The plant commonly referred to as blackjack is Bidens pilosa, an annual herb known by many regional names including beggar-ticks, Spanish needle, and cobbler’s pegs. This plant does not possess a venom-injecting apparatus, so it does not “sting” in the biological sense. Instead, the injury is a purely mechanical process, where specialized seeds embed themselves into the skin or clothing, causing a physical “prick.”
Identifying the Plant
Bidens pilosa is a fast-growing, erect plant that can reach heights of up to 1.5 meters in favorable conditions. Its stems are often noticeably square in cross-section and can range in color from green to a purplish hue. The leaves are arranged oppositely and are typically compound, featuring three to five leaflets with serrated margins.
The flower heads are small, usually about 7 to 8 millimeters across, and resemble miniature daisies. They feature yellow central disk florets, which are sometimes surrounded by four to seven small white or cream-colored ray florets. The plant is found globally as a naturalized or invasive species, thriving in disturbed habitats like gardens, roadsides, agricultural fields, and wasteland.
The plant is widely recognized due to its fruits, known as achenes or burrs. These seeds are linear, black, and relatively long, measuring up to 1.6 centimeters. Each seed is equipped with two to four stiff awns, which are the primary source of the painful pricking sensation. These awns facilitate seed dispersal by catching onto anything that brushes past the plant.
Prick Versus Sting: Understanding the Injury
The distinction between a prick and a sting lies in the mechanism of injury and the introduction of foreign substances. A biological sting, such as one from an insect, involves a specialized organ that injects venom or a toxin, which is the source of pain and reaction.
The injury inflicted by the blackjack seed is a simple puncture wound caused by its physical structure. These awns feature microscopic barbs that point backward, a structure known as retrorse barbing. This retrorse structure is why the seed adheres so tenaciously to fabric and skin and why removal feels difficult. When the awns contact the skin, they penetrate the epidermis, causing a sharp, immediate, mechanical pain. The resulting sensation of irritation is the body’s localized inflammatory response to this physical invasion of a foreign object.
Post-Contact Care and Irritation
Immediate action after contact involves carefully removing the embedded seeds to prevent further irritation or potential skin complications. For seeds that have only superficially attached to the skin or hair, a piece of strong adhesive tape can sometimes be pressed onto the area and then quickly pulled away to lift the seeds. If the awns are more deeply embedded, fine-tipped tweezers should be used.
Grasp the seed as close to the skin’s surface as possible, then pull gently and steadily in the direction opposite to the penetration. After removing the seed, the small puncture site should be cleaned immediately with soap and water or a mild antiseptic to reduce the risk of secondary bacterial infection. A common reaction is localized redness, minor swelling, or mild itching around the site of penetration, which is the body’s natural response to the physical trauma.
While the seed itself is not toxic, the plant contains compounds that can sometimes affect the skin. Bidens pilosa contains phenylheptatriyne, a phototoxic substance that may cause skin reactions when the plant material is crushed and exposed to sunlight, potentially worsening existing dermatological conditions. Medical attention should be sought if the area develops signs of infection, such as increasing pain, warmth, spreading redness, or the presence of pus.