Cocklebur

Who hasn’t had unkind thoughts about the cocklebur (Xanthium strumarium) while pulling the prickly burs from socks or dog’s fur? The bur is armed with strong, hooked spines that are perfectly engineered to grab and hang-on, hitching a free ride to some new location and ultimately spreading the species throughout the United States and around the world.

Although generally considered a noxious weed, the cocklebur has made a very practical contribution: 60 years ago, the annoyingly effective hooked design of the bur provided the inspiration for Velcro©.

Description ^{4,11,46,59,202}

Cocklebur is an upright annual herb that may reach six feet tall. It is extremely variable in specific characteristics. There is usually one stem from a tap root. The central stem often produces ascending lateral branches that decrease in length toward the top.  Stems are light green, with short, reddish streaks.

A basal rosette is absent. Cauline leaves are green and broadly ovate, up to seven inches (18 cm) long, with brownish petioles about as long as the leaf. Leaf margins are often shallowly and  irregularly toothed and may be palmately lobed (somewhat like a maple leaf). Leaves and stems are made rough to touch by short, upward pointing hairs.

The individual flowers (or florets) are highly modified and don’t look like flowers in the usual sense. Individual florets are either male or female and are clustered on separate flower heads that occur in branched terminal clusters and in shorter axillary clusters. Male flower heads are found on the upper or outer portion of a cluster. The tiny male florets occur on a hemispherical receptacle. Male florets lack a pappus and have a tiny, symmetrical corolla. Each has five stamens; their filaments are united into a column around a sterile pistil. The anthers unfurl outward, looking initially like tiny yellow egg whisks.  A female flower head consists of only two florets which lack pappus, petals or stamens and are enclosed by the egg-shaped receptacle. The receptacle is covered with overlapping leaf-like phyllaries, some of which are “hooked” outward at the tip. At the outer end of the green flower head are two larger projections, the “beak”, each of which is paired with a smaller tooth; between each beak and tooth, a forked style extrudes through a tiny pore. These are the only visible parts of the female florets. Cocklebur blooms July – October.¹

The bur is a dry, hard ellipsoid or ovoid structure, roughly an inch (2.5 – 3.0 cm) in length. The bur encloses two fruits, each with one elongate seed; the seed of one fruit is somewhat larger than the other. The spines that cover the bur are derived from the leaf-like phyllaries that have grown and hardened into the hooks that make the cocklebur so aggressive.

Classification ^11,44,49,143

Cocklebur is a dicot angiosperm in the sunflower family, the Asteraceae.² This is the largest family of vascular plants in the Northern Hemisphere. “Flowers” of Asteraceae are made up of one or both of two types of flowers (florets): symmetrical disk florets and strapped-shaped ray florets . These are crowded onto a common base (receptacle), and together are often assumed to be a single flower, which we call a flower head. Other familiar Asteraceae that occur in the Reserve include bush sunflower (Encelia californica),goldenbush (Isocoma menziesii), and coyote brush (Baccharis pilularis).⁴⁸

Members of the genus Xanthium have highly specialized female flower heads, and they can be told from almost every other member of the sunflower family by their distinctive burs. Other members of this family which also have burs with hooked spines are the burdocks^202,302 (Arctium spp) , but these have conspicuous thistle-like flowers, and are not known from the Reserve.⁴⁸

Cocklebur is a highly variable species, within which there are several complexes differing in geographical range and morphology.  At least 18 different species  have been described which, in 1959, were combined into one species, X. strumarium.²⁰²

Ecology  

Without colorful flowers, sweet nectar or an enticing scent, cocklebur is poorly equipped to attracted pollinators. It is pollinated by wind almost entirely and is often self-pollinated.⁵ In contrast, animals play an important role distributing the seeds. The small burs with their coat of hooked spines grab onto the fur and socks of passing mammals and are often transported considerable distances.^4,5

The two seeds in each bur are somewhat different sizes. The larger of the two usually sprouts the first year, or the spring of the following year. Sprouting of the second seed is delayed for another year, thereby increasing the chance that one of them will sprout during favorable conditions.^5,11,46,202

Human Uses  

Several North American Indian tribes developed medicinal uses for cocklebur; others ate the seeds or used the plant ceremoniously.²⁸² The Chumash from Santa Barbara and the Channel Islands boiled the leaves to wash cuts; teas were used for bladder problems.¹⁵ In Baja California, cocklebur was used to treat snakebite.¹⁵

Feeling creative? The grabby burs can be stuck together to make fanciful birds and animals.³⁰²

Interesting Facts ^303,304

In 1941, Swiss engineer and inventor George de Mestral returned from a trip through the fields with his dog, both covered with cockleburs. While pulling the burs he began to wonder what made them so tenacious. When a close inspection revealed the many tiny hooks, perfectly engineered to grab and hold loops of cloth or tangles of fur, de Mestral recognized the practical implications.

It took nearly a decade, working with a skilled French weaver, to design a system to manufacture a fabric of thousands of tiny hooks and another of tiny loops the perfect size to grab and hold the hooks, and release them on command. This was the origin of Velcro©, a patented name from the fabric name “velour” and the French word “crochet” meaning hook.