Growing up in the countryside, I accepted it as fact that dock leaves had the ability to soothe the irritation of stinging nettles. Only recently have I discovered that they have little to no affect.

Terminology

Urtica dioica is the scientific name for stinging nettles, a binomial nomenclature where Urtica is the genus (a group of closely related species with shared characteristics) and dioica /dye-oh-i-kah/ is the species. Urtica derives from the Latin for ‘sting’. The family Urticaceae which includes Urtica has 150 different stinging species.

Urticaria is the name of the skin rash produced by contact with stinging nettles, it is also referred to as irritant dermatitis.

The stinging hairs are often referred to as trichomes (outgrowths) or spicules (sharp-pointed structures).

Stinging as a defence mechanism

Stinging hairs are a great example of convergent evolution and have occurred several times in distant plant groups¹.

The primary driving force in stinging plant adaptation is mammalian herbivores¹. Nettles that have been grazed by cattle show higher numbers of stinging hairs. The quick acting stinging sensation deters mammals before the plant is consumed, although some such as rabbits, sheep and red deer do persist to consume nettles.

Stinging hairs

Some subspecies of stinging nettles have hollow hairs, which allow the plant to inject fluid into the body.

These are some images of the trichomes, taken with an electron microscope². The first image shows the spicule penetrating the skin and the second shows its hollow structure. The hairs are so tiny, around 1mm in length, that they can easily penetrate the skin when lightly brushed and subsequently break off from the leaf.

At the tip is a bulb shape which breaks off to expel the stinging fluid from the hair, only 4 nL. This image shows the tips of the trichomes before and after the tip is broken¹.

Other plants which don’t inject fluid may still irritate the skin when the mineralised tips of the hairs break off and remain under the skin. Others can irritate by creating a reaction on the skin without hairs.

Toxins

There does not appear to be a simple answer to the active components in the stinging fluid as of 2021¹.

Historically, in the 19th century, the first substance to be believed as the source of stinging was formic acid. However, more recent studies¹ identify neurotransmitters as the source of irritation. Specifically acetylcholine, histamine and serotonin, the ratios of which vary greatly between samples.

Similar neurotransmitters are present in wasp and hornet venom³. The body uses serotonin for an extensive number of functions, the majority of it is used by the gastrointestinal tract. Histamine and serotonin can directly stimulate sensory nerve endings. Histamine can cause itching, burning and pain while serotonin can amplify pain reception.

It is believed that these components alone do not cause the full effect of the stinging.

Other species of stinging nettle such as dendrocnide when dried can maintain their ability to sting for hundreds of years, serotonin degrades quickly and so cannot be the only active component in this species.

Dock leaves

It is often recited that dock leaves can relieve the irritation caused by stinging nettles. This idea is often expressed poetically: nature provides the remedy in close proximity to the irritant. As dock leaves commonly grow in the same area as nettles.

The ‘scientific’ explanation for this effect is usually that the stinging nettle is acidic and the alkali liquid in the dock leaf neutralises this. However, the fluid released by crushing dock leaves is also acidic. There is a lack of scientific literature about dock leaves alleviating the sensations caused by stinging nettles, partly due to its widespread understanding of being folklore⁴.

The perceived relief may be due to the liquid cooling the area by evaporation⁴, providing another source of stimulation, or purely a placebo. This is particularly affective when given to a child by a parent, when applied to the skin following the tales of its effectiveness. The remedy of a dock leaf is a physical application of tangible affect.

The reason for dock leaves and nettles growing together is that they both prefer the same qualities in soil; nitrogen-rich, damp and disturbed areas.