Winter always brings out the colors in our evergreens. Some evergreens turn purple or rust-colored due to the development of anthocyanin pigments. Some other evergreens become light-green or develop a yellow cast and sadly, some evergreens, especially pines turn white or light brown especially on the south and west side of the tree.
Anthocyanins are the cause of the purple and rusty color. These pigments are the major reason for the red color of beets and the purple and blues of flower petals, leaves, roots and seeds. The purple kernels of Indian corn are the result of genes that key for the development of anthocyanins.
So far, 1,000 different anthocyanins have been identified which accounts for profusion of shades of colors resulting from this group of pigments. Arborvitae and some of the upright junipers exhibit a bronze color in part due to the anthocyanin bronze gene (Bz1). The Eastern red cedar, Juniperus virginiana, develops a bronze cast during the winter to help protect it from the winter sun but turns green again in the spring when the green chlorophyll molecules increase in number and the anthocyanin pigment degrades. Not all evergreens have the full complement of active genes necessary to develop anthocyanins. Our two native upright junipers, Juniperus scopulorum and osteosperma, generally don't develop as large a concentration of anthocyanins and thus don't show as much bronze as their eastern cousin. -
Anthocyanin pigments increase when a tree is attacked by pathogens, exposed to cold temperatures or exposed to high levels of ultraviolet light. Anthocyanins help protect against these stresses. These flavonoids not only help protect plants but are the antioxidants your doctor recommends for your health. They also have been used to develop flexible organic solar cells because of their strong light harvesting, and their ability to convert light energy into electrical energy.
Additional pigments called carotenoids also appear in the winter. The carotenoids xanthophyll (yellow) and carotene (yellow-orange) pigments, however, are always present but don't appear until the green chlorophyll pigments degrade in the winter. The carotenoids develop along with chlorophyll in the chloroplasts, organelles where photosynthesis takes place. Sadly, the natural colorful anthocyanin and carotenoid pigments are not the only reason for a change in color of some our evergreens.
Some of the color change is due to dehydration of stems and needles. The needles of some pine trees are turning white from the tip back to the stem. The majority of this damage appears on the south and/or west side of the tree due to the high light intensity these needles are exposed to. Needles continue to give off moisture during the winter and if they can't replace what they lose due to a lack of adequate soil moisture they turn white or brown.-
In some cases the moisture loss from the tissue is so great any amount of soil moisture would be inadequate to replenish it. This is especially true with shrub-type junipers that were sheared last fall. The opens wounds caused by this practice allowed moisture to be pulled out of stems as well as the awn-like needles resulting in death. It was interesting to see the proliferation of lollipop- and marshmallow-shaped junipers appear through the Grand Valley last fall. This spring it will be interesting to do a tally on how many of these junipers exhibit severe winter dehydration. Ensuring these and or other evergreens have soil moisture they can utilize is critical to help avoid this winter injury.
A week ago I was checking the soil in the lavender research plots at Colorado State University's Research Center on Orchard Mesa. The soil had thawed to a depth of about 8 inches so I took up a handful and squeezed it into a ball. If the soil had stayed in a ball when I opened my hand it would have indicated sufficient moisture. As soon as I opened my hand the soil ball fell apart. This simple test told me if evergreens were growing in this soil, they would not be able to obtain sufficient water to replenish that lost from the needles. This test also told me I had better drag hoses and wet everything down to help reduce winter injury to these lavender plants.-
Dr. Curtis E. Swift is a retired horticulture agent with the CSU Extension. Reach him at Curtis.Swift@alumni.colostate.edu or check out his blog at http://SwiftsGardeningBlog.blogspot.com. He owns Swift Horticultural Consulting and High Altitude Lavender.