Burrs are specialized botanical structures that have evolved a highly effective method for travel by latching onto anything that brushes past them. Their ability to cling tenaciously to clothing, hair, and animal fur makes them a common, if irritating, encounter in natural environments. This strategy allows the plant to spread its offspring far beyond the immediate vicinity of the parent plant.
Biological Purpose and Structure
The primary function of a sticky burr is to facilitate a biological strategy known as epizoochory. This method is a form of co-evolution, where the plant has developed morphological features specifically to exploit passing mammals and birds. The plant’s goal is to ensure its seeds travel a sufficient distance to find new ground for germination and reduce competition with the parent plant.
The structure of these burrs is a testament to efficient natural engineering, featuring rigid spines, tiny hooks, or barbs. These protrusions are designed to mechanically interlock with the fibers of fur, feathers, or woven fabric. In some species, the contact separation force of a single burr may be relatively low, but the total force generated by multiple burrs covering a fruit can exceed the fruit’s weight by a factor of 4 to 20 times, ensuring a secure hold until the seed is successfully transported. The hooks are often capped with microscopic, recurved tips, creating a secure grip that resists easy removal.
Common Sticky Burrs and Their Sources
One of the most recognizable is the Burdock (Arctium spp.), a plant that produces round, brown burrs covered in stiff, curved spines. These burrs can be up to an inch in diameter and are frequently found along roadsides and in disturbed forest edges.
Cocklebur (Xanthium spp.) yields an oval-shaped, hard seedpod encased in numerous stiff, hooked spines. This plant is a hardy weed often found in agricultural fields and floodplains, and its burrs are particularly tenacious when embedded in animal fur. Smaller, but equally annoying, are the burrs of Beggar-ticks (Bidens spp.), sometimes called “sticktights” or “beggar’s lice.” These typically consist of a flattened seed with two to four sharp, barbed prongs at one end, which easily catch onto socks and pant legs.
The Sandbur (Cenchrus spp.) is a type of grass that produces a small, tough fruit covered in thick, long spines. Hedge Parsley (Torilis arvensis) also produces very small, velcro-like burrs, often referred to as beggar’s lice, which cling to clothing in high numbers.
The Practical Science of Adhesion
The mechanism by which burrs stick is purely mechanical, relying on the microscopic interaction between the burr’s structure and the loops in a material. This hook-and-loop principle is highly effective because it distributes the bonding force across many individual points of contact. A single burr may have hundreds of tiny hooks, each catching a loop in the fabric or a strand of animal hair. The collective strength of these numerous, small interlocks creates a surprisingly strong and reliable temporary bond.
In 1941, Swiss engineer George de Mestral was hiking with his dog and noticed how Burdock burrs clung to his pants and his dog’s fur. This observation led de Mestral to develop the fastening system known as Velcro. The resulting product mimics the burr’s design by using a strip of stiff plastic hooks and a complementary strip of soft, woven loops.
Dealing with Burrs: Removal and Prevention
Removal from Clothing
For clothing, tools with stiff edges work best, such as a fine-tooth comb, a wire brush, or a specialized burr remover. Adhesive materials like a strong lint roller or a strip of packing tape pressed firmly onto the affected area can effectively lift and remove multiple burrs at once. It is generally best to pull the burrs in the direction of the fabric weave to minimize snags.
Removal from Pet Fur
Burrs in pet fur require a gentle approach. Heavily matted burrs should first be lubricated, which can be done safely with a small amount of vegetable oil or a pet-safe detangling spray. After lubrication, the fur can be carefully teased apart using fingers and a wide-toothed metal comb, working from the outer edges of the tangle inward. For large or deeply embedded burrs, crushing them gently with a pair of pliers can break them into smaller, more manageable pieces for easier extraction. Prevention involves keeping pets leashed and avoiding areas of high burr concentration, such as tall, overgrown weeds or thickets along paths.