The Blue Spruce (Picea pungens) is a coniferous evergreen native to the Rocky Mountains of North America. It is sometimes called the Colorado Blue Spruce, reflecting its native range in states like Colorado, Utah, and New Mexico. The tree is popular for ornamental plantings due to its distinctive foliage and symmetrical pyramidal shape.
The Defining Hue of Blue Spruce
The color of the Blue Spruce is not a uniform blue but a spectrum of hues, ranging from silvery-blue to grayish-green. In the wild, the species exhibits wide variation, with some individuals appearing green while others display the bright, silvery-blue for which the tree is named. The most desirable color is often described as glaucous, denoting a pale, grayish-blue appearance.
The characteristic blue appearance comes from a waxy, powdery layer on the needles. This silvery-blue hue makes the tree a favorite for use as a specimen or focal point in gardens and parks. Cultivars selected for the nursery trade are specifically chosen for this intense, year-round coloration.
The Science Behind the Silvery Appearance
The silvery-blue visual effect is caused by epicuticular wax, which forms a waxy coating over the plant’s cuticle. This coating is composed of hydrophobic organic compounds that project as three-dimensional crystals from the needle surface. Structural elements of this wax are often tiny hollow nanotubes.
This layer of wax acts as a physical structure that interacts with light, reflecting specific wavelengths. The presence of these wax nano-objects affects how light penetrates the needle, causing the underlying green chlorophyll to appear blue or silver. If the wax layer is chemically removed, the needles become standard green, demonstrating that the wax is the direct cause of the blue color. The wax also serves several protective functions, including reflecting UV radiation and helping the tree retain moisture.
Factors Influencing Color Variation
The intensity of the blue color is highly variable and depends on genetics, age, and environmental conditions. Genetic factors are the most significant influence, as needle coloration is a highly heritable trait. Trees grown from naturally pollinated seeds often exhibit a wide range of colors, from deep green to bright silver-blue.
For a reliably intense blue color, most commercially available trees are grafted, named cultivars, such as ‘Hoopsii,’ ‘Fat Albert,’ or ‘Koster.’ Environmental factors can modify the color intensity, as the wax can wear off due to exposure to wind, heavy rain, or winter conditions. New growth typically produces a brighter color, and the intensity of the blue often increases during the tree’s first 7 to 10 years.