Blue Spruce (Picea pungens)

Blue Spruce, Colorado Spruce

The blue spruce (Picea pungens) is an evergreen conifer with a beautiful, thick crown. It gets the “blue” name because its needles have a bluish tint, unlike other pine trees whose needles are a simpler green. This unique appearance has helped to make the blue spruce one of the world’s favorite ornamental conifers, and it’s especially popular in Christmas tree production. Historically, these trees have also served other ornamental purposes.

I. Appearance and Characteristics 

The blue spruce (Picea pungens), also commonly known as green spruce, 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. 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.

Blue Spruce occurs at high elevation (above 6000 ft/1830m) in the Rocky Mountains of the west-central United States. It grows in mesic montane conifer forests, often associated with Douglas-fir, ponderosa pine, or white fir. It has a riparian affinity.

In the wild, Picea pungens grow to about 23 m (75 ft), but when planted in parks and gardens it seldom exceeds 15 m (49 ft) tall by 5 m (16 ft) wide. The tree can grow larger if the tip is cut when it is at least 3 years old. It is a columnar or conical evergreen conifer with densely growing horizontal branches. It has scaly grey bark on the trunk with yellowish-brown branches.

Waxy gray-green leaves, up to 3 cm (1 in) long, are arranged radially on the shoots which curve upwards. The pale brown cones are up to 10 cm (4 in) long. Male cones are found on the entire tree, whereas the female cones are found at the top of the tree. This helps to facilitate cross-pollination.

The specific epithet pungens means “sharply pointed”, referring to the leaves.

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

II. How to Grow and Care

Sunlight

Plant Colorado blue spruce trees in full sun, ensuring they get at least six hours of unfiltered sun daily to reach their full growth potential. Blue spruce can tolerate some shade, but planting in a low-light area can increase disease incidence and severity.

Temperature and Humidity

A native to high mountain areas, Colorado blue spruce, is tolerant of dry, cold weather. However, blue spruce will not thrive in extremely hot conditions, as its needles will turn brown and fall. Still, this tree variety can tolerate heat and humidity better than other spruces.

Watering

After transplanting blue spruce, water consecutively 5-7 times, once every 5-7 days. Mix a little rooting powder in the water; this will help the roots grow. During daily care, keep the soil moist but avoid leaving standing water, as this may cause the roots to rot. In general, water once every 10 days. If the leaves soften and droop, increase the watering frequency. The amount of watering can be adjusted depending on the weather conditions.

Soil

A spruce tree grows best in a location with moist, well-drained soil made fertile through soil amendments. However, this tree can adapt to loamy, sandy, or clay soils. As for soil pH, spruce thrive between 6.0 and 7.5, but its unfussy nature allows this tree to tolerate highly acidic or alkaline soils.

Fertilizing

Applying enough base fertilizers before transplanting can provide nutrients to blue spruce over a long period of growth. In the first year after transplanting, the nutrition-absorption ability of the tree’s roots is not very strong, so apply nitrogen and phosphorus fertilizer every 2 weeks. In the fall, apply a little potassium fertilizer to help the trunk grow thick and sturdy. After the plant matures, it’s best to fertilize it in spring and summer and only fertilize 2-4 times a year. In late fall, blue spruce slowly enters dormancy, so fertilizing should be reduced and stopped by the end of fall.

Organic fertilizer is the best choice for this slow-growing tree variety. It contains a full set of nutrients that can be utilized continually and reliably. It can also help optimize the soil texture and benefits the plants’ growth. If the soil turns dry after fertilizing, water promptly.

Planting Instructions

To grow blue spruce in a yard, purchase seedlings and transplant them in early spring. Get the planting pit ready one week before transplanting; its diameter needs to be about 20 cm longer than that of the root ball and have a depth of about 15 cm deeper. It should be no less than 6 m away from other plants. Add organic fertilizers to the bottom of the planting pit before transplanting and mix them well with the soil.

Place the seedling vertically into the pit, shovel 2/3 of the soil back, and water thoroughly once to make sure the moisture around the plant’s roots is sufficient. Then, fill the pit fully up with soil and water again. Stomp the soil firmly and keep the pit surface level with the ground. If it’s often windy at the planting site, support the young tree with wood sticks or metal poles in case a strong gale threatens to tilt it or knock it down.

Indoor potted blue spruce can be directly purchased. The tree grows slowly and usually doesn’t require repotting. Repotting is only necessary if the needles turn yellow and fall off the tree, the roots around the pot brim or at the bottom wither and dry up, or the soil in the pot compacts. Move the majority of the original soil to the new pot with the plant, and add small amounts of organic fertilizers and new soil to provide more nutrients for its growth.

Pruning

Blue spruce has lush terminal buds. The tree usually takes on a beautiful triangular shape and does not require pruning in general. When overly pruned, its incisions secrete rosin excessively which affects the tree’s normal growth. It’s best to trim off overly dense, diseased, and dried branches during the plant’s winter dormancy. Prune and beautify the treetop in early spring when buds sprout. About 1/2 of the young branches can be pruned off to create your desired shape. For large, mature trees, it’s a good idea to prune off all the branches on the lower trunk to reduce nutrition consumption.

When growing blue spruce for Christmas decorations, you need to control height and width. Prune the treetop and lateral branches appropriately short and continue to cut the newly-grown buds short, as well. Repeat this process many times. Seal pruning incisions with wax or duct tape to keep rosin from effusing.

Propagation

Colorado blue spruce grows well from mid-summer cuttings and is planted in the fall. Cuttings take a while to root (two to three months) and must be treated with a strong rooting hormone. Grow cuttings in cool and humid conditions for successful propagation. Here’s how:

  • Gather your garden shears, an alcohol wipe, a paintbrush, a potting container, peat, medium-grit sand, 0.6-percent indolebutyric acid rooting talc, and a spray bottle.
  • Combine equal parts peat and sand in a bucket. Cover the mix with water and let it sit until the peat swells. After that, fill your potting container with the mixture.
  • Clean the blades of your shears with alcohol and cut a tip 4 to 5 inches long from one of the tree’s side branches. Make an angled cut between two sets of needles.
  • Strip the needles from the bottom half of the cutting and peel the bark from the end. Brush the end with the rooting hormone.
  • Poke a hole in the potting container equal to half the length of your cutting. Insert the cutting into the hole and backfill it with the peat mixture.
  • Place the pot in a cool indoor spot with filtered light. Avoid direct sun.
  • Drizzle water regularly onto the peat’s surface, and mist the air above the cutting, allowing the water to fall onto the needles.
  • Check for roots after two months (it could take up to four months to root), and then move the pot outside into the direct sun to acclimatize for a week, bringing it in each night.
  • Transplant the cutting into the ground in early autumn by digging a small hole, placing the baby tree and the contents of the pot into the hole, backfilling it, and then spreading a thick layer of mulch around the base. Water the tree regularly for three years until it’s established.

Grow from Seed

With patience, Colorado blue spruce can be grown from seed after collecting and drying pine cones to obtain the seeds.

  • In spring, soak the seeds in water for 24 hours.
  • Wrap the seeds in a moist paper towel and store them in the refrigerator for six weeks.
  • Place a seed in a moist seed-starting mixture, cover it with 1/4 inch of the mix, place plastic wrap over the container, and relocate it to a sunny spot.
  • Keep the mixture moist until the seed germinates and grows about an inch, then remove the plastic wrap.
  • Harden off the seedling by bringing it outside into direct sunlight for several days.
  • Transplant the seedling to the ground once it’s acclimatized, mulch around the baby tree, and water seedlings regularly for three years.

Pests and Diseases

Common Pests & Diseases

The most common issues affecting Colorado blue spruce trees are fungal tip blight, fungal needle cast, beetle kill, aphids, the spruce spider mite, Cooley spruce gall adelgid, and canker.1 On the plus side, Colorado blue spruce trees are deer resistant.

The fungal issues will cause the needles to grow yellow blotches, needle drop, dieback, and sticky tips. This tree can also be affected by Cytospora canker.1 This fungal disease typically moves into trees that are 15 to 20 years old, causing the needles to turn brown and drop from the branches. Cytospora canker is a stress-induced disease, so manage it by amending the soil regularly and not overwatering.

Signs of spider mite insect activity include a cotton-like substance on the spruce’s branches. Another insect, the great spruce bark beetle, regularly affects wild spruce trees. This beetle can take full stands of trees by tunneling into the trees’ bark and laying eggs. The holes weaken the tree, eventually causing a die-off.

To ward off pests, hire a company to spray the tree with a non-toxic horticultural oil that won’t endangered birds, pets, or humans.

Common Problems 

An established Colorado blue spruce tree is usually easy to maintain. Since blue spruce trees need regular watering during their first several years, a common mistake is giving frequent water after the first three years, which can lead to overwatering.

Browning Needles and Die Off

Signs of overwatering include browning needles, wilting branches, and die-off. Never let the tree sit in standing water, and provide ample mulch to keep moisture around the tree’s base, which helps the tree between waterings and drought periods.

Red or Purple Needles

If you notice needles turning purple, red, brown, or dead leaves in the spring, it’s likely the result of winter injury. Winter injury is often caused by stress; in many cases, it could be under watering or drought conditions. If you live in a place with wintery weather, do not plant blue spruce in the fall since it does not give the tree enough time to establish its roots properly to brace for the winter weather.

Bottoms Appears to Be Dying

Spruce decline is a term for a dying spruce tree that usually will not recuperate due to one of several factors. This condition is often caused by temperatures that are too high, high humidity, low air circulation, too much moisture on the needles or in the soil, fungal diseases, or insect activity.

You can attempt to revive the tree by correcting the issue affecting the tree—often disease or insects. Some fungicides might effectively treat needle cast disease over the long term but can only protect new growth (it will not revive old growth). Canker diseases are rarely repaired with fungicide; removing the affected branches is usually the best way to stop the spread of that disease.

Overwintering

Colorado blue spruce is native to cold climates; therefore, established trees do not need special care in the winter, as the tree will go dormant during this season. To protect the tree against hungry animals, you can wrap its trunk in burlap.

Mulching around the tree’s base helps it retain moisture, prevents soil compaction, and allows roots to stay warmer in winter and warm up faster in spring, extending its growing season.

After a heavy or wet snowfall, shake the branches to prevent them from snapping due to the snow’s weight.

III. Uses and Benefits 

The intriguing grey-blue needles of the evergreen tree blue spruce (Picea pungens) make it a popular choice of ornamental tree. It is also often used as a Christmas tree. Blue spruce is a low-maintenance tree that adds great structure to gardens in winter and excels when planted with contrasting evergreen conifers and heathers. Other plants that share a love of acid soils, like bergenia, lily-of-the-valley, and lungwort, can be grown nearby.

Blue Spruce (Picea pungens) Details

Common name Blue Spruce, Colorado Spruce
Botanical name Picea pungens
Plant type Perennial
Hardiness zone 2a, 2b, 3a, 3b, 4a, 4b, 5a, 5b, 6a, 6b, 7a
Growth rate Slow
Height 30 ft. 0 in. - 60 ft. 0 in.
Width 30 ft. 0 in. - 60 ft. 0 in.
Sunlight Full sun (6 or more hours of direct sunlight a day)
Soil condition Clay
Leaf color Blue