One of the most annoying noxious weeds found here in Arizona is the Rough Cockleburr or Common Cocklebur (Xanthium strumarium). This native annual is usually found in washes, wetlands, disturbed areas, drainage channels, and in fields here in southeastern Arizona. Rough Cockleburr is a widespread species, and it is found throughout the United States in every state except Alaska.
Rough Cockleburrs can grow up to 5 feet (1.5 m) tall, and they have coarsely-toothed, heart-shaped or maple-like leaves. Their most notable characteristic is their annoying abundance of spiny burs. The hard, prickly, dried burs are brown in color and have hook-tipped spines that easily attach to fur, wool, or woven clothing like sweaters or socks. Their hitchhiking burs are likely how these Rough Cockleburrs came to be so widespread.
Rough Cockleburr burs are not only a nuisance with woolly livestock like sheep, they can also cause problems with pets as well. I've had to pull these nasty, painful burs out from between my poor dogs' toes and comb tangles of them out of my Golden Retriever's coat.
As if being extremely weedy and having spiny burs were not enough to make this plant truly noxious, the seeds and seedlings of Rough Cockleburr are also quite poisonous and contain the highly toxic glycoside carboxyatractyloside. Carboxyatractyloside is a plant growth inhibitor and is principally found in the seeds and cotyledons (seed leaves), and not in the rest of the plant. Carboxyatractyloside poisoning in animals or humans can cause severe hypoglycemia, multiple organ dysfunction, coma, and even death.
Because of the spiny burs, animals rarely eat the poisonous seeds inside them, but animals will unfortunately sometimes be sickened or killed from grazing on dense patches of hundreds of poisonous Rough Cockleburr seedlings. Humans (especially children and young people) have been made seriously ill or have died from consuming Rough Cockleburr seeds, which both look and taste rather like sunflower seeds.
An additional annoying characteristic of Rough Cockleburr is that it is capable of causing allergic contact dermatitis in susceptible persons. I'm obviously not one of those persons because I've brushed against these green plants numerous times without any effect.