The white spruce (Picea glauca) is a native to the northern reaches of North America, where it enjoys regular cool temperatures and moist conditions that keep it a bright green throughout the year. People throughout the country do not often get to enjoy these towering evergreens because of climate limitations. Horticulturalists, though, have created dozens of cultivars of Pilea glauca that have allowed people to use the species in their landscapes, from micro to weeping – you won’t be disappointed.
If you are among the lucky that can grow a white spruce in their native area, caring for them isn’t particularly difficult.
I. Appearance and Characteristics
Picea glauca, the white spruce, is a species of spruce native to the northern temperate and boreal forests in North America. Picea glauca is native from central Alaska all through the east, across western and southern/central Canada to the Avalon Peninsula in Newfoundland, and south to Montana, North Dakota, Minnesota, Wisconsin, Michigan, Upstate New York and Vermont, along with the mountainous and immediate coastal portions of New Hampshire and Maine, where temperatures are just barely cool and moist enough to support it.
There is also an isolated population in the Black Hills of South Dakota and Wyoming. It is also known as Canadian spruce, skunk spruce, cat spruce, Black Hills spruce, western white spruce, Alberta white spruce, and Porsild spruce.
The white spruce is a large evergreen conifer which normally grows to 15 to 30 metres (50 to 100 ft) tall, but can grow up to 40 m (130 ft) tall with a trunk diameter of up to 1 m (3 ft 3 in). The bark is thin and scaly, flaking off in small circular plates 5 to 10 centimetres (2 to 4 in) across. The crown is narrow – conical in young trees, becoming cylindrical in older trees. The shoots are pale buff-brown, glabrous in the east of the range, but often pubescent in the west, and with prominent pulvini. The leaves are needle-like, 12 to 20 millimetres long, rhombic in cross-section, glaucous blue-green above (hence glauca) with several thin lines of stomata, and blue-white below with two broad bands of stomata.
The cones are pendulous, slender, cylindrical, 3 to 7 cm long and 1.5 cm wide when closed, opening to 2.5 cm broad. They have thin, flexible scales 15 mm long with a smoothly rounded margin. They are green or reddish, maturing to pale brown 4 to 8 months after pollination. The seeds are black, 2 to 3 mm long, with a slender, 5 to 8 mm long pale brown wing.
- Seeds
Seeds are small, 2.5 to 5 mm long, oblong, and acute at the base. Determinations of the average number of sound seeds per white spruce cone have ranged from 32 to 130.
Common causes of empty seed are lack of pollination, abortion of the ovule, and insect damage.
The average weight per individual seed varies from 1.1 mg to 3.2 mg.
Each seed is clasped by a thin wing 2 to 4 times as long as the seed. Seed and wing are appended to the cone scale. Embryo and megagametophyte are soft and translucent at first; later the endosperm becomes firm and milky white, while the embryo becomes cream-coloured or light yellow. At maturity, the testa darkens rapidly from light brown to dark brown or black. Mature seeds “snap in two” when cut by a sharp knife on a firm surface.
White spruce cones reach their maximum size after 800 GDD. Cone moisture content decreases gradually after about 1000 GDD.
Cone color also can be used to help determine the degree of maturation, but cones may be red, pink or green. Collection and storage dates and conditions influence germination requirements and early seedling growth.
A bushel (35 L) of cones, which may contain 6500 to 8000 cones, yields 6 to 20 ounces (170 to 570 g) of clean seed.
Seed dispersal begins after cone scales reflex with cone maturation in the late summer or early fall of the year of formation. Cones open at moisture contents of 45% to 70% and specific gravities of 0.6 to 0.8. Weather affects both the initiation and pattern of seed dispersal, but cone opening and the pattern of seed dispersal can vary among trees in the same stand. Even after dispersal has begun, cold, damp weather will cause cone scales to close; they will reopen during dry weather. Most seed falls early rather than late, but dispersal may continue through fall and winter and even into the next growing season. Seed dispersal occurs mainly in late summer or early fall.
White spruce seed is initially dispersed through the air by wind. Both the initiation and pattern of seed dispersal depend on the weather, but these can vary among trees in the same stand. Small amounts of white spruce seed are normally dispersed beyond 100 m from the seed source, but exceptionally seeds have been found more than 300–400 m from the nearest seed source.
- Root system
The root system of white spruce is highly variable and adaptable, responding to a variety of edaphic factors, especially soil moisture, soil fertility, and mechanical impedance. On soils that limit rooting depth, the root system is plate-like, but it is a common misconception to assume that white spruce is genetically constrained to develop plate-like root systems irrespective of soil conditions. In the nursery, or naturally in the forest, white spruce usually develops several long ‘running’ roots just below the ground surface.
The structure of the tracheids in the long lateral roots of white spruce varies with soil nitrogen availability.
- Stem
White spruce can live for several hundred years, with an estimated average lifespan of 250 to 300 years.
Slow-growing trees in rigorous climates are also capable of great longevity. White spruce 6 to 10 m (20 to 33 ft) high on the shore of Urquhart Lake, Northwest Territories, were found to be more than 300 years old.
- Bark
The bark of mature white spruce is scaly or flaky, grey-brown or ash-brown, but silvery when freshly exposed. Resin blisters are normally lacking, but the Porsild spruce Picea glauca var. porsildii Raup has been credited with having smooth resin-blistered bark.
White spruce bark is mostly less than 8 mm and not more than 9.5 mm thick.
- Chemistry
Isorhapontin can be found in spruce species such as the white spruce.
P. glauca has three different genomes; a nuclear genome, a mitochondrial genome, and a plastid (i.e. chloroplast) genome. The large (20 Gbp) nuclear genome of P. glauca (genotype WS77111) was published in 2015, and the organellar (plastid and mitochondrial) genomes (genotype PG29) were published in SD Jackman et al. 2015. The plastid genome of P. glauca (genotype WS77111) has also been published.
II. Types of White Spruce
If you want a tree with many cultivars to explore, there may not be a better choice than the white spruce. Like many conifers, the white spruce has been cultivated into dozens, if not hundreds, of cultivars, allowing landscapers and home gardeners to choose from trees in all kinds of colors, sizes, forms, and growth rates.
Looking for a cultivar of white spruce can be confusing because they are often hidden behind the various common names for the species Picea glauca goes by—namely white spruce, Alberta spruce, skunk spruce, Canada spruce, cat spruce, and Porsild spruce. This is why botanical names are so important.
Below are just a few of the more common cultivars out there, but a search will find many more.
- Picea glauca ‘Conica’ is a growing dwarf variety that becomes conical over time.
- Picea glauca ‘Pendula’ is a weeping variety with a slow growth rate.
- Picea glauca ‘Green Carpet’ is a cultivar with blue-green needles and a flat growing habit.
- Picea glauca’ Spruce Lane’ is a very colorful cultivar with gray-green needles tipped in yellow. Cones are a vibrant red that fades to yellow at the end.
III. How to Grow and Care
Sunlight
A white spruce will grow and perform its best when planted in full sun. The species can also manage in partial shade without a noticeable impact. Certain cultivars will perform better in certain conditions, which should be noted before you purchase one.
Cold hardiness
White spruce is extremely hardy to low temperatures, provided the plant is in a state of winter dormancy. Throughout the greater part of its range, white spruce routinely survives and is undamaged by winter temperatures of −50 °C (−58 °F), and even lower temperatures occur in parts of the range. Boreal Picea are among the few extremely hardy conifers in which the bud primordia are able to survive temperatures down to −70 °C, −94 °F.
Especially important in determining the response of white spruce to low temperatures is the physiological state of the various tissues, notably the degree of “hardening” or dormancy. A natural progression of hardening and dehardening occurs in concert with the seasons. While different tissues vary in ability to tolerate exposure to stressful temperatures, white spruce, as with woody plants in general, has necessarily developed sufficient winter hardiness in its various tissues to enable them to survive the minimum temperatures experienced in the distribution range.
White spruce is subject to severe damage from spring frosts. Newly flushed shoots of white spruce are very sensitive to spring frost. This sensitivity is a major constraint affecting young trees planted without overstorey nurses in boreal climates.
Watering
Water can be an issue for the white spruce in certain areas, especially those that can become too warm. Supplemental watering may be needed to keep your white spruce vigorous and attractive if you are in the southern ranges of its habitable zone or are experiencing long stretches of dry weather.
Soil
Other than the ability of its soil to retain and drain moisture, the white spruce is very adaptable when it comes to soil. Unlike some conifers, it is not fussy regarding pH and soil consistency.
Fertilizing
The white spruce does not require any supplemental fertilizer.
Pruning
There is no need to prune white spruce to shape or form it. It only needs maintenance pruning (removing dead, dying, and damaged branches). They will grow as intended and will only need to be pruned if branches are creating a hazard.
Propagation
White spruce is most easily propagated by cutting or grown from seed. While it is possible to propagate through grafting, this is difficult, so using cuttings and seeds is the most common route for the typical home gardener to propagate the white spruce.
Here’s how to propagate white spruce using cuttings, a process best done in winter or early spring:
- Choose a healthy and green branch; use only established growth.
- Separate the cutting from the main branch with sharp shears. Look for a “node” and lively growth on a branch, then cut between the two.
- Scrape the cutting about an inch from the bottom to remove some bark, then dip the exposed piece in rooting hormone.
- Place the cut end of the branch in a pot filled with damp peat moss and tamp the medium down thoroughly around the base. Use pots at least 5 inches deep and with holes for drainage to allow for better root growth. The cuttings might take up to 6 months to develop their root systems.
- When there is ample root growth, repot to larger pots.
Overwintering
The white spruce will thrive at very low temperatures, and no overwintering is needed. Immature trees will often do better if a thin layer of mulch is placed around their base to help insult their developing roots; mature trees will not need such protection.
Pests and Diseases
Relatively disease-resistant trees when in the right conditions, white spruce can become susceptible to plant diseases when exposed to drought or higher than usual temperatures.
Two common fungal diseases that can arise in these conditions are rhizosphaera needle cast (the most common white spruce disease) and cytospora canker. Both diseases share the same symptoms: needles will turn brown in late winter to early spring and begin to drop by summer. Usually, these diseases are not fatal but can be very unsightly.
Ensure plenty of water during droughts to help stem these pathogens; be sure you’re watering the soil around the base of the tree without allowing a backsplash onto the needles. Planting with an appropriate space between trees will also defend against both diseases, as proper airflow will allow needles to dry between watering.
As far as insects go, the wild-type white spruce is very good at holding its own against pests when mature. Cultivars can be more prone to insect damage, so you must take more care and inspect them for signs of pests more often. The most common pests you will find are spider mites and bagworms. Botanical oils and insecticidal soaps can help with the problem.
III. Uses and Benefits
The wood of white spruce is of a lower quality than that of Engelmann spruce, but is stronger. It was used to make shelters and as firewood by Native Americans and European settlers in Alaska, where lodgepole pine does not grow. The wood is of major economic importance in Canada, being harvested for paper and construction. It is also used as a Christmas tree.
The wood is also exported to Japan where, known as “shin-kaya”, it is used to make go boards as a substitute for the rare kaya wood.
Black Hills spruce (Picea glauca var. densata) is used for bonsai.
White spruce is the provincial tree of Manitoba and the state tree of South Dakota.
The new growth or tips of white spruce is used in beer making, gin production, flavouring pop, candy making or in pickles and preserves.