In the Garden: Kalmia, the Un-True Laurel

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By Charles Kidder

Kalmia latifolia
Kalmia latifolia

Occasionally you run across plant names containing the modifier “false,” such as False Hellebore (Veratrum viride). For some strange reason, this sets me to imagining some Shakespearean-type hero exclaiming, “Oh, thou un-true Hellebore, thou most false Hellebore!” Not sure who Hellebore was, but obviously not some one to be trusted.

In the plant world, the terms “true” and “false” don’t carry the moral weight they might in literature. For example, a true Laurel is simply a species in the genus Laurus, just one of the several genera in the family Lauraceae. There are many plants that have “laurel” somewhere in their common names, but alas, most are not true Laurels.

Some of the laurel pretenders are found in the genus Kalmia, and are members of the heath family (Ericaceae), which includes azaleas, rhododendrons and blueberries. The seven Kalmia species—or eight, depending on which taxonomist you believe—are exclusively American. Most are found in eastern North America, with one found in the western part of the continent and another in Cuba. Some species are attractive plants, but have never made inroads into the gardening world. Several occur farther to the south, primarily in the Carolinas, while one has a particularly interesting natural distribution: the New Jersey Pine Barrens, the North Carolina Coastal Plain, and a few rocky mountain tops in the southernmost peaks of the Blue Ridge—but nowhere in between.

We’re most familiar with the Mountain Laurel (Kalmia latifolia), one of those Maine-to-Florida plants that ranges from the southern corner of the first state to the northwest corner of the latter. And as is usual in plants with such a wide distribution, those native to the northern extremes are likely genetically far removed from their Deep South cousins.

Irrespective of its origins, Mountain Laurel is one of our most beautiful native shrubs. Its irregular branches can reach 10 to 15 feet in height, even up to 25’ in the largest individuals. Never a tidy meatball of a plant, mountain laurel will become scraggly-picturesque with age. In forest conditions, its sinewy branches weave around in the understory. I recall my goddaughter referring to them as “vine-trees” during a hike in the mountains.

Mountain Laurel is best known for its gorgeous flowers. Even as white or pinkish buds they are quite showy, and only become more striking as they open up into ¾” white bells that are held in clusters of a couple of dozen flowers. There are dozens of cultivars on the market, some with deep red buds, others with pink, red or purplish flowers, and some with bands of contrasting color in the flower chalices. Many fine cultivars were developed at the University of Connecticut, but I have read that some of these don’t perform well in the south. I can’t really say if that includes us in the Upper South, but you might consider the cultivar ‘Pristine’, discovered in a native population near Aiken, South Carolina.

At its best, Mountain Laurel leaves are a glossy green. If planted in too much sun, however, entire leaves can become somewhat yellowish in winter. If the leaf veins remain green but the rest of the leaf turns yellow, this is a sign of chlorosis, a lack of chlorophyll in the leaves caused by high pH soil. Kalmias, like most heath family members, want acid soil. This is not usually a problem in our area, but if you have any doubts, run a soil test to determine the pH. It should be no higher than 6.0. You’re most likely to have a higher pH around fresh mortar in building foundations, so planting a Mountain Laurel right by your house might not be the best idea.

Various fungal diseases can produce spots on the foliage of Mountain Laurels, both in the wild and in gardens. Aside from using nasty fungicides for control—not recommended—good horticultural practices are the best way to head off problems.

Good air circulation is important to warding off fungal diseases, and this is a tricky balancing act. You want your plant in a place where it’s not crowded by other shrubs, and the air is free to move, but you don’t want it exposed to excessively drying winds. High overhead shade is best, especially from late morning through afternoon. Too much moisture on the leaf surfaces can promote fungal growth; on the other hand, you do want the plant’s roots to receive adequate moisture. Once again, a balancing act.

Indeed, Mountain Laurel has been referred to as a “finicky” plant by one authority. To a certain degree, this gives lie to such truisms as, “Native plants are tough survivors. Lower maintenance, lower water needs, etc.” Any plant needs to be in the right place. You may see Mountain Laurel growing in many places in the wild locally, but there are many places you will not see it. In addition to the cultural caveats given above, good drainage is key to successfully growing these shrubs. A moist, north-or east-facing slope would be ideal.

But what if you want to try growing Mountain Laurel in less than ideal conditions? I’ve heard a couple of different strategies to address the drainage issue. One is to plant on top of the ground, presumably scratching up the soil surface first. Then you pile pine bark around the root ball. This would certainly give you the desired drainage, but I would worry about the plant drying out quickly. Frequent watering would be indicated, especially in the first year. A similar, but less radical, strategy, involves planting high, with about half the root ball above the ground. The planting hole should be wide and amended with pine bark or a similar material; then, more pine bark is placed around the root ball as in the first example.

A note of caution about Mountain Laurel and the other Kalmias: they are poisonous to livestock and humans, evidenced by such common names as Sheep-Kill and Lamb-Kill. Enjoy them, but don’t plant where critters can munch.