We have Trouble in River City, Trouble with a Capital T, spelled Tribulus terrestris: AKA puncture vine; goat head; bullhead; caltrops; cat head; devil’s eyelashes; devil’s thorn; devil’s weed; and tack weed-and those are just the English names. It’s a tender herbaceous perennial that grows as a summer annual in our area. It grows below lawn mowers and spreads its thorny seeds under feet and bike tires, which they are hard and sharp enough to puncture, thus the name puncture vine.
Its flowers are lemon-yellow with 5 petals, about ¼ inch wide. Its leaves are about 1 inch long, pinnately divided, with leaflets less than ¼ inch long; the vines can spread over a yard wide; they pile up to a few inches, but do not climb. Flowers are followed in a week by five-sided fruit that easily falls apart into 5 tack-shaped nutlets that become quite hard and sharp.
It didn’t grow here more than 20 years ago, but has since come in on tires. It appears to prefer dry or lightly moistened ground, probably because of the lack of early competition; it can take considerable water. Its mode of travel being on feet and tires, it tends to grow along roads and paths. It can invade even well-watered lawns if they have bare spots or are cut too short. Cutting no shorter than 3 inches favors our northern grasses and discourages many weeds, including puncture vine.
Pulling the plants and picking up any seeds is the only effective way to quickly remove this plant and the menace of its seeds. One need not pull the entire taproot; the crown from which the stems radiate is what matters. Run a gloved hand over the ground to find loose seeds, which readily fall off the plant before they are even dry. Recognizing and pulling it before it flowers is most effective.
Its seeds can last 5 years on the ground, but it is possible to kill them by steaming them under clear plastic: water the area; cover it with clear plastic; seal the edges with boards or rocks; let it sit in full sun for a few days in warm weather. The steam should sterilize the top inch or two of soil, where the seeds lie.
Roundup and other glyphosate herbicides might seem a quick and easy solution, but it is a powerful fertilizer of broadleaf weeds, high in nitrogen and phosphorus, that will feed the next crop of whatever weed seeds are present or fly in on the wind. It kills young plants, but will ripen the seeds of flowering plants as they die. Since puncture vine with seed must be pulled dead or alive, it is best to avoid herbicide and its weed-fertilizing effects altogether.