Photo by Dinesh Valke from Thane, India licensed under CC BY-SA 2.0
Spend enough time in disturbed areas and you will certainly cross paths with a cocklebur (Xanthium strumarium). As anyone with a dog can tell you, this plant has no problems getting around. It is such a common occurrence in my life that I honestly never stopped long enough to think about its place on the taxonomic tree. I always assumed it was a cousin of the amaranths. You can imagine my surprise then when I recently learned that this hardy species is actually a member of the sunflower family (Asteraceae).
Cocklebur doesn't seem to fit with most of its composite relatives. For starters, its flowers are not all clustered together into a single flower head. Instead, male and female flowers are borne separately on the same plant. Male flower clusters are produced at the top of the flowering stem. Being wind pollinated, they quickly dump mass quantities of pollen into the air and wither away. The female flowers are clustered lower on the stem and consist of two pistillate florets situated atop a cluster of spiny bracts.
After fertilization, these bracts swell to form the burs that so many of us have had to dig out of the fur of our loved ones. Inside that bur resides the seeds. Cocklebur is a bit strange in the seed department as well. Instead of producing multiple seeds complete with hairy parachutes, the cocklebur produces two relatively large seeds within each bur. There is a "top" seed, which sits along the curved, convex side of the bur, and a "bottom" seed that sits along the inner flat surface of the bur. Studies performed over a century ago demonstrated that these two seeds are quite important in maintaining cocklebur on the landscape.
Photo by Dinesh Valke from Thane, India licensed under CC BY-SA 2.0
You see, cocklebur is an annual. It only has one season to germinate, grow, flower, and produce the next generation. We often think of annual plants as being hardy but in reality, they are often a bit picky about when and where they will grow. For that reason, seed banking is super important. Not every year will produce favorable growing conditions so dormant seeds lying in the soil act as an insurance policy.
Whereas the bottom seed germinates within a year and maintains the plants presence when times are good, the top seed appears to have a much longer dormancy period. These long-lived seeds can sit in the soil for decades before they decide to germinate. Before humans, when disturbance regimes were a lot less hectic, this strategy likely assured that cocklebur would manage to stick around in any given area for the long term. Whereas fast germinating seeds might have been killed off, the seeds within the seed bank could pop up whenever favorable conditions finally presented themselves.
Today cocklebur seems to be over-insured. It is a common weed anywhere soil disturbance produces bare soils with poor drainage. The plant seems equally at home growing along scoured stream banks as it does roadsides and farm fields. It is an incredibly plastic species, tuning its growth habit to best fit whatever conditions come its way. As a result, numerous subspecies, varieties, and types have been described over the years but most are not recognized in any serious fashion.
Sadly, cocklebur can become the villain as its burs get hopelessly tangled in hair and fur. Also, every part of the plant is extremely toxic to mammals. This plant has caused many a death in both livestock and humans. It is an ironic situation to consider that we are so good at creating the exact kind of conditions needed for this species to thrive. Love it or hate it, it is a plant worth some respect.