Kalmia latifolia L—Mountain laurel
FAMILY: Ericaceae—the Heath Family
Widely distributed on acid soils, members of this family are found mostly in the northern temperate region. Characteristics include: flowers: regular, bisexual, and perfect; petals: usually united; leaves: alternate, opposite, or whorled on the stems; stamens: as many or twice as many as the petals; calyx: 4-7 lobed, often S; corolla: often urceolate; anthers: often appendaged, frequently opening by a terminal pore; pistil: 1; carpels: 5; style: 1; fruit: a capsule.
PHENOLOGY: Mountain laurel flowers May through July.
DISTRIBUTION: Woodlands on rocky or sandy acidic soil.
PLANT CHARACTERISTICS: Shrubs or small trees, 2 to 10 m high; petioles: 1-2 cm; leaves: evergreen, alternate, glabrous, 5-10 cm long, margin entire, dark green above, bright green below; flowers: terminal; corolla: white to rose with purple markings; anthers: held in chambers on the corolla tube until pollination; fruit: a dry, 5-celled septicidal capsule.
POISONOUS PARTS: Flowers, twigs, pollen grains, and green plant parts cause toxicity. Percentages of Kalmia (relative to animal's body weight) needed to produce symptoms, but not death, are: 0.15% (sheep), 0.2-0.4% (cattle and goats), and 1.3% (deer).
SYMPTOMS: In order of appearance, symptoms are: anorexia; repeated swallowing or eructation and swallowing of cud without mastication; profuse salivation; watering of the mouth, eyes, and nose; loss of energy; slow pulse; low blood pressure; incoordination; dullness; depression; vomiting; and frequent defecation. As poisoning progresses, animals become weak and prostrate. Difficulty in breathing is common and there is no pupillary refex; death is preceded by coma. Symptoms are similar for all classes of livestock; the time for the appearance of symptoms averages 6 hours. Postmortem: nonspecific gastrointestinal irritation and hemorrhage.
POISONOUS PRINCIPLES: Andromedotoxin and arbutin are responsible for toxicity. Andromedotoxin, a resinoid, causes vomiting by directly stimulating the vomition center; its structure is not fully known. Arbutin is a glucoside by hydroquinone.
CONFUSED TAXA: Three species of Kalmia occur in Pennsylvania: K. angustifolia L. with lateral flower clusters and K polifolia Wang and K latifolia L. with terminal flower clusters Kalmia polifolia has opposite leaf arrangement (as does K. Iatifolia, which can also have ternate leaves), whereas K. latifolia has alternate leaves.
SPECIES OF ANIMALS AFFECTED: Apparently all species of animals can be poisoned by mountain laurel. Sheep are the most susceptible of animals, cattle are next. Monkeys, angora goats, and humans have been poisoned by mountain laurel.
TREATMENT: (11a)(b); (1); (5); ( 12)
OF INTEREST: The Delaware Indians used laurel for suicide. Experiments indicate that fowl can eat relatively large quantities of mountain laurel without developing symptoms. When fed to cats, the meat was toxic. Humans have been poisoned by andromedotoxin in pollen after eating honey suspected to have been made from members of the Ericaceae.
The toxin arbutin is used commercially as a stabilizer for color photographic images and in veterinary science as a diuretic and urinary anti-infective. Many members of this family contain these or similar toxins. Evergreen plants are more commonly the cause of poisoning than the deciduous species. Plants in this family that occur in Pennsylvania and cause sickness or death similar to that described above include Ledum groenlandicum Oed., Labrador tea; Pieris japonica (Thunb,) D, Don, Lily-of-the-valley bush (commonly cultivated); and possibly Menziesia pilosa (Michx.) Juss.