Onionweed plant USDA
Onionweed is a pretty plant, and it was once grown as an ornamental in Arizona.

NPS Photo

Invasive Plant Species: Onionweed

Onionweed (Asphodelus fistulosus), a member of the Lily family, is a species that prefers wet washes or riparian areas in the Park. It is a pretty plant and was grown as an ornamental prior to it being listed on the federal noxious weed list. Areas in Australia with rainfall similar to ours, there are onionweed infestations that have damaged rangelands. This signals potential negative impacts in our ecosystem.

How did it get here?

Onionweed was new to the Park in 2006. It was most likely planted as an ornamental by homesteaders in the Rincon Creek area. Seeds were spread into the Park during the 2006 flood event.

What is the threat?

Onionweed takes moisture, minerals, and nutrients from the soil, denying them to native plant species. The thick mats of leaves provide dense shade, blocking sunlight, and discouraging germination of native plant seeds.

Management goals and strategies

Onionweed is easily removed by hand. During the spring, we visit the infested sites and remove any new plants we find. We are constantly on the look out for new infestations.We would like to work with our upstream neighbors and private land owners along Rincon Creek to remove it from their property. We believe that, with the help of our park neighbors, onionweed can be eliminated from the Park.

Onionweed flower J Randall
Onionweed flowers in late winter. The flowers have six petals with a dark stripe on each.

J. Randall The Nature Conservancy

Progress to date

Each spring, we survey the Park for this invasive weed. When seed sources from outside the park boundary are eliminated, the Park will be rid of onionweed.

How to Identify Onionweed

Onionweed has many hollow, three-sided cylindrical leaves, up to 14 inches long, coming from the base. Many flowering stems emerge from the base and can be branching, reaching up to 28 inches. Flowers are white to light pink, a half-inch to an inch across, with a dark vein down the length of each petal. The roots are fibrous and form dense mats and no bulb. Onionweed is similar to the native wild onion except onionweed does not smell like an onion and does not form a bulb.

In the Rincon Creek area, onionweed is found in sandy soil where the canopy is open.

How you can help

Onionweed can easily be removed from your yard by hand pulling. Don't plant the seeds or relocate any plants you find. If you think you may have onionweed on your property and would like someone from the Park to confirm it, please send a picture to sagu_invasive@nps.gov.

Last updated: July 7, 2023

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