Fast growing trees are generally native to a region. Good plants to try are other species related to natives, or hybrids bred from native trees. Cultural factors are important, too. Research a tree's optimal conditions and meet their requirements for best growth. In the Pacific Northwest suitable trees tend to reach their full size because of the favorable growing climate.
Few trees grow as quickly as the elderberry. The Pacific native elderberry (Sambucus cerulea) may suffer some transplant shock before it begins to grow faster. Containerized elderberry trees will take off quicker. All hybrid elderberries are fast growing in the Pacific Northwest. They are deciduous multi-stemmed, under-story trees reaching from 6 to 20 feet depending on the variety. Hybrid elderberry trees may have red, purple, yellow and variegated foliage. A dark-leaved hybrid 'Thundercloud' is one of the fastest and tallest of the elderberries. The fruit of most elderberries are edible. The native red berried elder (Sambucus racemosa) has caused some stomach irritation and is used strictly as an ornamental.
The popular Japanese maples are reliable in the Northwest, but are not known as fast growing. However, the two native maples are fast growing. The smaller multi-stemmed vine maple (Acer circinatum) remains small enough to grow in any sized landscape. It will reach 10 feet easy, but can be pruned to any height. The larger big-leaf maple should be reserved for the more expansive landscape. This tree can reach up to 75 feet.
Evergreen conifers are at home in the moist Northwest climate. Most conifers hardy to USDA zone 8 will grow well there. Examples of conifers with extra fast growth are: Douglas fir (Pseudotsuga menziesii), Western red cedar (Thuja plicata) and leyland cypress (Cupressocyparis). These conifers can reach from 100 to 200 feet. Another large conifer that grows well is incense cedar (Calocedrus decurrens). Incense cedar is still large, but will top out at around 75 feet. There are several smaller varieties of Western red cedar more suitable for smaller areas.
Birch trees grow quickly in the Pacific Northwest; and its white peeling bark is an asset. A quality native full-sized birch is the paper birch (Betula papyrifera). A popular smaller landscape birch is the Himalayan white birch (Betula jacquemontii). The bark of this birch turns bright white at a much younger age than most birches. Birch trees love moisture--and will do fine with regular water. They are tall and narrow so the only restrictions are height. If there are no vertical obstructions--such as power lines--they are a good choice. Birches are sold singly or in clumps. The clump form has three trunks or more and has become a popular choice.