A couple who were building a home in Tennessee asked me (as a Master Gardener) why their landscaper advised them not to use spruce trees (Picea species) in their Tennessee landscape. Was there a serious disease afflicting spruce trees here? I didn’t have an answer to those questions but promised to research the issue then let them know what I found out.
Results of my research: Spruce trees are in the botanic genus Picea. Cold tolerance USDA hardiness zones 2-7, heat tolerance American Horticulture Society Heat Zones 7-2. These coniferous, needled evergreens are found in various cold forested regions in the world. Some are native to North America (blue or Colorado spruce, Brewers spruce, Engleman spruce, Sitka spruce, white, red and black spruce ). Some are very large (up to 200 feet). The dwarf Alberta spruce grows slowly (to 10 feet in 25 years). They are popular as Christmas trees and landscape ornamentals (particularly blue spruce).
Spruce trees prefer sunny locations in a cold climate. In general, place spruces in moist, acidic, deep loam soils. They will tolerate less than ideal soils if moisture is adequate. Moisture is an essential in the first few years.
I have personally experienced spider mite infestations with dwarf Alberta spruce. The perfect cone shape is spoiled by bare spots from mite damage unless the tree is pre-treated to prevent such infestations. I don’t use landscape plants requiring lots of pesticides nor do I recommend them to homeowners. A brochure on "Spruce Problem Diagnosis for Yard Trees" by the Minnesota Forestry Department of Natural Resources (http://files.dnr.state.mn.us/assistance/backyard/treecare/forest_health/whitesprucediagnosis.pdf) has great pictures showing damage and recommending remedies for non-fatal pest problems, and/or diseases.
There were no indications that serious diseases of spruce trees mentioned in reference books or on internet websites were especially bad in our state. Why had the nurseryman dealing with these homeowners insisted spruce trees were not good yard trees for Tennessee? I got my answer when I checked with the UT Extension agriculture agent in Coffee County (who taught the urban forestry class during my Master Gardener training).
He explained that spruce trees, even though some are indigenous to North America, don’t like the warmth and humidity of our southern climate. Each plant has a range of high and low temperatures to which it is best suited. This varies from species to species. In Tennessee, Picea varieties are at the southern edge of their natural thermal range. Spruce trees grow naturally in Tennessee forests only at high elevations in the eastern mountains. Planting spruce in shady spots to get them out of the hot sun does not usually remedy the problem.
Heat damage to spruce trees (as well as to other species at the extreme threshold of their heat tolerance range) is not as noticeable as cold injury. Typically, the plant just fails to thrive. The specimen is more likely to sustain serious damage or die during hot, dry spells than species with better heat tolerance.
Another aspect of constant heat stress is that the plant’s natural defense mechanisms can’t resist disease infections and insect pest infestations effectively. The plant may slowly decline then die after a few years or linger on suffering one insect or disease problem after another until it finally succumbs. If we see temperature increases throughout the 21st century due to global warning, heat stressed species, like the spruce may not survive (let alone thrive) in Tennessee forests, yards and gardens.
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Plateau Gardening is written by Master Gardeners for gardeners in Tennessee’s Upper Cumberland Region. UT Extension Cumberland County at P.O. Box 483, Crossville, TN 38557 (931-484-6743) has answers horticulture questions, free publications and details on how to become a Master Gardener. Send email comments or yard & garden inquiries to Master Gardener Rae, email@example.com.