CORVALLIS, Ore. – Plants are put on the Oregon noxious weed list for a variety of reasons – some crowd out native or beneficial vegetation; others might harm livestock or farming efforts.
The Oregon State Weed Board defines a noxious weed as "a plant that is injurious to public health, agriculture, recreation, wildlife, or any public or private property."
Puncturevine (Tribulus terrestris) is listed as noxious because of its seeds. It produces seed burs with spines so sharp and hard they commonly cause bicycle flats and can even penetrate your sneakers, explained Rich Affeldt, crop scientist with the Oregon State University Extension Service.
"The seeds are injurious to livestock if they are eaten or stepped on and they damage wool," said Affeldt. "Processing crops, like snap beans, are also at risk for contamination because burs are easily caught in harvesting equipment. The spiny burs do an excellent job of dispersing seed via tires, shoes, and hooves."
Puncturevine, also known as goathead, Mexican sandbur or Texas sandbur, is a summer annual, usually prostrate, with stems as long as eight feet. Its leaves are divided into four to eight pairs of dark or grayish green oval leaflets, which are covered with hair, as are the stems. It produces yellow flowers with five petals from July to September that are about a half-inch wide and pollinated by insects.
Native to southern Europe, puncturevine seeds may have been introduced to North America in wool just after 1900. The plant is now widely scattered throughout the United States. In Oregon, it's abundant in the eastern part of the state and is becoming more common west of the Cascades.
Pictures of the plant, distribution maps and opportunities to report an infestation can be found at the following website: oregon.gov/ODA/PLANT/WEEDS/profile_puncturevine.shtml. Or, download the Punturevine Patrol document.
"You can kill puncturevine easily with glyphosate, 2,4-D or other postemergence herbicides," said Affeldt. "Try to spray before flowering. Once the plant produces the spiny burs, the seeds can remain dormant in the soil for several years. If you find a plant that's already seeded, dig it up, being careful to capture the seeds. Dispose of it by burning or in your garbage. Don't recycle it with yard waste or compost."
Small infestations can easily be controlled by hand weeding or by using a propane weed burner.
"A persistent effort will be necessary to eliminate a puncturevine infestation if you use post-emergence herbicides or hand weeding," Affeldt said. "Puncturevine germinates throughout the summer and can produce viable seed within three to four weeks of emerging, depending on temperatures.
“In other words, to prevent puncturevine from making more seed, you need to spray or hoe every three weeks in the summer."
In areas where there's a sizable population of puncturevine and the winters aren't too cold, puncturevine weevils that feed entirely on puncturevine seeds have been introduced to help control the spread of the plant. Until recently, these weevils were found only in southern states and were believed not to be able to survive the colder winters in the northwest. However, populations of puncturevine weevils are now surviving from year to year in some of the northeast counties of Oregon. The Oregon Department of Agriculture has approved the release of these weevils to control puncturevine.
"The weevils are only partially effective because they attack only ripening burs," warned Affeldt. "They can be part of a control strategy, but should not be counted on to save the day."
Luckily, puncturevine doesn't compete well with most other vegetation. It's most likely to be found in sparse dry areas, which is why it is more prevalent in eastern Oregon than west of the Cascades. In western Oregon it mostly shows up on bare ground or in gravel driveways. Establishing a vegetative cover, like creeping red fescue, in problem areas will go a long way in limiting a puncturevine infestation.