Mountain Laurel


Mountain Laurel N Mt Marshall 1-24-04
The rime-edged leaves of Mountain Laurel attest to its hardiness


Common Name: Mountain Laurel, Calico-bush, Lambkill, Spoonwood, Ivy-bush – The entire, alternate leaves are similar in appearance to the leaves of the Bay Laurel (Laurus nobilis) native to the Mediterranean region; bay (laurel) leaves are added as an aromatic herb to many Italian dishes. It is a dominant understory species in mesic upland regions; the general notion is to convey the idea of a laurel-like shrub of the mountains.

Scientific Name: Kalmia latifolia – The generic name honors Pehr Kalm, a Swedish-Finnish botanist and explorer; one of Carolinus Linnaeus’ primary field agents, he traveled to Pennsylvania in 1748 to catalogue New World plants with the imprimatur of the scientist Ben Franklin and the naturalist John Bartram. The generic eponym was assigned by Linnaeus in recognition of Kalm’s contributions to taxonomy. Latifolia is from the Latin words for ‘wide leaf’ as a key distinguishing characteristic.

Potpourri: Mountain Laurel forms a virtually impenetrable thicket of gnarled evergreen shrubs that dominate the understory of mixed hardwood forests at higher elevations where appropriate environmental conditions prevail. A member of the heath or heather family (Ericaceae – erice is Latin for heath), it shares the heath distinguishing attributes; almost all ericaceous plants have evergreen leaves and shrubby growth to support a tenuous habitat of infertile acidic soil. The success of mountain laurel in particular and of heaths in general, is their adaptation to occupy an otherwise onerous and barren wasteland – the term heath conveys the ubiquity of the shrubby, axenic monoculture of the British Isles. Mountain laurel domination is an evolutionary triumph of adaptation that succeeded due to three factors: 1) Hermaphroditic pistol-cocked flowers to promote fertilization; 2) Specialized mycorrhizal fungi to extract soil resources; and 3) Toxic foliage and stems to forfend herbivore browsing and arthropod consumption.

Mountain Laurel Bloom AT Roanoke 150606
The alternate name Calico-bush suggests miniature summer dresses.

The pink-tinged bouquets of white flowers that surround densely packed mountain laurel shrubs convey nature’s spring festival with elven delicacy. The alternate name Calico-bush for the dyed cottons of Calcutta (now Kolkata) India suggests miniature summer dresses. Close inspection of the individual flower reveals a physiological complexity that belies the skin deep metaphor of beauty. Each flower has 10 stamens that are neatly tucked into 10 matching pits or niches that are radially and symmetrically placed so that there are two in each of the five petals that comprise the corolla. As the flowers open and expand, a tensile bending stress is imparted to each stamen, potential energy cocked and ready to become kinetic. Pollinators, drawn by the beckoning nectar deep within the style, land tentatively at the center of the flower; their movement provides the trigger that disrupts some or all of the ten stamens. The forager is unwittingly pummeled by pollen-laden anthers at the stamen tips and thereby dusted with the male pollen of fertilization deposited at the next blossom to complete the vicarious sexual act at the pistil and stigma. The force imparted to the pollen by the snapping of the stamen is quite substantial; individual pollen spores can travel more than five inches from the source. The end result is that the mountain laurel is a proficient pollen launcher, enhancing successful fertilization. As if this were not enough to ensure propagation, the pollen does not even need to travel from one flower to another, as mountain laurel has perfect flowers; they have the ability to hermaphroditically self-fertilize. While this survival strategy would seem both practical and admirable, it is in most cases eschewed by angiosperms due to the lack of genetic diversity that it provides; living things that don’t have the genetic mutations of heterogeneous ancestry are less likely to survive environmental stressing according to Darwinian principles. The myriad flowers arrayed in a branching corymb yield copious quantities of seed ready to initiate germination for subsequent generations.

Mountain Laurel Seeds Robinson Mountain 161203
The prodigious seeds ensure regeneration and dominance

Most plants share their nutrients with fungi, receiving water and minerals from the extension of their roots to the fungal hyphae in return. This mutualistic relationship is known as a mycorrhiza from the Greek mykes meaning fungus and rhiza meaning root; literally fungus root. The mycorrhizal relationship is thought to be of primordial origin, the photosynthetic plants that originated in the sea needed a means of extracting nutrients from the comparatively barren land and the fungi that had specialized in epigeal growth in root-like hyphae provided it. Lichens are another example of a plant kingdom – fungi kingdom partnership as they are by definition a combination of the two. Ericaceous plants have their own specialized mycorrhizal fungi with which they partnered about 140 million years ago, the specialization necessitated by the marginal soil conditions and acidity to which both are adapted. Ericoid mycorrhizas differ from the more common endomycorrhizas that penetrate the root cells of a host plant and ectomycorrhizas that cover the outside of (primarily tree) root cells in that they form coils that penetrate the cortex layer of the root but do not actually penetrate the plant cells. The identification of the specific fungi allied with ericaceous plants and the manner in which nutrients are exchanged is the subject of current research; a part of the nascent understanding of the nature of the complex relationships between fungi and plants. One notably ubiquitous candidate is Rhizoscyphus ericae, the species name indicating the singularity of the ericoid singularity.

Mountain Laurel Robinson Mtn 161203
Mountain Laurel dominates the trailside on Robinson Mountain

Mountain Laurel is poisonous to many animals including humans; the common name Lambkill is not arbitrary, as sheep are among the most susceptible to its toxins. While likely apocryphal, it is common lore that Pehr Kalm discovered its virulence when several sheep in his expeditionary party nearly succumbed to the debilitating effects of its browse in the 18th Century. Once spread by the many seeds from perfect and prodigious flowers and nourished by specialized ericoid fungi, its bitter poisons deter herbaceous animals; mountain laurel burgeons in nearly impenetrable copses. It is a potent poison: about 0.15 percent of mountain laurel parts relative to animal body weight induces symptoms in sheep; for a 100 pound sheep that would be one ounce or about 20 leaves. The critical dose rises to 0.2 percent for cows, 1.3 percent for deer and 2 percent for goats. Birds are apparently immune, but cats that eat the birds are affected. It is rarely fatal to humans. The symptoms, which vary somewhat according to animal type and dosage, are anorexia (lack of appetite), eructation (belching), watery eyes and loss of energy. With time and increased dosage, symptoms progress to breathing difficulties and eventually comatose death due to gastrointestinal hemorrhage.


The poisons of Kalmia latifolia are andromedotoxin and arbutin; they are common to other ericaceous plants such as those of the genus Rhododendron. The shared plant chemistry that provides a measure of protection against predation suggest that the heath plants share a common ancestor that initially evolved the toxic mix to survive in an environment of large herbivores. Poisons are also an evolutionary result of the need for insect repellent, leaf-eating insects are the bane of the woods. The ubiquity of heath plants suggests a success against the entire animal kingdom. Arbutin is a sugar based compound called a glycoside that is used as a color stabilizer and as a skin lightening agent (it represses melanin). Andromedotoxin is a diterpene associated with plant resin that disrupts the sodium ion channels that are critical to nerve impulse signaling; its variegated disruption effects are evidence of some breach in controlling functionality. These poisons are important for another reason; the human vector is insidiously apian. Honey bees are drawn to the nectar of mountain laurel flowers to collect it as part of their quotidian routine. The honey generated for hive nutrition and surreptitiously garnered for human consumption is laced with the concentrated poisons. The result, which has earned the sobriquet “mad honey,” can have a variety of effects according to the particular heath family plant from which it was derived and the degree to which that plant was used exclusively in the specific honey formulation. In extreme cases, the honey can cause diaphragm paralysis and concomitant respiratory cessation. In the case of honey made from mountain laurel or the rhododendrons, mild to severe inebriation results. Mad honey figures prominently in European culture and history. The Roman naturalist Pliny the Elder (Gaius Secundus) wrote of the defeat of the invincible Romans by the Anatolian Heptakometes, the former reduced to delirium and nausea due to eating mad honey.

The complementary yin and yang of poison and medicine is a matter of dosage and degree of susceptibility. Poisons can be useful in killing or driving away harmful microbes and parasites and would therefore be used as human medicines. Mountain laurel is a case in point. As an endemic North American plant, it was the autochthonous Native American peoples who first experimented with and discovered by trial and error what it might be useful in treating. While records are few, the Cherokee and the Cree are recorded as the most skilled practitioners; mountain laurel was used primarily as a wash for rheumatic joint pain, as a topical disinfectant treatment against vermin and, as a poison, to commit suicide. Mountain laurel wood was carved into small utensils; the alternative name Spoonwood is one vestige of this. The early colonists, faced with the same frontier ailments of the Indians, adopted their pharmaceuticals and expanded on their use. The King’s American Dispensary of 1898 provides that “In immoderate doses, sheep laurel is a poisonous narcotic” but “In medicinal doses, it is antisyphilitic, sedative to the heart, and somewhat astringent.” It was used as a remedy for the scourge of syphilis before the discovery of penicillin by stewing the leaves in lard to make an ointment that was topically applied to the resultant chancres with reportedly salutary results; the salve was also good for rheumatism. A dilute tincture of Kalmia was administered in droplet dosage for “febrile and inflammatory diseases and hypertrophy of the heart.” Brandy and whiskey were offered as antidotes to the poison of overdose, which may have given rise to the epithet “dead drunk.”