According to the ASPCA website's list of plants known to be toxic (or non-toxic) to various animals, the mountain laurel [kalmia latifolia] is indeed highly toxic to dogs, cats, and horses.
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Laurel shrubs are plants from a number of different families. While one type is grown as a spice, many are grown purely for horticultural value. The bay laurel can grow to be a large tree, but is often grown by cooks as a shrub or container plant, so that its leaves can be used to season foods. Mountain laurel and cherry laurel shrubs are grown as horticultural plants. The growth requirements of these plants are quite different.
The bay laurel shrub, Laurus nobillis — sometimes also known as the true laurel — is a large shrub that can reach 33 to 59 ft (10 to 18 m) high in Mediterranean regions. It is usually pruned to be a shrub rather than a tree. In cool areas, it is often grown as a potted plant. The bay laurel is one of the oldest cultivated plants, and is mentioned in Biblical and ancient Chinese and Greek writings.
Generally, laurel shrubs prefer soil that is kept somewhat moist. They prefer sun, but can tolerate some shade. The tough, leathery leaves have a high quantity of essential oils, which makes them a desirable spice. The older leaves are of higher quality than the younger ones, in terms of cooking.
The leaves of some other laurel shrubs are poisonous to livestock or humans. For instance, the mountain laurel shrub, Kalmia latifolia, is grown for its beautiful flowers and lush foliage. These leaves, however, are poisonous to horses and cattle, but not dogs and cats. Mountain laurel shrubs usually prefer shaded conditions with moist, acidic soil, and should be fertilized once a year with a product made for acidic plants.
The plants, which do well in temperate regions, usually grow to about 8 ft (2.4 m) tall. New cultivars have become available in a wide range of colors and flowering times. The mountain laurel shrub, for example, blooms between April and June. They do not need to be pruned, but it is helpful to remove flowers to encourage more buds the following year. If the plants get too large, they can be cut down almost entirely to encourage new growth.
Another laurel shrub that is grown horticulturally, but has toxic leaves, is the cherry laurel shrub. This plant is also known, in North America, as the English laurel. Also known as Prunus laurocerasus, it looks quite a bit like the bay laurel shrub, but is unrelated. It is capable of growing to 59 ft (18 m) tall, but is usually kept small through pruning. The whitish flowers bloom in the summer, and are followed by an edible fruit that looks like a cherry.
These evergreen shrubs are widely grown in temperate regions around the world, often to mask undesirable views or as a ground cover. The plants adapt well to difficult growing conditions, including shade and drought. Due to this, they have become invasive in parts of their range.
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