Bog laurel (Kalmia polifolia) is native to northeastern North America. It is pollinated by bees, but when the bees have pollinated this plant, the honey they produce is poisonous. The plant is highly toxic to humans and animals. Caribou, however, can eat most plants, including this one.
I. Appearance and CharacteristicsÂ
Kalmia polifolia, previously known as Kalmia glauca and commonly called bog laurel, swamp laurel, or pale laurel, is a perennial evergreen shrub of cold acidic bogs, in the family Ericaceae. It is native to north-eastern North America, from Newfoundland to Hudson Bay southwards.
The genus ‘Kalmia’ is named after Pehr Kalm, a Swedish-Finn botanist, who was a student of Linnaeus. The species name, ‘polifolia’, is Latin for ‘pole-leaves’ or ‘pole-petals’.
The former species name, ‘glauca’, is Latin for gleaming or gray, a word ultimately derived from the Ancient Greek ‘γλαυκός’, meaning blue-green or blue-gray.
The plant was first described by Friedrich Adam Julius von Wangenheim, a German botanist.
Kalmia polifolia flowers in April and is pollinated by bees. Bees, however, after pollinating this plant, produce a poisonous honey. Its seeds ripen in September. These seeds are five-parted, round, and woody.
Kalmia polifolia can grow to be two feet tall. Its leaves are arranged oppositely upon its branch and grow to be an inch to an inch and a half in length and tend to be waxy with an entire and revolving margin. Below each leaf base there are ridges, where it appears as though a part of the leaf is curled around the circumference of the stem. This is especially noticeable lower on the plant.
The base of the petiole is pressed against the stem as its flowers cluster in a single terminal bunch, which appears to be pink or purple in color; the near cup-shaped flower spans about three-eighths of an inch in diameter.
II. How to Grow and Care
Bog-laurel thrives in acidic, moist but well-drained soil, with a preference for cool climates. Special care is required in ensuring soil maintenance to prevent root rot and providing partial shade to mimic bog-laurel’s natural understory habitat. Regular watering that keeps the soil consistently moist, without becoming waterlogged, is crucial. Despite these specific needs, bog-laurel is considered easy to care for due to its hardiness and low maintenance once the appropriate conditions are met.
Sunlight
The bog laurel prefers partially shaded conditions due to its origins as a mountain woodland plant. It will grow in both full sun or deep shade; however, deep shade is likely to inhibit flowering. Ideally, your bog-laurel should be provided with dappled shade.
Temperature
The bog-laurel comprises several different genera of plants, with the most hardy able to tolerate temperatures down to -32 ℃. For the most part, the bog-laurel knows how to look after itself and will adapt to local conditions. In severe cold, it may experience damage; measures should be taken to protect the plant from freezing conditions or strong, cold winds. The bog-laurel needs more water than most shrubs due to its shallow root system. However, it is important to provide adequate drainage as it can be prone to root rot.
Watering
Bog laurel thrives in its native moist environments, exhibiting a high tolerance for wet conditions. This species typically prefers consistent moisture and can withstand brief periods of water saturation. It is accustomed to watering once every 2-3 weeks, maintaining a balance between hydration and drainage. Given its preference for humid climates, bog-laurel is often found flourishing outdoors where it can benefit from natural rainfall patterns. Its growth cycle is closely tied to water availability, with optimal development occurring in regions where moisture levels are naturally regulated by the environment.
Soil
The bog laurel prefers moist acid or sterile soils with good drainage. Some varieties, such as Mountain bog-laurel, are sensitive to heavy clays, although Bog laurel will only grow in wet or poorly-drained sites. Mountain bog-laurel will often do better in a raised bed that allows for extra soil to provide better drainage. Most varieties do best in soils rich in nutrients and organic matter.
Soil acidity is very important to the bog-laurel, particularly when the plant is young, with an ideal soil pH of between 4.5 and 7.5. pH testing will show soil acidity; these can either be bought from home stores, or a home-made kit can be made using red cabbage. If the soil shows insufficient acidity, a layer of wood chips or evergreen bark mulch should be added. In the spring, plant food formulated for acid-loving shrubs, such as rhododendron food, should be used. However, care should be taken using plant food when the plants are young, to prevent burning the plant’s roots.
Fertilizing
Fertilizing bog laurel with a balanced nutrition fertilizer or high nitrogen variant promotes robust health and flowering. For optimal growth, apply fertilizer sparingly in spring, as excess can harm. Utilize a formulation designed for acid-loving plants, applying according to label guidelines to avoid over-fertilization. Seasonal changes impact nutritional demands; lighter feeding is required in fall. Always water bog laurel after fertilizing to distribute nutrients. Ensuring the right fertilizer balance is key to bog-laurel’s vitality.
Planting Instructions
The bog laurel can be planted from early spring once the danger of late frost has passed, and also during the summer. The soil should be cool when the bog laurel is planted, with shrubs spaced 1.5 to 2 m apart. The bog-laurel should be situated away from direct strong winds, particularly in colder climates. The shrub’s crown (where the trunk meets the roots) should be at or slightly above ground level to avoid rot.
Pruning
Leggy plants can be avoided by pruning heavy branches to between 30 cm and 1 m in length. They can be pruned all the way back to 30 cm off the ground if needed, which will result in a bushier plant within a few years. The removal of fruit capsules as they develop during flowering will also encourage vigorous plant growth and blooming. If the aesthetic of flower clusters or the appearance of berries is undesirable, flower clusters can be carefully removed.
Because the plant’s buds set in the spring and early summer, pruning of the bog-laurel should be done straight after flowering in the spring/summer. Deadheading expired flower blooms throughout the season will help flower production the following year.
Propagation
Native to North America, bog-laurel thrives in moist, well-drained acidic soils typical of boggy areas. This evergreen shrub is best propagated through cuttings. Gardeners should select healthy semi-hardwood stems and treat them with rooting hormone to encourage growth. Ensuring a high humidity environment, like a greenhouse or under a plastic cover, will help cuttings develop roots effectively. The key to success is maintaining consistent moisture without waterlogging, which can lead to rot. Gently firming the soil around the new cuttings aids in stability and root establishment.
III. Uses and BenefitsÂ
- Medicinal uses
Kalmia polifolia can be used topically for skin wounds, disease, and inflammation, while internal uses may address bleeding and diarrhoea.
- Culinary uses
While caribou do not have specialized food habits, they can eat most plants – preferring fungi, green leaves of deciduous shrubs, and new spring growth of sedges. They often eat Kalmia polifolia in the spring and summer; the plant comprises 11% of their dietary dry-matter protein.
- Other uses
As poison
Every part of Kalmia polifolia is highly toxic and poisonous to animals and persons. Although related to Kalmia angustifolia (lambkill), it is less toxic.
Certain indigenous groups have used the toxicity of the plant’s leaves to commit suicide.
According to Alaback et al., Kalmia polifolia contains a grayanotoxin, which when ingested lowers blood pressure, and may cause respiratory problems, dizziness, vomiting, or diarrhea. According to Lewis and Elvin-Lewis, the active compounds within Kalmia polifolia include andromedotoxin and arbutin. These compounds cause a slowing of pulse, a lowering of blood pressure, progressive paralysis and death. Kalmia polifolia has poisoned cattle, goats and sheep. For poisoning to occur, the animal must consume 0.3% of its body weight, while a dosage of 2% of an would cause severe sickness.
Symptoms for affected goats include depression, nausea, salivation, vomiting, and grating of teeth.
Symptoms for affected sheep include depression, staggering, nausea, recubency, salivation, and vomiting.
For aesthetics
Kalmia polifolia was planted in Montreal’s First Nation Garden, among another 300 species planted.
IV. Harvesting and Storage
Seeds are easy to collect and can be stored for many years before planting. Seed capsules can be harvested as soon as they have turned brown and dry, to be put in a paper bag for several weeks whilst they dry further. The fully dried capsules will open, allowing the seeds to be shaken loose. Stored at room temperature and out of light, seeds will remain viable for several years. Before planting, seeds can be pre-treated with gibberellic acid to aid in germination.
