The term “prickly weeds” is an informal description for diverse plants that have developed defensive or dispersal structures. Identifying the specific plant is the first step toward understanding its life cycle and how to manage its presence. These plants are broadly categorized based on the structure and location of their sharp parts.
Upright Weeds with Sharp Foliage (The Thistle Family)
Many taller prickly plants belong to the thistle family, characterized by sharp leaves and stems. Two prominent examples are Bull Thistle (Cirsium vulgare) and Canada Thistle (Cirsium arvense). Bull Thistle is typically a biennial plant reaching heights of 6.5 feet, featuring large, rose to purple flowers and stout, branched stems with “spiny wings.” Its leaves are deeply lobed and prickly-hairy on the upper surface, with a woolly texture underneath.
Canada Thistle, a perennial species, tends to be shorter, usually growing between one and four feet tall. Unlike Bull Thistle, its stems are generally smooth and lack the prominent spiny wings. Canada Thistle leaves have spines along their wavy margins but are often smoother and lighter green. Its flower heads are also smaller, ranging from 0.5 to 1 inch in diameter, and appear in clusters rather than singly.
The sharp structures on these thistles are botanically classified as spines, which are modified leaves or parts of leaves containing vascular bundles. This differs from prickles, which are outgrowths of the plant’s epidermis, like those found on a rose stem. For Bull Thistle, the sharp projections are considered true spines because they emerge from the leaf’s internal veinlets. This distinction highlights the plant’s investment in rigid defensive structures.
Ground-Level Weeds with Painful Seed Pods (The Sticker Group)
A different group of prickly weeds causes trouble at ground level, where the main source of irritation comes from their reproductive structures rather than foliage. These plants often grow low in dense mats, making them difficult to spot until stepped on. Puncture Vine (Tribulus terrestris), also known as Goathead, is a common example of this low-growing, mat-forming plant.
The plant itself has small, hairy leaves and small yellow flowers, but its danger lies in the fruit, which matures into a hard, tack-like burr. Each seedpod typically breaks into five segments, and each segment, or burr, is armed with two sharp, stiff spines. These burrs are notorious for puncturing bicycle tires and embedding themselves into shoes and animal paws. Another member of the sticker group is the Sandbur (Cenchrus species), an annual grass that produces a different kind of painful seed casing.
Sandbur burs are oval to spherical, measuring about 0.2 to 0.4 inches long, and are densely covered with numerous stiff spines that can range in number from 8 to 75, depending on the species. The plant’s overall appearance is that of a tufted grass, often found in dry, sandy soils, but its spiny burrs detach readily upon maturity. These seed-containing burrs are often discovered when they stick to clothing, socks, or pet fur, enabling their spread. The bur itself protects the seed, allowing it to remain viable in the soil for extended periods.
The Biological Purpose of Prickles and Burrs
The development of sharp structures in plants is not random but represents a significant evolutionary investment to ensure survival and reproduction. These physical defenses, whether spines on a thistle or burrs on a goathead, serve a dual purpose: protection and dispersal. The most immediate function of spines and prickles on stems and leaves is herbivore deterrence.
A plant commits energy to creating these rigid extensions to discourage animals from consuming its photosynthetic tissues. This defense is important in resource-poor environments where losing foliage to grazing is a severe setback. For a plant to dedicate resources to building a spine instead of an extra leaf, the protective benefit must outweigh the energy cost.
The burrs produced by plants like Puncture Vine and Sandbur also serve as a mechanical defense, but their primary function is seed dispersal. The hooks and spines are perfectly adapted for epizoochory, which is the process of seeds hitchhiking on the exterior of passing animals. The burr attaches firmly to fur, feathers, or clothing, allowing the seeds to be transported far from the parent plant. This mechanism ensures that the next generation of plants can colonize new territory, which is a highly effective reproductive strategy.