Plant Exchange Blog

You may find a green tree like this nearby. The Colorado Blue Spruce (Picea pugens) is found in many yards and in public spaces in the region. 

This one is about 30 ft. tall and nearly twenty years old. Evergreen trees add green to the winter landscape, while deciduous trees have lost leaves and are dormant until spring. 

Colorado Blue spruce is commonly planted on the Northern Plains, prized for its full-bodied shape and blue/gray needles. The tree tolerates the slightly alkaline soil found in the region. When the tree reaches maturity at about 30 years, it may lose its lower branches. Needles may begin to discolor as it becomes more susceptible to fungal and insect pests. The tree’s defenses may become overwhelmed. Another blue spruce with fewer pest issues is the Black Hills spruce (Picea glauca var. densata) is an alternative. (Trees! Field Guide, by John Ball)

Conifers like the Colorado blue spruce continue to grow in winter. Winters of fluctuating, cold temperatures and lack of sufficient moisture challenge all evergreens. What are some of the spruce defenses?

One tree defense is communication by way of scent. Spruces register pain when limbs are cut by releasing scent compounds. If you bring a spruce bough indoors for winter décor, its wintergreen scent is welcome as you display the blue/gray needles. When cut or attached by insects, the tree releases the scent into the air. It alerts other Colorado blue spruces, other blue spruces, and other trees around it that its limb is cut. The scent is released quickly and doesn’t travel many feet away. Alerting nearby trees gives them a chance to respond according to their defenses.

Tree-produced scents that are slower acting may engage the whole tree’s defense. When a specific insect pest is a problem in summer, some trees respond by releasing a pheromone that draws beneficial insect predators. 

Other defenses besides scent include storing oils in the spruce needles that act like antifreeze for the tree in winter. Producing bitter chemicals like tannin in oak leaves deter tasting by pests. Communication can occur between tree roots, sometimes aided by a fungal network in the soil. Other tree defenses include the social interaction of tree groves and bark adaptations as the tree ages. More on this topic in The Hidden Life of Treesby Peter Wohlleben.

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