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Purple and Iberian Starthistle

Centaurea calcitrapa, C. iberica
Updated Feb 28, 2025
 
1

Make a Positive Identification

  • Purple and Iberian starthistle plants live for one, two or more years (short-lived perennial).
  • Seeds germinate fall through spring. Young plants are spiny rosettes and grow deep tap roots.
  • Throughout summer and fall plants grow stiff, spreading stems about 1-3 feet tall.
  • The purple flowerheads have long, sharp spines on bracts below the flowers.
Species: Purple and Iberian starthistle
Purple starthistle plant

Purple starthistle plants quickly dominate invaded areas. Single plants may produce many hundreds of seeds.

Species: Purple and Iberian starthistle
Purple starthistle leaves and stems

"Centaurea calcitrapa" by Alex Heyman is licensed under CC0 1.0

Upper leaves are narrow and undivided. Lower leaves are deeply divided, often finely hairy.

Species: Purple and Iberian starthistle
Purple starthistle flower and spines

There are about 25-40 purple flowers per head. Spines about 0.5-1 inch long cover the bottom of the flowerhead.

Species: Purple and Iberian starthistle
Purple starthistle rosette with other weeds

Seedlings germinate in the fall or early spring. They form spiny rosettes with deep taproots. Rosettes are often hairy and shoots become smooth with maturity.

Species: Purple starthistle
Purple starthistle fruits

D. Walters and C. Southwick, Table Grape Weed Disseminule ID, USDA APHIS PPQ, Bugwood.org

Seed is hairless and less than ¼ inch long. Seeds are streaked white or brown. Most seeds are viable and germinate within a year. Buried seeds can remain viable in the soil for 3 years.

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LOOK-ALIKE: YELLOW STARTHISTLE
Species: Yellow starthistle
Yellow starthistle flower and spines

"St Barnabys Thistle" by John Tann is licensed under CC BY 2.0.

Yellow starthistle (Centaurea solstitialis) has gray-green leaves and stems. The stems of both plants look winged. The plants look different when they flower. Yellow starthistle has yellow flowers.

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2

Purple and Iberian Starthistle Benefits

  • Purple and Iberian starthistle flowers attract pollinators, especially bees.
 

Purple and Iberian Starthistle Risks

  • Purple and Iberian starthistle spread quickly with seeds. They are difficult to control.
  • They degrade wildlife habitat and livestock forage if left un-managed.
  • Purple and Iberian starthistle will displace desired vegetation and dominate an area if not controlled.
  • Their spiny flowers are painful and limit human activity where they grow.

 

Risk Card
Does it cause harm?
Adults & Children
Some
Property
High
Pets
Some
Annoyance
High
Environment
High
Action Recommended
 
3

Take Action

If you have purple or Iberian starthistle on your property, take action to control it right away.

Do I need to take action?
Yes. Remove individual plants and small patches of purple and Iberian starthistle. Both small and large patches require several years of attention to control.

What if I do nothing?
Purple and Iberian starthistle dominate an area when left unmanaged.

 
4

Prevent Purple and Iberian Starthistle

Purple starthistle seedling

Steve Dewey, Utah State University, Bugwood.org

 

Look for Newly Germinated Seedlings and then Rosettes
  • Look for seedlings and rosettes in the late fall, winter, and early spring.
  • Take note of their location and plan to treat plants.
  • Continue to monitor the area each year. Control it as needed.
Gloved hand using metal brush to clean shovel

Weston Miller, Oregon State University

Remove Dirt from Shoes and Equipment
  • After working or traveling in an area with purple or Iberian starthistle, clean your boots and tools. Use a wire brush to remove all soil that may contain seeds.
  • If you drive into an area with purple or Iberian starthistle, clean your vehicle. Seeds are easily transported to new locations on vehicles and equipment.
Area damaged by invasive plants growth and removal replanted with native plants
Replant Larger Areas with Technical Support
  • Purple and Iberian starthistle are difficult to expel from an infested area. Replanting a previously infested area requires planning and effort. The photo shows an area that was replanted after invasive weeds were removed.
  • Create a multi-year re-vegetation plan. Plans include site preparation, planting details, and plant care. Also, follow-up control for Italian thistle and other weeds.
  • Plan for at least 2-3 years of monitoring and maintenance.
  • Contact your local Extension specialist, soil and water conservation district, or a professional re-vegetation specialist. They can suggest strategies for your area.
 
5
Solutions for Purple and Iberian Starthistle

Early Detection & Rapid Response

  • Purple and Iberian starthistle seeds are carried to new areas on contaminated vehicles, soil, or livestock.
  • Watch for purple and Iberian starthistle on property you manage. Remove it before it becomes a bigger problem.

Physically Remove Plants & Non-Chemical Options

  • Hand tools are effective to remove individual plants and small patches.
  • Mowing plants in the early flowering stage may reduce seed production.
  • Cultivation that cuts the taproot reduces or eliminates flowering. Repeat cultivation as needed to keep plants from flowering and setting seed.
  • Mowing or cultivation alone will not effectively control purple and Iberian starthistle.

Herbicides (Weed Killers)

  • Herbicides effectively kill purple and Iberian starthistle when used according to label directions.

Monitoring & Follow-Up

  • Return to purple and Iberian starthistle patches following control activities. Look for regrowth.
  • Look for rosettes in the winter and early spring.
  • Control it each year as needed.

NEED HELP?

Consider a licensed pest control company. Learn How to Hire a Pest Control Company.
Your local Extension Specialist in Oregon  and other states  can suggest other methods.

Jump to

Method Does it work? Is it safe? Recommendation
A
Physical removal
Effective
Low risk
Recommended
B
Cultivation and Mowing
Effective
Low risk
Recommended
C
Control with Herbicides
Effective
Moderate risk
Use if Necessary
D
If Using Herbicides, Protect Yourself & Minimize Risks
 
A

Physical removal

Non-Chemical Method

Weston Miller, Oregon State University

Physical removal

Recommended
  • Digging out plants with hand tools before flowering reduces or eliminates seed production. This effectively controls purple and Iberian starthistle.
  • Dig out individual plants and small patches in winter or early spring when seedlings and rosettes are present.
  • This works best when the soil is drying and no rain or irrigation will follow control action.
Does it work?
Effective
  • Several years of monitoring and effort are required to get rid of purple and Iberian starthistle.
  • Use preventive measures for best results.
How much effort?
High effort
  • Cut stems with loppers. Dig out the root crown with picks, shovels, or similar tools.
  • Remove as much root crown as possible from the soil.
  • Wear thick gloves and heavy clothing. Purple starthistle spines hurt.
  • Return to the area each year and take action as needed.
What's the risk?
Low risk
Possible risk of exposure or harm from chemicals
NONE
 
B

Cultivation and Mowing

Non-Chemical Method

sshepard, iStock

Cultivation and Mowing

Recommended

Cultivate and/or mow patches of purple and Iberian starthistle to keep plants from flowering and setting seed.

 

Does it work?
Effective
  • Well-timed cultivation and/or mowing reduces seed production and weakens a population of starthistle.
  • Use preventive measures for best results.
How much effort?
High effort
  • Cultivate purple and Iberian starthistle after the last rains and before flowering for best results.
  • This must be repeated every year to keep plants from blooming and setting seed.

 
What's the risk?
Low risk
Possible risk of exposure or harm from chemicals
NONE

Cultivating Starthistle TIPS

  • Cultivate purple or Iberian starthistle after the last rains and before flowering for best results. If you cultivate it too early (before the last rains), more seeds will sprout.
  • Cultivation will have to be repeated regularly to keep plants from blooming and setting seed.
  • Manage pastures to reduce bare ground. This will limit starthistle establishment.

Soil Disturbance & Erosion

  • Tilling large patches of established plants with disk attachment creates significant soil disturbance.
  • Minimize soil disturbance as much as possible when removing purple or Iberian starthistle.
  • Regrade the soil after digging out plants. Apply mulch (when appropriate).
  • Take steps to prevent erosion as needed.
  • Replant the area to shade purple or Iberian starthistle seedlings.

Mowing Starthistle

  • Carefully timed mowing limits purple or Iberian starthistle seed production.
  • Mow when flowerheads are in full bloom.
  • Plants will regrow stems and flowers after mowing.
  • You'll likely have to mow a second or third time to keep plants from flowering.
 
C

Control with Herbicides

Chemical Method: Use with caution

iStock

Control with Herbicides

Use if Necessary
Does it work?
Effective
  • Herbicides effectively control purple and Iberian starthistle when used according to label directions.
  • Use of herbicides on smaller plants is more effective than on larger plants
  • Use in combination with preventive measures for best results.

 

 
How much effort?
Moderate effort
  • Several years of monitoring and effort are required to get rid of purple and Iberian starthistle.
  • Treat individual purple and Iberian starthistle plants and patches.
  • Return to the area each year and take action as needed.
What's the risk?
Moderate risk
  • Herbicides come with real risks. ALWAYS read the entire label front to back. Review instructions even for brands you know.
  • Herbicides can move off your site into waterways and may harm wildlife. See How to Keep Pesticides Out of Waterways.
Possible risk of exposure or harm from chemicals
Using herbicides includes some amount of risk. The lowest risk comes with using alternative methods.

You may be exposed to an herbicide if you:

  • Get it on your skin
  • Breathe it in
  • Eat or smoke afterward without washing hands
  • Touch or eat plants that are wet with spray (you, pets, or children)
  • Bring it inside on your shoes or clothes

Follow directions closely to reduce risk.

Some herbicides are available for home use. A few are listed below.  These active ingredients are available alone or in products containing more than one. They are effective chemical treatments for purple and Iberian starthistle. Look for these chemical names in the “Active Ingredients” section of product labels.

  • Dicamba
  • Glyphosate, Isopropylamine salt
  • Triclopyr
  • Imazapyr
  • 2,4-D

Suggested active ingredients are from the Pesticide Information Center Online (PICOL) Database. They are permitted for HOME use in Oregon. Read the label for application directions and effectiveness information.

ONLY professional pesticide applicators can use Restricted-Use Pesticides (RUP) that may be more effective. For larger stands of purple and Iberian starthistle contact a licensed pesticide applicator with experience controlling purple and Iberian starthistle.

Purple starthistle rosette

"centaurea calcitrapa" by Ana Júlia Pereira is licensed under CC BY-NC 4.0

  • Consider using herbicides to kill purple and Iberian starthistle. Apply herbicides before plants flower, at the seedling and rosette stage.
  • The photo shows a purple starthistle in the rosette stage. Ideally, locate and treat plants at this stage, before they grow stems.
Herbicide label highlighting active ingredient triclopyr
Sample product label with Active Ingredient highlighted
  • The red box on this example product label highlights the active ingredient triclopyr. The text on the label states “Controls annual and perennial broadleaf weeds."
  • Triclopyr doesn’t injure most grasses. It is a good choice for treating purple and Iberian starthistle growing next to desired grasses in lawn, pasture, and meadow areas.
  • Some products with triclopyr (ester formulations) become a vapor when applied on hot days. The vapor can damage nearby plants. Check the label for temperature limits.

Herbicide Application Tips

  • Most of these ingredients will damage most plants and grasses. Don’t let the spray contact plants you want to keep.
  • Apply herbicides on seedlings and small rosettes.  This is more effective than application on larger plants. Seedlings may be killed with a single herbicide application.
  • Herbicides are most effective when the plants are actively growing, and the soil is moist. 
  • Repeated applications over 2-3 years will reduce or eliminate established purple and Iberian starthistle stands. However, dormant seeds may germinate after 3 years. Follow-up site visits will be needed.
  • For best results with a single treatment, apply herbicides late in the rainy season. Use a high herbicide rate based on label instructions.
  • Return to the treated area and look for regrowth. Re-treat as needed.
  • Pre-emergent herbicide products work by preventing purple and Iberian starthistle seeds in the soil from germinating.
  • Purple and Iberian starthistle will re-grow unless you take steps to encourage or plant desirable vegetation.
Minimize the potential impact of herbicides on bees and other pollinators. Treat purple and Iberian starthistle plants before they flower. Or apply herbicides in the morning or evening when bees are less active. Avoid spraying pollinators directly. For more information, see OSU’s How to Reduce Bee Poisoning from Pesticides  
 

If Using Herbicides, Protect Yourself & Minimize Risks

Chemical Method: Use with Caution
Great blue heron in marsh

BrianLasenby, iStock

Why is it important to read herbicide labels?

  • They have detailed information on how to use the product correctly and legally.
  • They contain information on potential hazards of the product.
  • They provide instructions you should follow for poisonings and spills.
  • Following label instructions helps you to minimize the risks and maximize the benefits.

Key Herbicide Safety Tips

  • Read the entire label front to back.
  • Follow the instructions.
  • Review the instructions even for brands you know.
  • Only apply the product where the label says it may be applied.
  • Be precise in your application. More is not better.

The Label is the Law

ALWAYS read the label before using herbicide products. The label is a legal document that provides information on how to safely use the herbicide. It includes appropriate personal protective gear and application methods. This helps avoid harm to human health and the environment. Using an herbicide in off-label ways is illegal. It can result in legal enforcement actions..

READ THE LABEL & Follow Instructions
It has instructions to protect you and the environment.

  • Labels are different for every product and they often change over time.
  • Use a magnifying glass for small print.
  • Pay attention to CAUTION, WARNING, and DANGER statements.
  • Pay attention to the PRECAUTIONARY STATEMENTS.
  • The law states you must read and follow herbicide instructions.

Protect Yourself
Eye, skin & lung irritants

  • Wear the right protective gear. This often includes, but is not limited to: a long-sleeved shirt, pants, socks, closed shoes, safety glasses, and chemical-resistant gloves. 
  • Mix outdoors or in a well-ventilated area.
  • Wash hands after mixing or applying, and before eating or smoking.
  • Take a shower immediately after handling herbicides.
  • Wash clothes worn while mixing or applying separately from other laundry.

Protect Children & Pets
Children and pets are at risk if they eat or touch the plants before they dry.

  • Keep them away during and after applying herbicides (read the label for how long).
  • Remove toys and pet dishes from the yard before applying.
  • Don’t track herbicide products into your home on shoes or clothes.

Protect Pollinators

  • Apply in the early morning or evening when bees are less active.
  • Kill weeds before they flower. Avoid spraying flowering plants.
  • Do not spray on bees or insects.

Protect Plants You Want to Keep

  • Glyphosate and other broad-spectrum herbicide ingredients can damage both grass and broadleaf plants.
  • Minimize spraying of foliage, stems, exposed roots, or the trunks of desirable shrubs or trees to avoid harm.
  • Follow the label to avoid damaging the roots of trees and shrubs.

Avoid Wet, Windy, or Hot Weather
Use during favorable weather for best results.

  • Don’t spray when it’s raining or when rain is expected in the next 24 hours.
  • Wind causes the spray to drift, which can get on you and desired plants.
  • Herbicides may be less effective in hot weather if the target plants are moisture-stressed.
  • Some herbicides can turn into a vapor in hot weather and damage nearby plants.

Don’t Spray into Water

  • It’s illegal to apply herbicides in a stream or slow-moving/wetland pool.
  • You need a product registered for aquatic areas. This includes waterways, ditches, drains, and other places where water collects.

Storage & Disposal

  • Store in a secure area away from children.
  • Don’t put unused herbicide products in the trash.
  • Never pour down any drain or waterway.
  • Take unused herbicides to a hazardous waste facility.

Call  1-800-CLEANUP (1-800-253-2687) to find out where to dispose of herbicides.

For the Portland metro region in Oregon, contact Metro’s Recycling Information. Call  503-234-3000, email   or visit Metro’s website  

More about:

About Using Pesticides on School Grounds in Oregon

If using pesticides on school grounds, there are special rules in Oregon. See School Integrated Pest Management  (Oregon Department of Agriculture).

NEED HELP?

The National Pesticide Information Center (NPIC)  can answer questions about pest control chemicals.
 1-800-858-7378 or npic@ace.orst.edu  

Consider using a licensed pest or weed control company. Learn How to Hire a Pest Control Company.

Your local Extension Specialist in Oregon  and other states  can suggest other methods.

Invasive Species Alert

  • Invasives are non-native species that spread aggressively and alter the environment.
  • Controlling unwanted purple and Iberian starthistle is costly.
  • Please do your part to control it on property you manage. It can spread beyond your property and impact your neighbors.

If you think you’ve found purple or Iberian starthistle in the grey areas of this map, please report it to the Oregon Invasive Species Hotline at:  1-866-INVADER (1-888-468-2337) or use their online reporting form  

open Map static invasive map
Invasive species data @ 2022, iMapInvasives (NatureServe)

If you find purple or Iberian starthistle in a new area (orange shows already reported cases), please report it

View Larger Map >
SEE OTHER COMMON PACIFIC NORTHWEST WEEDS

Content provided by editor Weston Miller and writer J. Jeremiah Mann. Pesticide safety information edited by Kaci Buhl.

  Peer review facilitated by OSU Department of Horticulture.

Photo of Weston Miller

Weston Miller

Project Founder and Content Writer

Weston Miller served as Community and Urban Horticulture faculty for Oregon State University Extension Service for Clackamas, Multnomah, and Washington Counties. Weston is an author for content for this website. He developed funding partnerships with Portland area agencies to initiate and build out the Solve Pest Problems website focused on this goals:

J. Jeremiah Mann

J. Jeremiah Mann

J. Jeremiah Mann completed a Physical Science undergraduate degree at Humboldt State University, and M.S, Ph.D focusing on plant science topics at UC Davis. He went on to work for the Natural Resources Conservation Service and in a leadership position serving a private agricultural technology company. He currently lives in Sacramento California where he consults on pest and property management topics.

Photo of Kaci Buhl

Kaci Buhl

At the state level, I lead the Pesticide Safety Education Program (PSEP). The program hosts live recertification events around the state, serving over 1,000 licensed pesticide applicators each year. We also produce web-based training modules and license-preparation study manuals. Special training for unlicensed pesticide applicators is also available through a grant from the USDA Specialty Crop Block Grant Program. The PSEP at OSU works closely with the Oregon Department of Agriculture's Pesticides Division.

PURPLE AND IBERIAN STARTHISTLE REFERENCES

Purple Starthistle
Washington State Noxious Weed Control Board

Purple and Iberian Starthistle (PDF)
Weed Control In Natural Areas of the Western United States, University of California

Starthistle
PNW Weed Management Handbook