Tree Profiles: Douglas-fir (Pseudotsuga menziesii), Part 2
This is the second of a two part series on Douglas-fir. The following examines seed propagation of Douglas-fir, its root system, preferred soil types, cultivation, damaging agents and pests, allergenic potential, and uses.
Douglas-fir (Pseudotsuga menziesii) , also referred to as Douglas fir, Doug-fir, Douglas tree, Douglas pine, Douglas spruce, Oregon pine, Pino Oregon, red fir, and yellow fir, is an evergreen conifer species. Throughout its history, Douglas-fir has been designated a pine, a spruce, a hemlock, and a true fir. In 1867, Douglas-fir was assigned its own genus, Pseudotsuga, which translates to false hemlock.
Douglas-fir was first documented in 1791 by Archibald Menzies, a Scottish physician and naturalist, who discovered it on Vancouver Island. The latin name menziesii is a reference to Menzies. The tree species became popular in 1826 when it was introduced to cultivation by Scottish botanist and explorer David Douglas. It is now considered one of the world’s most important trees.
Douglas-fir produces its seeds within cones. Each seed has a single wing, which enables it to be dispersed by the wind. The wind transports the seed to the ground, where it potentially germinates. This is how Douglas-firs propagate naturally. In commercial settings, Douglas-fir is typically grown through seed propagation. This is the preferred method of propagation due to its high rate of success, and overall cost efficiency. Root cutting is another common propagation technique. This approach allows for the selection of cuttings based on desired characteristics, but is inefficient, seldom resulting in success. Other forms of propagation include grafting and tissue culture. Grafting is rarely performed, as it is laborious, and requires constant maintenance. Tissue culture is generally avoided due to its high cost, and the amount of time required to achieve success.
To propagate seeds, collect a cone from a Douglas-fir tree. Extract the seeds from the cone. Gently rub the seeds in between your fingers, and remove the wings. Fill up a bowl of water, and let the seeds soak in it for 24 hours. This will stimulate the seeds to initiate the germination process. Once the seeds have been soaked, transfer them to a paper towel or cloth, and allow them to dry for 24 hours. After the seeds have dried, wrap them in a moist paper towel, and place them inside of a thin plastic bag. To encourage germination, refrigerate the seeds for 30 days. Once the thirty days have expired, clean several small containers, and fill them with at least six inches of potting soil. In each container, create a 2 to 3 inch deep hole. Drop each seed into a hole, and cover it with soil. Transport the containers to a warm area with temperatures around 70°F. Ensure that the soil within the containers remains moist. Monitor the containers for growth. As the seedlings sprout, thin them out until only one remains in each pot. Transfer the trees into larger containers as necessary.
When young, Douglas-fir establishes a taproot in the soil. The taproot continues to penetrate into the soil for the next ten years. As the tree ages, it produces a slew of lateral roots. The main lateral roots breach the soil at a sharp angle. The depth of the root system is related to the soil structure and texture. In areas that receive abundant rainfall, the roots may burrow 60 to 100 cm into the earth, with some progressing even deeper. On well-drained soils, the lateral roots may reach up to 1 meter beyond the crown. As the taproot and lateral roots branch out, they create a heart-shaped root system. In locations that are prone to drought, the root system will become denser. Douglas-firs growing in soil frequently exposed to groundwater or runoff will often develop shallow rooting systems.
Soil and Topography
Douglas-fir can tolerate most soil types, but thrives on well-aerated, deep soils that have a moderately acidic to neutral pH level. Douglas-fir’s root system is adaptable, allowing it to grow in myriad environments. The tree species can be found from shallow slopes to steep mountains.
Douglas-fir is a popular tree used for cultivation. Some of the most notable cultivars are ‘Anguina’, ‘Brevifolia’, ‘Compacta’, ‘Fastigiata’, ‘Fretsii’, ‘Fletcheri’, ‘Graceful Grace’, ‘Glauca’, ‘Little Jon’, ‘Loggerhead’, ‘Nana’, ‘Pendula’, ‘Revoluta’, and ‘Stairii’.
Damaging Agents and Pests
The red tree vole and the spotted owl inhabit forests replete with mature Douglas-firs. Pairs of spotted owls require at least 990 acres of forest to settle. Red tree voles nest primarily in the foliage of Douglas-fir. They constructs nests that are 5 to 165 feet above the ground. The red tree vole feeds on Douglas-fir needles, and can cause minor defoliation.
Douglas-fir dwarf mistletoe frequently parasizes the tree. The needles can be infested by the wooly conifer aphid Adelges cooleyi. The aphid secretes a characteristic waxy spot on the underside of vulnerable leaves. The aphids feed from within the waxy constructs, extracting sap and other plant liquids from the tree. Dense populations can partially defoliate host trees. Numerous other insects infest Douglas-fir. Some of the most common examples include black pine leaf scale, black vine weevil, cooley spruce gall adelgid, elongate hemlock scale, fall webworm, pine needle scale, and spruce spider mite.
Douglas-fir may be infected by a bevy of diseases, including Annosus root rot, Armillaria root rot, bleeding sap rot, blue stain fungus, brown trunk rot, Cytospora canker, laminated root rot, red ring rot, Rhabdocline needle cast, shoestring root rot, and swiss needle cast. Root rot occurs frequently on Douglas-firs that are rooted in clay, and other wet soils. Needles that have been infected by fungal pathogens will experience discoloration, before being cast from the tree. Douglas-fir is susceptible to desiccation from heavy winds.
Douglas-fir is not considered a serious allergen. It does not produce seeds in winter.
- Douglas-fir can be used as an ornamental tree in parks, gardens, and landscapes.
- Many countries use Douglas-fir wood as timber.
- The buds have been used to flavor eau de vie, a colorless fruit brandy.
- Native Hawaiians historically utilized Douglas-fir to build wa’a kaulua, or a double-hulled canoe.
- Douglas-fir has been a staple of Christmas since the 1920s.
- Douglas-firs are exported to the Hawaiian Islands, Guam, and several Asian countries.
- Douglas-fir is one of the most strongest, and most reliable softwoods. As such, it makes for excellent structural material. It has been used in the construction of items as diverse as boxes, cabinets, ladders, railroad ties, telephone poles, flooring, interior trim, pallets, laminated timbers, plywood, and high grade veneer.
- During World War II, Douglas-fir was used by soldiers to create portable huts, and makeshift stretchers.
- In 1925, the masts of the USS Constitution were replaced with more durable ones made from Douglas-fir.
- Douglas-fir was used by Native Americans to build structures, and baskets. It was also used as a remedy for headaches, rhuematism, indigestion, and head colds.
- The seeds are consumed by blue grouse, songbirds, squirrels, rabbits, and other small animals. Antelope, deer, elk, mountain goats, and mountain sheep feast on the twigs and foliage.
- Douglas-fir is featured as the state tree of Oregon.
Photo courtesy of Jon D. Anderson CC-by-2.0