Blue spruce explained

The blue spruce (Picea pungens), also commonly known as Colorado spruce or Colorado blue spruce, is a species of spruce tree native to North America in Arizona, Colorado, Idaho, New Mexico, Utah and Wyoming.^[1] It is noted for its blue-green colored needles, and has therefore been used as an ornamental tree in many places far beyond its native range.^[2]

Description

In the wild, Picea pungens grows to as much as 500NaN0 in height,^[3] but more typically tall. When planted in parks and gardens it most often grows tall with a spread of .^[4] It has scaly grey-brown bark with a slight amount of a cinnamon-red undertone on its trunk, not as rough as an Engelmann spruce. On older trees the trunk bark will be deeply furrowed and scaly. The diameter of the trunk may reach as much as .

Blue spruces are conifers with a pyramidal or conical crown when young, but more open and irregular in shape as they become older. The stout branches grow out horizontally in well defined whorls, but lower branches droop downwards as trees age.^[5] Young twigs are never hang downwards and are yellow-brown in color.

The narrow, needle like, evergreen leaves are quite sharply pointed and may be dull green, blue, or pale white. Each of the needles is four sided with stomata on every side, stiff, and NaNfrac=4NaNfrac=4 long. The needles are attached radially to their shoots, but curve upward. The leaf buds are golden brown and cone shaped. The buds may be 6to in size and the tip may either be blunt or pointed.

The pollen producing cones, more properly strobili, develop throughout the crown of blue spruce trees, but are more common in the upper half of the crown.^[6] Pollen cones are mainly yellow with a touch of red and average long. The seed cones begin growing in May or June and release their mature seeds in the autumn of the same year in which they start to grow. When young they are purple-brown in color. When fully mature they are light brown with thin, papery scales and are often curved. Overall they are longer than they are wide, between 8and long, and circular in cross section. The seed cones are only found at the top of the tree. This helps to facilitate cross-pollination.^[7]

The seeds are dark brown. They average 4 mm in length with the papery wing extending beyond the tip almost twice this length.

Chemistry

The phytochemistry of the blue spruce is relatively little studied.^[8] The ripe seeds have a 1.17% yield of essential oils while the cones produce only 0.38% when steam distilled for four hours. The main component, over 40%, of the essential oils is limonene with β-Pinene and α-Pinene the next most significant.

Taxonomy

Picea pungens was given its first valid scientific description by George Engelmann in 1879. He had previously named it Abies menziesii in 1862 and then as Picea menziesii in 1863 after, but both those names had already been used making them illegitimate names.^[9]

Names

Picea, the genus name, is thought to come from the Latin word pix meaning "pitch", a reference to the typical sticky resin in spruce bark. The specific epithet pungens means "sharply pointed", referring to the leaves.

The most frequently used common name in English is blue spruce. It was first used for other trees in 1817 and is still used for any spruce tree with a glaucous blue color to their needles, but most frequently meaning Picea pungens. Though this is the most common name, in the wild only part of the population has the waxy blue-gray coating for which the tree is named. Less frequently, but still common, is Colorado blue spruce, a name first used in 1912. The usage of Colorado spruce dates to 1881, but is less frequent than the longer alternate. Occasionally encountered are the names Parry's spruce, prickly spruce, silver spruce, and white spruce. Blue spruces are also rarely called silvertip fir, but this name is also applied to Abies magnifica especially when sold as Christmas trees. In addition it is sometimes labeled as "Colorado green spruce" or "green spruce" by plant nurseries or tree farms.^{[10] [11]}

Similar to the meaning of the scientific name, the Navajo name for this species is a compound c’ó deniní with c’ó meaning spruce and deniní meaning "it is sharp".

Ecology

Blue Spruce occurs at high elevations, NaN-1NaN-1 in the forests of the South Central Rockies and NaN-1NaN-1 in the Southern Rocky Mountains. It grows in mesic montane conifer forests, often associating with Douglas-fir, ponderosa pine, or white fir. It has a riparian affinity, preferring moist soils such as those along streams or at the edges of wet meadows. The Douglas-fir or ponderosa pine only become associated with streams at lower, warmer elevations. It also may be found alongside the quaking aspen (Populus tremuloides) in the high mountain habitats of desert ranges in the Intermountain West.

Climate

Blue spruce usually grows in cool and humid climatic zones where the annual precipitation mainly occurs in the summer.^[12]

Blue spruce is most common in Colorado and the Southwest. The annual average temperature ranges from 3.9 to 6.1 degrees C (39 to 43 degrees F). And ranges from - 3.9 to - 2.8 degrees C (25 to 27 degrees F) in January. In July, the average temperature ranges from 13.9 to 15.0 degrees C (57 to 59 degrees F). The average minimum temperature in January ranges from - 11.1 to 8.9 degrees C (12 to 16 degrees F), and the average maximum temperature in July ranges from 21.1 to 22.2 C (70 to 72 degrees F). There is a frost-free period of about 55 to 60 days from June to August.^{[13] [14]}

Annual mean precipitation generally vary from 460 to 610 mm (18 to 24 in). Winter is the season with the poorest rainfall, the precipitation is usually less than 20 percent of the annual moisture falling from December to March. Fifty percent of the annual precipitation occurs during the growing season of the plants.^[15]

Blue spruce is generally considered to grow best with abundant moisture. Nevertheless, this species can withstand drought better than any other spruce. It can withstand extremely low temperatures (-40 degrees C) as well. Furthermore, this species is more resistant to high insolation and frost damage compared to other associated species.

Distributed soil types and topography

Blue spruce generally exists on gentle uplands and sub irrigated slopes, in well-watered tributary drainage, extending down intermittent streams, and on lower northerly slopes.^[16]

Blue spruce always grow naturally in the soils which are in the order Mollisols, and the soil will also be in the orders histosols and inceptisols in a lesser extent.^[17]

Blue spruce is considered as a pioneer tree species in moist soil in Utah.^[18]

Rooting habits

Blue spruce seedlings have shallow roots that penetrate approximately 2.5inches into the soil during the first year of growth. Although freezing can't damage much in blue spruce, frost heaving will cause seedling loss. Shadows in late spring and early autumn minimize this frost heaving loss.^{[19] [20]} Despite the shallow roots, blue spruce is able to resist strong winds. Five years before transplanting, the total root surface area of 2-meter-high trees was doubled by pruning the roots of blue spruce. It also increases the root concentration in drip irrigation pipeline from 40% to 60%, which is an advantage in landscape greening.^[21]

Pests and diseases

The blue spruce is attacked by two species of Adelges, an aphid-like insect that causes galls to form. Nymphs of the pineapple gall adelgid form galls at the base of twigs which resemble miniature pineapples and those of the Cooley's spruce gall adelgid cause cone-shaped galls at the tips of branches. The larva of the spruce budworm eat the buds and growing shoots while the spruce needle miner hollows out the needles and makes them coalesce in a webbed mass. An elongated white scale insect, the pine needle scale feeds on the needles causing fluffy white patches on the twigs and aphids also suck sap from the needles and may cause them to fall and possibly dieback. Mites can also infest the blue spruce, especially in a dry summer, causing yellowing of the oldest needles.^{[22] [23]} Another insect pest is the spruce beetle (Dendroctonus rufipennis) which bores under the bark. It often first attacks trees which have blown over by the wind and when the larvae mature two years afterwards, a major outbreak occurs and vast numbers of beetles attack nearby standing trees.^[24]

The blue spruce is susceptible to several needle casting diseases which cause the needles to turn yellow, mottled or brown before they fall off. Various rust diseases also affect the tree causing yellowing of the needles as well as needle fall. Canker caused by Cytospora attacks one of the lower branches first and progressively makes its way higher up the tree. The first symptom is the needles turning reddish-brown and falling off. Meanwhile, patches of white resin appear on the bark and the branch eventually dies.

It is also relatively intolerant of light pollution and when planted near street lights or other outdoor lighting its preparation for winter can be delayed and parts of the tree may be damaged.

Range

The native range of the blue spruce is largely in the Central and Southern Rocky Mountains and moist mountain valleys and canyons to the west. In New Mexico it only grows naturally in the higher mountain ranges of the state such as the Sandia–Manzano Mountains, Sangre de Cristo Mountains, and San Juan Mountains, as well as on Sierra Blanca Peak to the south. In Arizona the range is even more limited, growing in just Coconio and Apache counties.^[25] In Apache County it is found in the White Mountains in central eastern Arizona and the Lukachukai Mountains in the northeastern corner of the state. In Coconino County they only grow on the Kaibab Plateau. The blue spruce grows in every county in the western two-thirds of Colorado; approximately half of natural range of the species is in the mountains of Colorado. In Utah they are a locally common part of forests in the Uinta Mountains. West of the Uintas blue spruces are less frequent in canyons south of Salt Lake City.^[26]

The blue spruce has become naturalized outside of its native range. In North America has escaped from cultivation in the states of Minnesota and New York. It has also become established to some extent in many western and northern European countries including Iceland, Norway, Sweden, the United Kingdom, France, and Belgium. In middle and southern Europe it is found in Germany, Switzerland, Austria, the former Czechoslovakia, and mainland Italy. To the east it grows in European portions of Russia, the Caucasus, and Bulgaria.

Notable trees

The tallest documented blue spruce tree is an individual in the San Juan Mountains of southern Colorado in the Hermosa Creek area. When measured by Matt Markworth in 2015 it was 54.9m (180.1feet) tall. Just three years later in 2018 it was threatened by the 416 Fire. Though the fire killed a shorter American champion tree with a larger trunk and crown spread the tall tree was spared due to being located in a sheltered valley.^[27]

Cultivation

Picea pungens and its many cultivars are often grown as ornamental trees in gardens and parks. It is also grown for the Christmas tree industry. It grows best in USDA growing zones 1 through 7,^[28] though it also does well in zones warmer than 7 where summer heat is moderate, as at San Francisco.^[29]

Common cultivars (those marked have gained the Royal Horticultural Society's Award of Garden Merit):^[30]

• 'Baby Blue Eyes', 'Baby Blueeyes', or 'Baby Blue'^[31] – This is a semi-dwarf cultivar that grows slowly, but may eventually reach 15– in height. It has a pyramidal shape and holds its color well.^[32]
• 'Fat Albert' – compact perfect cone to 10– of a silver blue color^[33]
• 'Globosa' ^[34] – shrub from 3– in height^[35]
• 'Hoopsii' ^[36] – A full size variety with a dense pyramidal habit known for "excellent" silver-blue color of its foliage. It reaches 30– tall when full grown.^[37]
• 'Koster' – A medium sized cultivar that will reach 8–^[38]
• 'Montgomery' – a slow growing dwarf variety. It will typically only grow 3– tall in eight years, but may eventually reach a height of over 8order=flipNaNorder=flip.^[39]
• 'Pendula' – drooping branches, spreads to about 4– wide by 2– tall^[40]
• 'Sester's Dwarf' – denser foliage than the species, slowly grows to about 6– tall^[41]

Culture

The Navajo and Keres Native Americans use this tree as a traditional medicinal plant and a ceremonial item, and twigs are given as gifts to bring good fortune. In traditional medicine, an infusion of the needles is used to treat colds and settle the stomach. This liquid is also used externally for rheumatic pains.^[42]

The blue spruce is the state tree of Colorado.^[43] It officially became Colorado's state tree on 7 March 1939 when House Joint Resolution 7 was enacted by the legislature. Previously a vote of the state's school children was taken on Arbor Day in 1892 expressing their preference for the blue spruce as the state tree.^[44]

From 1933 until 2014 the blue spruce was also the state tree of Utah. It was replaced by the quaking aspen because the aspen is a great deal more common than the blue spruce in Utah, making up 10% of the state's tree cover.^{[45] [46]}

See also

References

Books

• Book: Ackerfield . Jennifer . Lipscomb . 2015 . Flora of Colorado . en . First . Fort Worth, Texas . Botanical Research Institute of Texas . 61 . 978-1-889878-45-4 . 910162216.
• Book: Barnes . Burton V. . Burton V. Barnes . Wagner . Warren H. Jr. . Warren H. Wagner . Illustrations by Sarah Phelps . 1981 . 1913 . Michigan Trees: A Guide to the Trees of Michigan and the Great Lakes Region . registration . en . Revised and Enlarged . Ann Arbor, Michigan . University of Michigan Press . 78–79 . 978-0-472-08018-2 . 80039717 . 7170918 . 1 October 2024.
• Book: Cronquist . Arthur . Arthur Cronquist . Holmgren . Arthur H. . Holmgren . Noel H. . Reveal . James L. . James L. Reveal . 1972 . Intermountain Flora : Vascular Plants of the Intermountain West, U.S.A. . registration . en . 1. Geological and Botanical History of the Region, Its Plant Geography and a Glossary. The Vascular Cryptogams and the Gymnosperms . First . Bronx, New York . New York Botanical Garden . 320442 . 30 November 2024 . .
• Book: Goor . Amihud Y. . Barney . Charles Wesley . 1968 . Forest Tree Planting in Arid Zones . registration . en . 330 . 419436 . 3 December 2024.
• Book: Harrison . S.G. . Dallimore . William . William Dallimore . Jackson . A. Bruce . Albert Bruce Jackson . 1966 . A Handbook of Coniferae and Ginkgoaceae . registration . en . Forth . London . . 373 . 1703374 . 22 September 2024.
• Book: Harrison . Lorraine . 2012 . RHS Latin for Gardeners . en . First . London . Quid Publishing . 172 . 978-1-84533-731-5 . 797981038.
• Book: Heil . Kenneth D. . O'Kane, Jr. . Steve L. . Reeves . Linda Mary . Clifford . Arnold . 2013 . Flora of the Four Corners Region: Vascular Plants of the San Juan River Drainage, Arizona, Colorado, New Mexico, and Utah . en . First . St. Louis, Missouri . . 89 . 978-1-930723-84-9 . 0161-1542 . 2012949654 . 859541992 . 6 October 2024 . .
• Book: Johnson . David W. . Stimm . Bernd . Roloff . Andreas . Andreas Roloff . Lang . Ulla M. . Weisgerber . Horst . 2014 . Picea pungens . Enzyklopädie der Holzgewächse: Handbuch und Atlas der Dendrologie . Encyclopedia of Woody Plants: Handbook and Atlas of Dendrology . de . Weinheim, Germany . Wiley . 1–8 . 10.1002/9783527678518.ehg1996012 . 978-3-527678518 . 900416230.
• Book: Jones . John R. . 1973 . Southwestern Mixed Conifers . Silvicultural systems for the Major Forest Types of the United States . Agriculture Handbook No. 445 . en . First . Washington, D.C. . U.S. Department of Agriculture . 47 . 5 October 2024.
• Book: Krüssmann . Gerd . Johann Gerd Krüssmann . 1972 . Handbuch der Nadelgehölze . Handbook of Conifers . registration . de . Berlin . Paul Parey . 216–217 . 3-489-71422-9 . 304738 . 23 September 2024.
• Book: Little . Elbert L. . Elbert Luther Little . Southwestern Trees : A Guide to the Native Species of New Mexico and Arizona . 1950 . U.S. Department of Agriculture . Washington, D.C. . 17-18 . en . First . registration . 5 October 2024.
• Book: Quattrocchi . Umberto . 2012 . CRC World Dictionary of Medicinal and Poisonous Plants . en . Boca Raton, Florida . . 2923 . 978-1-4822-5064-0 . 774639599.
• Book: Sargent . Charles Sprague . Charles Sprague Sargent . . 1898 . The Silva of North America: A Description of the Trees Which Grow Naturally in North America Exclusive of Mexico . en . XII Coniferae (Abietinece after Pinus) . First . Boston, Massachusetts . Houghton, Mifflin and Company . 47–49, Tab DC . 1077591401 . 27 November 2024.
• Book: Sjöman . Henrik . Anderson . Arit . 2023 . The Essential Tree Selection Guide: For Climate Resilience, Carbon Storage, Species Diversity and Other Ecosystem Benefits . en . First . Bath, England . Filbert Press . 161–163 . 978-1-7399039-4-7 . 1400102923.
• Book: Stuart . John David . Sawyer . John O. . Illustrated by Andrea J. Pickart . 2001 . Trees and Shrubs of California . en . First . Berkeley, California . University of California Press . 31–32 . 978-0-520-22109-3 . 44267780.
• Book: Vedel . Helge . Lange . Johan . Hillman . C.H.R. . Edlin . H.L. . 1960 . 1958 . Trees and Bushes in Wood and Hedgerow . registration . en . First English . London . Methuen & Co . 119–120 . 978-0-416-61780-1 . 1151411431 . 2 December 2024.
• Book: Wooton . Elmer Ottis . E. O. Wooton . Standley . Paul Carpenter . Paul Carpenter Standley . 1915 . Flora of New Mexico . Contributions from the United States National Herbarium, Volume 19 . en . First . Washington, D.C. . Smithsonian Institution . 34 . agr15001072 . 3786500 . 5 October 2024.
• Book: Young . Robert W. . Robert W. Young . Morgan . William . William Morgan (Navajo scholar) . 1980 . The Navajo Language : A Grammar and Colloquial Dictionary . registration . en, nv . First . Albuquerque, New Mexico . University of New Mexico Press . 296 . 978-0-8263-0536-7 . 6597162 . 5 October 2024.

External links

• Conifers.org: Picea pungens (blue spruce) description
• Lady Bird Johnson Wildflower Center NPIN — Native Plant Information Network
• Interactive Distribution Map of Picea pungens
• Picea pungens — U.C. Photo Gallery

Notes and References

1. Flora of North America Editorial Committee, editor. 1993. Flora of North America North of Mexico. Volume 2. Pteridiophytes and Gymnosperms. Oxford University Press, New York, 475 pp
2. Web site: Picea pungens (blue spruce) description . Earle . Christopher J. . 26 February 2023 . The Gymnosperm Database . en . 24 September 2024 . live . https://web.archive.org/web/20231211081155/https://www.conifers.org/pi/Picea_pungens.php . 11 December 2023.
3. Web site: Taylor . onald J. . Picea pungens . . 21 September 2024 . 5 November 2020.
4. Web site: Picea pungens . Plant Finder . . St. Louis, Missouri . en . 21 September 2024 . live . https://web.archive.org/web/20240523174142/https://www.missouribotanicalgarden.org/PlantFinder/PlantFinderDetails.aspx?taxonid=284991 . 23 May 2024.
5. Web site: Picea pungens (Blue Spruce, Colorado Spruce) . North Carolina Extension Gardener Plant Toolbox . . 22 September 2024 . en.
6. Web site: Picea pungens Engelm. . Burns . Russell M. . Honkala . Barbara H. . December 1990 . Silvics of North America . . . Washington, D.C. . en . 9780160271458 . 91600537 . 25008780 . 24 September 2024 . live . https://web.archive.org/web/20240722121855/https://www.srs.fs.usda.gov/pubs/misc/ag_654/volume_1/picea/pungens.htm . 22 July 2024.
7. Web site: 2021 . Yale Nature Walk, Blue Spruce . 2021-02-21.
8. Wajs‐Bonikowska . Anna . Szoka . Łukasz . Karna . Ewa . Wiktorowska‐Owczarek . Anna . Sienkiewicz . Monika . Composition and Biological Activity of Picea pungens and Picea orientalis Seed and Cone Essential Oils . Chemistry & Biodiversity . March 2017 . 14 . 3 . 10.1002/cbdv.201600264.
9. 30095300-2 . Picea pungens Engelm. . 24 September 2024.
10. Web site: Colorado Green Spruce (Picea Pungens) . Forest Start . 25 September 2024.
11. Web site: Picea pungens: Blue Spruce, Colorado Blue Spruce, or Green Spruce (Pinaceae - Pine Family) . Department of Horticulture and Crop Science . OSU PocketGardener . . Columbus, Ohio . 25 September 2024 . live . https://web.archive.org/web/20240605220156/http://hvp.osu.edu/pocketgardener/source/description/pi_ngens.html . 5 June 2024.
12. Web site: Fechner . Gilbert H. . Blue Spruce . 2018-11-23.
13. Bates . Carlos C. . Forest types in the central Rocky Mountains as affected by climate and soils . U.S. Department of Agriculture, Bulletin 1233 . 152p.
14. Pearson . G. A. . Forest types in the Southwest as determined by climate and soil. . U.S. Department of Agriculture, Technical Bulletin 247 . 144p.
15. Pearson . G. A. . Forest types in the Southwest as determined by climate and soil . U.S. Department of Agriculture, Technical Bulletin 247 . 144p.
16. Web site: Fechner . Gilbert H. . Blue Spruce . 2018-11-22.
17. Moir . William H. . Ludwig . John A. . A classification of spruce-fir mixed conifer habitat types of Arizona and New Mexico. . USDA Forest Service, Research Paper RM-207..
18. Dixon . Helen . 1935 . Ecological Studies on the High Plateaus of Utah. . Botanical Gazette . 97 . 2 . 272–320 . 10.1086/334554 . 2471603 . 84538967.
19. Alexander . Robert R. . 1974 . Silviculture of central and southern Rocky Mountain forests : a summary of the status of our knowledge by timber types . USDA Forest Service Research Paper . en . Fort Collins, Colorado . Rocky Mountain Forest and Range Experiment Station . RM-120 . 19 . 5 October 2024.
20. Pearson . G. A. . Forest types in the Southwest as determined by climate and soil. . U.S. Department of Agriculture, Technical Bulletin 247..
21. Watson . Gary . Sydnor . T. Davis . 1 May 1987 . The Effect of Root Pruning on the Root System of Nursery Trees . Arboriculture & Urban Forestry . en . 13 . 5 . 126–130 . 10.48044/jauf.1987.027 . free . 5 October 2024.
22. Web site: Gilman, Edward F. . Watson, Dennis G. . 2011-05-01 . Picea pungens: Colorado Spruce . 2013-10-06 . EDIS . IFAS Extension Service: University of Florida.
23. Web site: Cranshaw, W. S. . 2013-06-13 . Scale Insects Affecting Conifers . 2013-10-05 . Colorado State University Extension.
24. Web site: Ciesla, Bill . 2013-04-19 . Spruce Beetle Threatens High Country Spruce Forests . 2013-10-05 . Colorado State University Extension.
25. PIPU . Picea pungens . 5 October 2024.
26. Mauk . Ronald L. . Henderson . Jan A. . 1984 . Coniferous Forest Habitat Types of Northern Utah . USDA Forest Service General Technical Report . en . Ogden, Utah . Intermountain Forest and Range Experiment Station . INT-170 . 31–32 . 9 October 2024.
27. Web site: A Tale of Two Trees and a Wildfire . Harper . Liz . American Forests . en . 30 November 2024 . live . https://web.archive.org/web/20241130045318/https://www.americanforests.org/article/a-tale-of-two-trees-and-a-wildfire/ . 30 November 2024.
28. Web site: Colorado Blue Spruce . 2018-11-20 . treecanada.ca.
29. Web site: 11 February 2023 . Blue spruce (Picea pungens) .
30. Web site: July 2017 . AGM Plants - Ornamental . 25 April 2018 . Royal Horticultural Society . 78.
31. Web site: Baby Blue Colorado Spruce - Picea pungens 'Baby Blue Eyes' . WSU Clark County Extension – PNW Plants . . Pullman, Washington . en . 24 September 2024 . live . https://web.archive.org/web/20240713230735/https://pnwplants.wsu.edu/PlantDisplay.aspx?PlantID=243 . 13 July 2024.
32. Web site: Picea pungens 'Baby Blueeyes' . Plant Finder . . St. Louis, Missouri . en . 24 September 2024 . live . https://web.archive.org/web/20240924214557/https://www.missouribotanicalgarden.org/PlantFinder/PlantFinderDetails.aspx?taxonid=267995 . 24 September 2024.
33. Web site: Picea pungens 'Fat Albert' . Plant Finder . . St. Louis, Missouri . en . 24 September 2024 . live . https://web.archive.org/web/20230228205616/https://www.missouribotanicalgarden.org/PlantFinder/PlantFinderDetails.aspx?kempercode=e660 . 28 February 2023.
34. Web site: Picea pungens (Glauca Group) 'Globosa' – Colorado spruce 'Globosa' . Conifers/RHS . . London . en . 24 September 2024 . live . https://web.archive.org/web/20240228133347/https://www.rhs.org.uk/plants/91537/picea-pungens-(glauca-group)-globosa/details . 28 February 2024.
35. Web site: Picea pungens 'Glauca Globosa' . Plant Finder . . St. Louis, Missouri . en . 24 September 2024 . live . https://web.archive.org/web/20240523081243/http://www.missouribotanicalgarden.org/PlantFinder/PlantFinderDetails.aspx?kempercode=c825 . 23 May 2024.
36. Web site: Picea pungens (Glauca Group) 'Hoopsii' – Colorado spruce 'Hoopsii' . Conifers/RHS . . London . en . 24 September 2024 . live . https://web.archive.org/web/20211208133848/https://www.rhs.org.uk/plants/98036/picea-pungens-(glauca-group)-hoopsii/details . 8 December 2021.
37. Web site: Picea pungens 'Hoopsii' . Plant Finder . . St. Louis, Missouri . en . 24 September 2024 . live . https://web.archive.org/web/20240703052242/https://www.missouribotanicalgarden.org/PlantFinder/PlantFinderDetails.aspx?taxonid=255100 . 3 July 2024.
38. with a conical shapeWeb site: Picea pungens (Glauca Group) 'Koster' – Colorado spruce 'Koster' . Conifers/RHS . . London . en . 24 September 2024 . live . https://web.archive.org/web/20240924213602/https://www.rhs.org.uk/plants/59991/picea-pungens-(glauca-group)-koster/details . 24 September 2024.
39. Web site: Picea pungens 'Montgomery' . Plant Finder . . St. Louis, Missouri . en . 24 September 2024 . live . https://web.archive.org/web/20240921173347/https://www.missouribotanicalgarden.org/PlantFinder/PlantFinderDetails.aspx?taxonid=263630 . 21 September 2024.
40. Web site: Picea pungens 'Pendula' . North Carolina Extension Gardener Plant Toolbox . . Raleigh, North Carolina . en . 24 September 2024 . live . https://web.archive.org/web/20240922202453/https://plants.ces.ncsu.edu/plants/picea-pungens-pendula/ . 22 September 2024.
41. Web site: Sester's Dwarf Blue Spruce - Picea pungens 'Sester's Dwarf' . WSU Clark County Extension – PNW Plants . . Pullman, Washington . en . 24 September 2024 . live . https://web.archive.org/web/20240707110222/https://www.pnwplants.wsu.edu/PlantDisplay.aspx?PlantID=253 . 7 July 2024.
42. http://naeb.brit.org/uses/species/2936/ U. Michigan-Dearborn: Ethnobotany
43. Web site: June 11, 2009 . State Trees & State Flowers . dead . https://web.archive.org/web/20101206125016/http://www.usna.usda.gov/Gardens/collections/statetreeflower.html . 2010-12-06 . 2010-05-26 . United States National Arboretum.
44. Web site: Symbols & Emblems . Colorado State Archives . . 25 September 2024 . live . https://web.archive.org/web/20240818200929/https://archives.colorado.gov/collections/symbols-emblems . 18 August 2024.
45. News: Randle . Sterling . 13 February 2014 . Quaking Aspen moves closer to replacing Blue Spruce as State Tree . live . BYU Daily Universe . en . . https://web.archive.org/web/20240603120256/https://universe.byu.edu/2014/02/13/quaking-aspen-moves-closer-to-replacing-blue-spruce-as-state-tree/ . 3 June 2024 . 25 September 2024.
46. News: Wood . Benjamin . 26 March 2024 . Student-advocated state tree bill signed into law . live . Deseret News . Monroe, Utah . https://web.archive.org/web/20221123173256/https://www.deseret.com/2014/3/26/20538192/student-advocated-state-tree-bill-signed-into-law . 23 November 2022 .