Noxious Weeds and Invasive Plants in Southern Arizona

They say the difference between a wildflower and a weed is whether it is wanted or not. But when it comes to noxious or invasive weeds, there are plenty of reasons not to want these plants growing in Southern Arizona.

From a grass that is causing the destruction of saguaros to a flower that can poison you if you touch it to a seed that hurts more to step on than a Lego brick, learn why some plants have been classified as “noxious.”

In this article, we’ll go over several different kinds of noxious or invasive weeds found in Tucson and the surrounding areas, describe why they are more harmful than helpful, and give you tips for how you can spot and remove them.

What is the reason some plants are classified as noxious?

In the state of Arizona, there are 80 different classifications of plants that are considered introduced, invasive, and noxious. We’ve written before about the importance of using native trees and plants in your landscape whenever possible, not only because they have the best chance of surviving in our area, but also because they benefit the local wildlife, provide the right nectar for pollinators, and work well with the other plants in our area.

When non-native plants are introduced to our area, bad things can happen. For instance, olive and mulberry trees were brought here and planted as popular street trees in the early 20th century. However, they caused such severe allergic reactions that they are now prohibited in many areas of southern Arizona.

According to the University of Arizona Cooperative Extension,

“The spread of noxious weeds on public, state trust and private lands in southeastern Arizona poses risks to native and rangeland animals, threatens biodiversity and native plant species, damages park land and natural resources, and causes economic hardship for farmers, ranchers, and municipalities.”

What are some noxious weeds that should be removed immediately?

See the full list of noxious weeds in Arizona (classified as Class A, B, or C) from the Arizona Department of Agriculture. We will mostly be focused on some of the noxious weeds in Class C in this article. One plant, stinknet, is currently classified as a Class B noxious weed, but will get worse if it continues to spread.

A Class C Noxious Weed is “a species of plant that is widespread but may be recommended for active control based on risk assessment.” In other words, there are enough of them in Arizona to be causing a problem.

In this article, we’ll focus on five:

  • Buffelgrass (Cenchrus ciliaris)
  • Stinknet or Globe Chamomile (Oncosiphon piluliferum)
  • Fountain Grass (Pennisetum setaceum)
  • Goat Head or Puncture Vine (Tribulus terrestris)
  • Morning Glory (Ipomoea spp.)
Buffelgrass clump in Tucson, Arizona.

Buffelgrass is an invasive grass from Africa that is stealing nutrients from native plants and making Southern Arizona’s wildfires much more deadly and dangerous.

Buffelgrass (Cenchrus ciliaris)

You know how some things that sound like a good idea at the time end up being a really, really bad idea in the long run? Buffelgrass is the perfect example of that. It was planted here in the 1970s and 80s as a form of erosion control.

Originally from Africa, it was first introduced in the U.S. in the 1920s and 30s. Here in the Sonoran desert, however, it has quickly taken over many of our native plants. It uses up water, nutrients, and sunlight that other plants (including saguaros) desperately need.

Removing buffelgrass can help the ecosystem in Southern Arizona and reduce wildfires.

Remove buffelgrass whenever possible. By U.S. Fish and Wildlife Service Headquarters – Buffle Grass, an invasive species. Uploaded by Dolovis, CC BY 2.0, https://commons.wikimedia.org/w/index.php?curid=31314769

On top of that, once it grows into a thick clump, each clump can produce THOUSANDS of seeds.

Once the temperatures rise, buffelgrass goes dormant and becomes brown, dry, and brittle. When ignited, it burns at over 1600 degrees F – you can melt most metal at that temperature. The wildfires that have ripped through Arizona – easily spread because of the buffelgrass – destroy native species that take years and years to regrow. Guess what pops right up again after a fire, though? You guessed it – buffelgrass.

Before buffelgrass, our desert was not impacted by wildfires – because our native plants do not naturally burn. Buffelgrass has caused a huge environmental issue to not only the “wild” areas of Tucson, but the city as well.

Suffice to say that buffelgrass is bad, bad news. No wonder there are now organizations devoted to its removal! (Learn more about that under “how you can help”).

Laws or ordinances relating to buffelgrass

  • Pima County ordinance – Requires removal of buffelgrass
  • Department of Environmental Quality Buffelgrass Complaint Form – Report sites with buffelgrass in unincorporated areas
  • Many neighborhoods prohibit weeds or dried grasses, including buffelgrass.
  • Local fire code – Dried grass is often prohibited due to fire mitigation efforts. Learn more from your local fire department.

How you can help

Closeup of Stinknet or globe chamomile flowers.

Despite being new to the area, stinknet or globe chamomile is already causing countless issues in Tucson. By Kevin Thiele from Perth, Australia – Onchosiphon piluliferum, CC BY 2.0, https://commons.wikimedia.org/w/index.php?curid=63516824

Stinknet or Globe Chamomile (Oncosiphon piluliferum)

This plant may look quirky and unusual, like something that would grow on another planet, but it is by no means harmless. It can cause severe breathing problems – so severe that you may be hospitalized!

Stinknet can also cause severe skin reactions when handled. The name “stinknet” comes from the strong odor that sometimes comes from the plant, especially when some of the leaves are crushed.

You can recognize stinknet throughout the year by its dark green leaves that look almost like the top of carrots. Starting in February and continuing through May, the plant has bright yellow, round flowers that have no petals. These ball-shaped blooms may look fun, but the issues they cause are not worth keeping these dangerous plants around.

A collected stinknet plant showing the leaves and yellow ball-like flowers of this noxious weed.

A collected stinknet plant showing the leaves and yellow ball-like flowers of this noxious weed. Photo by Dean Wm. Taylor – Oncosiphon piluliferum JEPS100417. Uploaded by Magnus Manske, CC BY 2.0, https://commons.wikimedia.org/w/index.php?curid=24443662

Not only does stinknet cause severe allergic reactions, it displaces native vegetation, is highly flammable when dry, and the smoke is caustic. Stinknet was one of the reasons the wildfire season in Cave Creek was so bad last year, according to The Foothills Focus.

Stinknet was first spotted in Tucson in 2015 (at the northwest corner of Prince Road and I-10, to be exact), but this South African plant has been taking over Phoenix and other parts of Maricopa County since the 1990s. In the Phoenix area it has spread so much, thanks to its tiny seeds, that it is considered impossible to stop. But here in Tucson, it can still be eradicated.

I’ve seen it growing in my own neighborhood, so I can attest to its presence here. The good news is that it is easy to identify, especially once it is flowering. But the bad news is that it seems to come back no matter what you do.

See photos of stinknet from an Orange County bulletin >>

Laws or ordinances relating to stinket

How you can help

  • Learn how to spot it (this brochure or these slides from Arizona Native Plant Society may help).
  • Help the Tucson Audubon Society track stinknet.
  • If you spot stinknet, it has to be removed or treated with herbicide – several times. Even if it is removed with the roots (which should be attempted), it will grow back. Persistence is key when it comes to getting rid of this invasive plant! Remember to wear gloves when handling stinknet to prevent any kind of reaction.
  • Point it out to others so they know its dangers to the Sonoran Desert (and humans)!

Fountain grass seed heads at sunset with palm trees in the background.

Fountain Grass (Pennisetum setaceum)

Fountain grass is a close relative to buffelgrass. Also from Africa, it is still sold in nurseries here, but is considered a problematic invasive species in Arizona, Nevada, Hawaii, and California. It can be found all over the western United States.

You can recognize fountain grass by its pink or purple flowers that resemble a bottle brush. The plant grows in bunches that can reach 2 to 3 feet tall and turns tan when dried out.

See photos and identifying characteristics of fountain grass from Saguaro National Park >>

Grown as an ornamental plant in Tucson since 1940, the seeds are easily spread by animals, wind, water, and even humans. It is planted in many medians, where the seeds can be easily spread. This can cause it to form dense clusters, which is bad news during wildfire season.

Similar to buffelgrass, fountain grass increases the intensity and spread of fire, which can damage our native plants that are not used to fires, much less high-temperature fires. Fountain grass, on the other hand, is fire-adapted and grows back easily after a wildfire.

Fountain grass has also been known to displace native grasses, block the natural flow of water in our washes, and disrupt animal habitat.

See detailed photos of fountain grass on the Cabeza Prieta Natural History Association website >>

Seed heads of reddish fountain grass in Tucson.

Reddish fountain grass is used in yards and medians throughout Tucson.

What about “sterile” fountain grass?

Some nurseries will sell a version of fountain grass labeled as sterile, which is usually Pennisetum rubrum. While a sterile version is admittedly better than fountain grass that will produce many seeds, these types do still produce viable seeds that could potentially be spread. Consider carefully before planting any kind of fountain grass on your property.

Laws or ordinances related to fountain grass

How you can help

Goats head seeds with sharp burs or thorns that can injure pedestrians and pets.

Goat Head or Puncture Vine (Tribulus terrestris)

Sometimes also referred to as the “horrible evil spiky seeds of death,” goat head is a small plant with yellow flowers. The plant, however, isn’t the nuisance – the seeds are. They can injure pet paws, be painful to step on, and have caused endless bike tire flats.

The seeds have also been known to injure grazing animals – including around their mouths.

See a photo of someone’s shoe who had a goat head infestation in their rental backyard >>

Goat head plant with yellow flowers.

Puncture vine is not native to Tucson but comes from the Mediterranean. It can flower throughout the spring, summer, or fall, and the flowers are followed by fruit with spines. The seeds slightly resemble a goat head (hence its most well-known name). The plant itself is considered a noxious weed in Arizona.

Other less common names for goat head include:

  • Mexican sandbur
  • Texas sandbur
  • Bullhead
  • Caltrop
  • Devil’s eyelashes
  • Devil’s-thorn
  • Devil’s-weed
  • Tackweed
  • Cat’s-head

Laws or ordinances related to goat head

How you can help

Remove goat head on your property by removing the entire plant, including the taproot. You can also spray with herbicides, including pre-emergent herbicides (which are applied before the seeds germinate, in late winter to early spring).

Purple flowers of morning glory, Ipomoea purpurea, classified as a noxious weed in Arizona

Ipomoea purpurea, or morning glory, is classified as a noxious weed in Arizona.

Morning Glories (Ipomoea spp.)

Morning Glories have a complicated history with our area. Up until January 4, 2020, all morning glories (including native kinds) were classified as noxious weeds and it was illegal to sell any type of morning glory seeds or plants to Tucson residents.

It is almost unheard of for native species to be classified as noxious weeds. Now, however, many native kinds have been removed from the list.

Extreme caution should be used before introducing any morning glory species to your yard, however – there was a reason they were classified as noxious weeds for so long!

While we see the beautiful, colorful flowers that make up the morning glory plant, underneath is an entirely different story. Morning glories are notorious for their tangling, extensive root system, which will overtake anything else in the area.

You probably know that the 5 Cs of Arizona are copper, citrus, cattle, cotton, and climate. If morning glories start to grow in a field of cotton, they will overtake the cotton plant, creating a dense, thick mass of vines that are close to impossible to remove.

So you can start to see why, if these flowers are slowly attacking one of the main crops of our state, they might be classified as “noxious,” “invasive,” and “aggressive.”

Ipomoea pummerae, a native species of morning glory in Arizona, flowering in a field.

Ipomoea pummerae, or Huachuca Mountain Morning Glory, is a native species of morning glory in Arizona.

Another downside of morning glories is their ability to poison anyone who gets too close. The poison can enter through your skin if the plant comes in contact with your hands, for example, during pruning or another yard activity. This is called “transdermal poisoning,” and the effects can range from migraine headaches to hallucinations to days-long dizziness…and, in extreme cases, death. Be sure you are familiar with how to avoid morning glory poisoning if you have this plant in your yard. Wear gloves and long sleeves if you plan on working on or near them, and be extra cautious if you have any cuts or scrapes on your hands and arms.

You can read about one gardener’s uncomfortable toxic experience with morning glories in this HGTV article, “What’s the Story, Morning Glory?”

Native Morning Glories

According to the University of Arizona Cooperative Extension, these are the NATIVE species of morning glories in Arizona:

  • Canyon mountain glory (I.barbatisepala)
  • Heartleaf morning glory (I.cardiophylia)
  • Star-glory morning glory (I.coccinea)
  • Crestrib morning glory (I. costellata)
  • Tripleleaf morning glory (I. leptotoma)
  • Pinkthroat morning glory (I. longifolia)
  • Huachuca Mountain morning glory (I. plummerae)
  • Silky morning glory (I. pubescens)
  • Tall morning glory (I. purpurea)still classified as a Class C Noxious Weed, do not plant
  • Spiderlearf morning glory (I.tenuiloba)
  • Thurber’s morning glory (I. thurberi)
  • Littlebell morning glory (I. triloba)

Because of their extensive root systems, morning glories can be extremely difficult to eradicate. Many non-native species are perennials, meaning that they will return every year. The seeds of morning glories have been known to remain viable for up to 50 years. Morning glories tend to grow between other plants, which is one reason they can become so difficult to remove or to even treat with chemicals.

Several types of morning glories are still on the noxious weed list in Arizona and should not be bought, sold, planted, or transplanted.

DO NOT ADD THESE TO YOUR PROPERTY:

  • Garden or common morning glory (Ipomoea purpurea)
  • Grannyvine (Ipomoea tricolor)
  • Ivy-leaf morning glory (Ipomoea hederacea)
  • Morning glory (Ipomoea triloba)
  • Morning glory (Ipomoea x leucantha)

Did you know that sweet potatoes are part of the morning glory family? Luckily, they are on the approved list.

Laws or ordinances related to morning glories

  • Arizona Prohibited Noxious Weed Seeds
  • This website is a little outdated (from 2009), but it reiterates that any morning glory seeds that you see in a store (especially a big box store) are probably a type prohibited here. Always buy native plants and seeds when possible! (Native Seeds/SEARCH is a good local source of seed for the southwest)
  • Several kinds of morning glories are listed as Class C Noxious Weeds on the Arizona Department of Agriculture website.

How you can help

  • The best course of action is not to add any type of morning glory to your property. If, however, you are determined to add some to your yard, use only the approved, native varieties listed above.
  • If you see morning glory seeds for sale in a large box sale, do not buy them. Chances are, they are on the noxious weeds list.
  • Removing morning glory roots can be difficult due to their aggressive nature, but there are tips online if you are trying to remove this noxious weed from your yard.
  • Remember to wear gloves and long sleeves anytime you work in or near any morning glory plants to avoid morning glory poisoning.

In Summary

While the Sonoran Desert and the plants that have grown here for generations are able to withstand hot temperatures, seasons of drought, and monsoons, plants and trees that have been introduced from other areas of the world are making it harder for native plants to survive. Some, like buffelgrass, fountain grass, and stinknet, are creating fuel for wildfires that would ordinarily not reach the desert, leading to the burning and destruction of saguaros and other desert plants that take decades to grow. Noxious weeds steal nutrients and aggressively overtake areas that were previously inhabited by native plants and animals. Invasive grasses are even altering the flow of water in our washes and displacing animals such as toads and frogs.

Aggressive root systems can muscle out plants that are needed for food or shelter by birds, tortoises, and other desert creatures. And plants like goats head can injure people and animals who walk anywhere where their seeds may fall.

It’s always good to know which plants are native to an area, and which can cause problems. We hope this article has made you more informed about what you should plant (and what you shouldn’t) in your yard, and what plants to look out for when you’re in the wilderness of Pima County.

If you’re looking for ideas for native and desert-adapted plants for the Tucson area, check out our Tucson Plants page.

Were you surprised by any of the noxious weeds on our list? Are there any that you would add? Be sure to weigh in on our Facebook group page, Tucson Yard and Garden Resources: Projects, Advice, Inspiration.

Sarah Bohl

Sarah loves exploring all that Tucson has to offer. She enjoys hiking on Mount Lemmon, trying out the new restaurants, meeting new friends at Tucson Meet Yourself, and finding new things to appreciate about the Old Pueblo.

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