants not only in vegetable gardens, but ornamental plots as well, such as the popular Allium hollandicum, an onion whose flowering bulbs are a dramatic stalk broadly crowned by bright purple. This type has been bred in a variety of sizes and flower colors. On the other hand, some species of the genus are invasive gardening weeds that attract destructive insects, such as the larvae of moths and butterfly.
The sulfoxide compounds that many of these plants produce are a natural deterrent to predators and garden pests. Some allium species are toxic to dogs and cats. Humans consume onions, but sometimes suffer from watery eyes when handling raw plants. In rare medical cases, allergy to these plants can develop. Ironically, allium cepa, an extract of the common grocery store red onion, is sometimes used as a homeopathic remedy for hay fever and other allergic reactions.