Mature plant in flower. Photo: Mark Dimmitt What is it?
| Mature plant in flower. Photo: Mark Dimmitt |
Onionweed is an herbaceous perennial in the lily family (Liliaceae) to about a foot tall and almost as wide. Clusters of long, tapering, round, hollow leaves very much resemble chives or scallions. Leaves sprout after winter rains. Flowers appear in spring. Plants die to the ground during dry season.
The flat flowers are less than a half-inch across and have six petaloid parts, each white with pink center line. They are sparsely distributed on branched spikes to almost two feet tall.
Fruits are 1/8-inch round capsules.Onionweed might be confused with some native onions (Allium spp.) Allium macropetalum (desert onion) is a much shorter plant with leaves rarely more than four inches tall. Taller native onions grow in different habitats than onionweed.
 Closeup of flower. Photo: Mark Dimmitt |
 Dried plant with seed capsules. Photo: USDA |
Why is it a Threat?
Onionweed is an aggressive invasive species. Introduced as an ornamental, it easily escapes cultivation into surrounding unirrigated land. It seeds prolifically and can establish large populations quickly. It is unpalatable to cattle and apparently to most wildlife, so it is very persistent once established. To date it tends to invade disturbed ground, so it is unclear whether it will be a threat to natural communities.
Distribution
Native to southern Europe, Mediterranean Africa, and Western Asia. In the United States onionweed occurs in California (in several coastal southern counties), Arizona, New Mexico, and Texas. Also known to be in Mexico. It is a noxious weed causing problems in Australia. Arizona infestations are primarily in the southeastern corner of the state. A few small infestations have been found in Tucson. Reported but unverified in Ajo.
 Onionweed invading a roadside in southeastern Arizona. Photo: USDA |
In the Sonoran Desert region this weed seems to do best in areas above the desert that receive moderate winter rainfall. A well-established population in suburban Tucson was reduced to only two plants after the severe drought of 2002. Plants have been found in Arizona from about 2000 feet elevation to at least 4500 feet.
History
Plants introduced into the United States in the 1980s may be the founders of our invasion. They were offered for sale in Alpine, Texas and Phoenix, Arizona as early as 1984. Some of the original US plants were collected from a naturalized population near Saltillo, Coahuila, Mexico, where the species was documented in 1930.
 A flowering plant in desert grassland with mesquite. Photo: USDA APHIS Archives, www.forestryimages.org |
What can be Done
Some nurseries offer onionweed as an ornamental even though it is a prohibited noxious weed in many states as well as a federally listed noxious weed. Do not buy it or plant it, and eradicate it if it is established on your property.
Pulling the plant is usually not effective. The top breaks off leaving the tuberous roots underground. They must be dug up by the roots or sprayed with herbicide.
Other common names: pink asphodel, hollow-stemmed asphodel
Links
InvasiveSpecies.org onionweed page
The Nature Conservancy's Invasive Species Initiative
References
(The literature on this species is very sparse.)
