Curly dock has all the traits of a super weed
With high levels of oxalic acid, like that in Brussels sprouts, and with a proliferation of seed dispersal, the plant easily establishes itself everywhere except Greenland
During a recent trip to photograph landscapes on the Navajo Nation, I was surprised to see the prairie visually dominated by a robust plant, 3 to 4 feet tall, with thick stems, large leaves and showy plumes of red flowers. On the Colorado Plateau, this species is referred to as Navajo tobacco, but in most places in its immense range it is curly dock, Rumex crispus.
When early botanists compiled lists of species from colonial herbarium collections in North America, curly dock was not found before 1788. However, in lists compiled in 1814 and 1848, curly dock was described as common and very common.
Curly dock is native to northern Europe and Asia and was probably introduced accidentally, as a contaminant in shipments of grain to North America in the 1700s. Today, it is found everywhere except Greenland.
Curly dock grows without herbivore pressure, making it a formidable invader.
The genus Rumex has approximately 200 species, referred to as docks and sorrels. Some docks have small geographic ranges due to exceedingly precise ecological requirements, while others are cosmopolitan due to their ability to thrive in a wide range of habitats.
Some docks are endemics, while others are weedy or invasive and curly dock may be the most invasive. What makes curly dock such a successful invasive species? Common characteristics of invasive species are ability to become established in disturbed environments, good defenses against herbivores, ability to adapt to a wide range of environments and relatively high fecundity.
I stopped at a field to walk among the plants to gain some insight and soon noted that the plants were healthy with no evidence of herbivory by insects, cows, deer or antelope and no obvious seed predation by birds. All of these signs made me suspect that this species synthesized an effective chemical defense.
Curly dock has a familiar defense system, for it synthesizes the same chemical compound, oxalic acid, as brussels sprouts, broccoli, cauliflower, Swiss chard and spinach. Young leaves have tolerable levels of oxalic acid or oxalate, but as the leaves and seeds mature, oxalate levels soar, making them bitter and toxic.
Oxalate binds with calcium ions to form an insoluble compound that damages kidneys. Oxalates in leaves and seeds are toxic to poultry and dangerous to cattle, sheep and horses. Most mammals simply avoid curly dock.
Curly dock quickly becomes established in disturbed environments such as dirt roads, overgrazed fields, or areas swept by floods. Once established, it is extremely difficult to remove. It has a branching taproot that reaches over 3 feet deep and the taproot can form more taproots that feed multiple crowns.
Like dandelions, if the stems are broken off at the surface, roots quickly grow fresh stems and leaves. Each seed is attached to the calyx of its flower, which helps it float in water or attach to animal fur for dispersal. A single plant can produce thousands of seeds, each containing a chemical that prevents microbial decay so the seeds can be viable for 50 years in undisturbed soil. The numbers of seeds in a field may be as high as 5 million per acre.
Curly dock, known as Navajo tobacco on the Colorado Plateau, grows everywhere except Greenland.
Curly dock is remarkably adaptive. It can thrive in disturbed environments, roadsides, on mud in tidal estuaries, meadows, forest edges and shorelines. It hybridizes with other Rumex, both here and in Europe, and while the hybrids do not linger for long, these exchanges could conceivably help curly dock gather genes to adapt to specific environments.
It is also distributed across an incredible range of habitats, from southern Texas and Florida to northern Alaska. It seems that this super weed can go anywhere except Greenland.
While curly dock is a formidable threat to native plant communities, it has some positive aspects. If the leaves are picked while they are young, oxalate is still at low concentrations. If young leaves are boiled with several changes of water to extract oxalate, dock leaves are edible and a good source of iron, potassium and vitamins A and C. Finally, Rumex is a larval host for local copper butterflies in the genus Lycaena, including the purplish copper, blue copper, ruddy copper and bronze copper.
Did you enjoy this article? Passionate about ecology and evolutionary biology? Show your support.