When do goathead weeds seed

Northern Arizona Invasive Plants

a northern arizona homeowner’s guide to identifying and managing invasive plants


Common name(s): Goathead, puncturevine

Scientific name: Tribulus terrestris

Family: Caltrop family (Zygophyllaceae)

Reasons for concern: This plant easily outcompetes native plants, resulting in dense monocultures and a reduction in native plant diversity very important to wildlife and pollinators. The seeds germinate quickly and can lie dormant in the soil for many years, prolonging the life of an established population. Sharp burrs cause serious injury to people, pets, wildlife, bicycle and vehicle tires, and livestock, and are easily spread by vehicles, pedestrians, and animals, resulting in even larger populations.

Classification: Non-native. Included on the Arizona Noxious Weed List as a Class C noxious weed, which means it is widespread and “may be recommended for active control based on risk assessment.”

Botanical description: Mat-forming, fast-growing broadleaf plant.

Leaves: Each small, hairy leaf is subdivided into 4 to 8 pairs of smaller leaflets, opposite each other on stems.

Stem(s): Grows low to the ground, forming dense mats 2 to 5 feet in diameter. Can grow almost erect in dense vegetation. Hairy, trailing stems radiate out from a central point at the taproot.

Flowers: Bright yellow, with five petals. Appear in the axil, where the leaf meets the stem. Open only in the morning. Blooms July through September.

Seeds: Seedpod is a cluster of 5 flat spiny burrs, containing up to 5 seeds. It breaks apart at maturity.

Roots: Deep taproot branching into network of fine rootlets.

Native to: Southern Europe

Where it grows: Dry or gravely sites. Roadsides, waste places, pastures, fields, railroad tracks. Prefers dry, well-drained, sandy sites below 7,000 feet in elevation.

Life cycle: Summer annual

Reproduction: By seed Goathead habit.

Weedy characteristics: Goathead thrives in hot and dry conditions where other plants cannot. Its dense mat smothers out other species. It can start flowering within 3 weeks of germination, and continues to flower all summer. Depending on the moisture available, it typically produces 200 to 5,000 seeds in one growing season. Seeds can remain dormant in the soil for 5 years. Its roots are hard to remove.

Control strategies: Do not let them flower and go to seed! Prevent the seeds from spreading. Seedlings are easy to remove by hand pulling, and older plants can be pulled or dug out. Rake or sweep up any burrs that may have dropped. Tilling can be effective before seed production. Frequently monitor a population for new plants. Plant desirable native species to outcompete invasives. Herbicides are effective on small, actively growing plants. Contact your local county extension office for more information on chemical control.

Images: Click on an image to enlarge and see the image citation.

Three-step plan will eliminate goathead stickers

Dear Neil: We have an entire yard that’s being taken over by painful, prolific goathead stickers. How does one completely eradicate their yard of these plants so we can use our outdoor space again?

Answer: They are bad! Anyone who has experienced goatheads firsthand (or firstfoot) knows they are major league versions of the lightweight little grassburs. Goatheads can puncture bicycle tires and footballs. I’ve been there. Use a broadleafed weedkiller (containing 2,4-D) as a spray onto their foliage. It’s most effective when the weeds are growing most actively, and even then, it takes 10 to 15 days for it to eradicate the weeds. Of course, the goatheads themselves are the seeds for next year, and they’re going to persist, whether or not you kill the plants. Your best bets next year would be threefold. First, apply a pre-emergent weedkiller containing Gallery in early March and again in early June. Second, if you still see a few of the plants germinate and grow next spring and early summer, apply the broadleafed 2,4-D spray earlier next year — before they go to seed. Third, do whatever you can to foster stronger turfgrass. Hopefully rainfall will be better, which should allow your grass to thicken and crowd out the weeds. (For the record, broadleafed weedkillers will not help control grassburs at all. Your only recourse there is with a pre-emergent such as Balan, Halts, Team or Dimension.)

Dear Neil: I have three 5-year-old crape myrtles growing in full sun. They’re in the same area, and they’re treated the very same way, yet for the past two years, they’ve had many buds that have failed to open. They’ve been watered with an automatic sprinkling system. What can we do to correct that?

Answer: This year specifically, many crape myrtles have stalled out during their blooming cycles. I work a lot with a crape myrtle project being done by a not-for-profit organization, and I’ve watched about 8,000 plants of some 40 varieties. This year, you actually can see differences in how the varieties have handled the heat and drought. So, that heat may be part of your issue, even with proper watering. That said, for all of my career, I’ve been asked this very question, and prior to this year, every one of the dozens of cases I’ve actually seen in person ended up being seed heads that had formed immediately following the flowering. People would be gone for a week or two, and they’d come home to find their crape myrtles looking the same as when they left. Strange as it seems, crape myrtles’ seed heads look very much like their swollen flower buds. You might try cutting through one of them with a single-edged razor blade to see if you see seeds developing, or if you see primordial petals of an actual flower bud. This is a wide-ranging answer, and I hope some part of it is of help.

Dear Neil: My Knockout roses have had a traumatic summer. Between the heat, drought and grasshoppers, they’re almost bare. Should I trim them back?

Answer: It’s fine to tidy them up by pruning them as much as 25 percent, but I wouldn’t do anything more than that this late in the season. If you can reshape them gently, apply a high-nitrogen fertilizer to promote new fall growth, and water them deeply to get the fertilizer into the root zone, you should see good bloom within six to eight weeks.