Sheep Laurel (Kalmia angustifolia)

Sheep laurel (Kalmia angustifolia) is an evergreen shrub that grows to 91 cm tall. It is a poisonous plant with no edible qualities and can be found growing in bogs, swamps and other wetlands. It blooms late spring and early summer with clusters of deep pink flowers, shaped like saucers.

I. Appearance and Characteristics 

Kalmia angustifolia is a flowering shrub in the family Ericaceae, commonly known as sheep laurel. It is distributed in eastern North America from Ontario and Quebec south to Virginia. It grows commonly in dry habitats in the boreal forest, and may become dominant over large areas after fire or logging. Like many plant species of infertile habitats it has evergreen leaves and mycorrhizal associations with fungi. It is also found in drier areas of peat bogs.

The attractive, small, deep crimson-pink flowers are produced in early summer. Each has five sepals, with a corolla of five fused petals, and ten stamens fused to the corolla. They are pollinated by bumblebees and solitary bees. Each mature capsule contains about 180 seeds.

In the wild the plant may vary in height from 15–90 cm (6–35 in). New shoots arise from dormant buds on buried rhizomes. This process is stimulated by fire. The narrow evergreen leaves, pale on the underside, have a tendency to emerge from the stem in groups of three. The Latin specific epithet angustifolia means “narrow-leaved”. A peculiarity of the plant is that clusters of leaves usually terminate the woody stem, for the flowers grow in whorls or in clusters below the stem apex.

Kalmia contains a glycoside, known as andromedotoxin. It is poisonous to mammals. Hence, it can be unwelcome in pastures. Several of its folk-names testify to the plant’s toxicity: ‘lamb-kill’, ‘sheep-kill’, ‘calf-kill’, ‘pig laurel’, ‘sheep-laurel’ and ‘sheep-poison’. It is also known as narrow-leaved laurel and dwarf laurel.

II. How to Grow and Care

Sheep laurel is a resilient plant that flourishes in well-drained, acid soils, and requires consistent moisture without being waterlogged. Special care points include its sensitivity to high pH levels and the need for partial shade in regions with strong sun. Regular mulching helps maintain soil moisture and cool the roots, crucial for sheep laurel’s health.

Sunlight

The sheep 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 sheep laurel should be provided with dappled shade.

Temperature

The sheep laurel comprises several different genera of plants, with the most hardy able to tolerate temperatures down to -32 ℃. For the most part, the sheep 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 sheep 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

Sheep laurel thrives in its native moist, acidic soils of North American bogs and pine barrens, where it has adapted to periodic dry spells. This species exhibits a preference for consistent moisture but is also known for its moderate drought tolerance once established. It benefits from watering once every two weeks to maintain optimal hydration without waterlogging. Typically grown outdoors due to its natural habitat, sheep laurel particularly favors the cool, humid conditions found in higher latitude climates, which can influence both its flowering potential and overall growth vigor.

Soil

The sheep laurel prefers moist acid or sterile soils with good drainage. Some varieties, such as Mountain sheep laurel, are sensitive to heavy clays, although Bog sheep laurel will only grow in wet or poorly-drained sites. Mountain sheep 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 sheep 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

For sheep laurel, a balanced nutrition fertilizer is ideal, promoting vigorous growth and abundant blooming. Annually, in early spring, apply a slow-release formulation, suited to acid-loving plants. Use sparingly, as sheep laurel is sensitive to over-fertilization. Adjust amounts based on plant size and label guidance. In its growing season, a diluted liquid feed monthly can supplement nutrients. Avoid fertilization in late fall to prevent new growth susceptible to frost damage. Implementing these targeted practices ensures a thriving sheep laurel.

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 sheep 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

Sheep laurel is a broadleaf evergreen shrub known for its vibrant flowers and glossy foliage. Propagation of sheep laurel is effectively achieved through cuttings, specifically semi-hardwood cuttings taken in late summer. Gardeners should ensure cuttings are healthy, around 4-6 inches long, and include several leaf nodes. After cutting, the bottom leaves are removed, and the base is treated with a rooting hormone to encourage growth. The cuttings should then be planted in a well-draining, sterile medium, kept moist, and provided with indirect light. Root development typically occurs within a few weeks, and after a strong root system is established, transplanting into the garden or a permanent pot can proceed.

Transplanting

The best time to transplant sheep laurel is typically during the late autumn to early winter (S2-S3), as the plant can effectively establish roots in this cool, moist period. While transplanting, choose a location with partial shade and well-drained soil. When necessary, press firmly around the root zone to keep soil contact.

III. Uses and Benefits 

Sheep laurel is a bushy shrub often used to create borders or as a landscaping plant. It also works well in rock and cottage gardens, and in woodland areas to provide summer color. The plant produces clusters of small flowers throughout the warmer months. The evergreen shrub also provides year-long greenery when other plants have died back.

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.