Across the acidic-soiled woodlands and mountains of the Eastern United States grows an evergreen shrub called mountain laurel (Kalmia latifolia). Rather mundane most of the year, it bursts into bud and flower anytime from middle to late spring, bearing rosy pink to white flowers. Among North America's most ornamental native shrubs, gardeners grow mountain laurels in U.S. Department of Agriculture plant hardiness zones 5 through 8.
Mountain laurel grows where the winters are cool enough to force a dormancy. The temperatures dip anywhere into the 15 to minus 15 degrees Fahrenheit range. The native range of the shrub extends from Southeast Mississippi northward to Indiana and Ohio and northeastward to Norfolk, Virginia, and up to Southern Maine. It occurs in lowlands and in mountains, affecting its precise timing of flowering each spring. It grows well in gardens in other parts of the country with acidic soils, humid air and appropriate temperatures.
The lower the elevation and latitude, the earlier the mountain laurel blooms. Shrubs bloom anytime from mid-April to the summer solstice. In the Deep South, in USDA zone 8, flowering commences in mid-April and peaks just before May 1. In zone 7, flowering starts around May 1 and peaks in the second week. In higher elevations and farther north in zone 6, blooming peaks in middle to late May, and into early June in areas with higher elevation or farthest north in latitude. Across inland New England, mostly in USDA zone 5, flowering occurs around mid-June.
Factors Affecting Flowering Time
The precise timing of mountain laurel flowering varies year to year based on seasonal weather patterns. While average flowering time frames may be deduced for any region, a cool, wet and cloudy spring can delay flowering one or two weeks. Conversely, an unusually warm spring with lots of sunny weather moves the flowering season earlier seven to 14 days. Once flowers open, temperatures between 50 and 75 F make blossoms last the longest. Heat, wind and heavy downpours shorten flower cluster longevity.
The pointed flower buds develop and swell on the mountain laurel over two weeks prior to opening. The buds themselves lend an attractive textural and color contrast to the oval green leaves. Buds often are dark red to rosy pink and open to reveal lighter colored blossoms that range from coral red to medium pink or white. Flowers individually open in the bud cluster.