Globe chamomile in Arizona: Your guide to the invasive weed


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May 20, 2023

Globe chamomile in Arizona: Your guide to the invasive weed

If you are out on a nature walk and smell something a bit off it may be from a

If you are out on a nature walk and smell something a bit off it may be from a yellow-flowered weed with a name that fits the bill: stinknet — an invasive weed native to South Africa that has popped up in Arizona — harming the Valley's ecosystem.

It grows quickly and, if left unchecked, it could destroy much of the desert landscape it blankets.

Stinknet migrated from South Africa to southern California before spreading to Arizona in 1997. Stinknet, also known as globe chamomile, began to spread rapidly in metro Phoenix and started to spread outward toward southern Arizona.

The weed arrived innocuously enough with delicate, yellow globular flowers and feathery carrot-like leaves. It was imported as an ornamental plant and decorative filler for floral arrangements. But don't be fooled by its attractive and vibrant colors — stinknet is detrimental to new environments.

Stinknet is easily recognized by its dark green "carrot-like" leaves and unique rounded flowers. The leaves have a pungent odor. Stinknet can be highly allergenic, both dermal and respiratory, according to the Arizona Native Plant Society.

It's part of the sunflower family and can grow up to 2 feet tall.

Stinknet outcompetes other plants, taking away the balance of soils in large, dense, continuous patches and creating vast amounts of flammable material that allow wildfires to spread faster, according to the Mohave Desert Land Trust.

Michael Chamberland is the assistant agent for Urban Horticulture with the University of Arizona Maricopa County Cooperative Extension.

"(Stinknet) crowds out our native plants," Chamberland said. "Then those plants are no longer available for the wildlife and the ecosystem that would be happening."

The U.S. Department of Agriculture is raising awareness about the weed as summer approaches.

Chamberland noted that Phoenix is seeing a larger influx of stinknet due to wetness from previous seasons.

"(Stinknet) plant is a winter grower that grows with the winter rain," Chamberland said. "So, when we get a good, wet winter, like the one we just had, it's good for both the native wildflowers and sometimes can be described as a super bloom when our native wildflowers are all blooming."

After a very wet winter, Phoenix natives may see a lot more stinknet as this plant loves the wet conditions, ultimately allowing it to grow larger and produce more seeds, Chamberland said.

Stinknet primarily grows between November and March. After its growing season, the plant dries, dies and becomes prime wildfire fuel.

The foul scents are not the only negative effect of stinknet. After it flowers from February to May, it turns from yellow to brown, exposing the flower's seed, which can become a source for wildfires over the summer.

When the plant, flower, stems and foliage of the stinknet dry, it turns a brown paper bag color. The stinknet leaves behind dried bush and stems that can fuel wildfires, Chamberland said.

Phoenix residents should be aware when traveling as stinknet seeds can get attached to clothing and shoes. The weed can travel, spread and develop in more areas when seeds are blown by the wind or attached to people.

Arizonans can do their part to prevent stinknet from spreading by killing the plant before it dries and turns to seed in the early summer. Resistant to some herbicides, this weed is tough to eradicate. Digging it out by hand or using professional chemical control can stem infestations.

Stinknet plants can live months through the winter before eventually turning to seed, Chamberland said.

"So if you're on top of (monitoring the growth of stinknet), that's a good window of opportunity to get out there and kill stinknet (before it turns it turns to seed)," Chamberland said.

Stinknet does not set seed until all the flowers on the top of the plant have blossomed.

There are many everyday tools from the shed or toolbox ideal for eliminating stinknet. Chamberland suggests that the best way to go about it is with a hula hoe, rake or weed whacker.

University of California Weed Science suggests "milestone, Capstone and glyphosate are all highly effective at controlling stinknet, but only before the plants have flowered. Often if herbicides are applied after flowering, stinknet can finish flowering before the herbicides have killed the plant."