A “sticker plant” is a general term for various species that produce sharp, rigid structures capable of causing irritation or injury. These specialized appendages evolved primarily for defense against grazing animals and for effective seed dispersal. While many ornamental plants feature these structures, the term commonly refers to weeds whose burs or spines pose a hazard to feet, tires, and clothing. These plants vary widely in appearance, ranging from low-growing mats that hug the ground to tall, robust, upright stalks.
Categorizing the Types of Sticker Structures
The painful structures found on these plants differ in their origin and composition. Thorns are modified stems that emerge from the plant’s woody tissue. They contain vascular bundles, making them essentially hardened, pointed branches that are difficult to snap off.
Spines are modifications of leaves, parts of leaves, or leaf stipules, and also contain internal vascular tissue. For example, a cactus spine is a highly modified leaf designed to reduce water loss and deter herbivores. Both true thorns and spines are structurally integrated into the plant’s core tissue.
Prickles, such as those found on rose stems, are simpler outgrowths of the plant’s outer skin layer (epidermis). They do not contain vascular tissue, which is why they tend to break off more easily and irregularly than thorns or spines. Burs are specialized seed pods or fruit casings covered in hooks, barbs, or spines designed to catch onto the fur of passing animals or clothing for dispersal.
Appearance of Common Ground-Level Sticker Plants
Ground-level sticker plants are typically low-profile annual weeds that create a hazard underfoot, with Puncture Vine being a common example. Puncture Vine (Tribulus terrestris), also known as Goathead, is a prostrate plant whose stems spread radially from a central taproot to form a flat, dense mat. The stems are often green to reddish-brown, reaching lengths of one to five feet.
The leaves are compound, resembling tiny ferns, with small, oval leaflets arranged oppositely. The plant produces small, five-petaled yellow flowers, often visible only on sunny mornings. After flowering, the fruit matures into a woody, tack-like bur, approximately a quarter-inch wide, which splits into five segments. Each segment features two strong, sharp spines, one of which is always oriented upward, capable of puncturing thin rubber or skin.
Another nuisance is the Sandbur (Cenchrus species), which has a grass-like appearance, growing in tufts or clumps. The plant features flat leaf blades and can grow either upright or along the ground. Sandbur is identified by its small, densely spined seed heads, which are technically a raceme of burs attached to the main stalk. These burs are green when unripe, turning yellow or brown as they mature, and easily detach to stick to anything that brushes past.
Appearance of Common Upright Thistle and Prickly Plants
Upright sticker plants, primarily thistles, use their sharp structures as a defense mechanism on their foliage and stems. Bull Thistle (Cirsium vulgare) is a biennial plant that can grow quite tall, often reaching three to seven feet in its second year. The leaves are deeply lobed and dark green, with a rough texture on the top surface and a woolly, lighter underside.
The most noticeable feature is the prominent, stiff spines that line the edges of the leaves and often extend down the stem, creating spiny wings. Bull Thistle produces large, globe-shaped flower heads, about two inches across, which are typically pink to reddish-purple. These flowers are encased in a base of thorny, green bracts.
Canada Thistle (Cirsium arvense) is a perennial species that also grows tall, typically reaching five feet. Its leaves have wavy, prickly margins, but the main stem is smooth and lacks the spiny wings found on Bull Thistle. The flowers of Canada Thistle are smaller, usually only one to two centimeters wide, and cluster at the top of the plant. These flowers are pink to purple and sit in bases that are notably spineless, helping differentiate the two common upright thistle varieties.