The state of Washington is a mixture of USDA Hardiness zones 5 to 8. The Cascade Mountain range splits the state into two main growing areas. Mild temperatures develop west of the Cascades because of the marine influence from the Pacific Ocean. East of the cascades the climate is drier. This is because of the rain shadow from moisture-blocking mountains. Plants in the western part of the state enjoy cool summers, mild winters and abundant rainfall. The eastern part of Washington has warmer summers, colder winters and snow. Washington is a haven for dense forests with a variety of trees.
Douglas fir (Pseudotsuga menziesii) grows a straight trunk ending with a spire-like crown. This evergreen tree reaches more than 300 feet tall in good-draining, nutrient-rich forest soil. The upper branches tend to grow upward while the lower branches droop. Douglas fir bark is gray to reddish brown in color with thick grooves. Flattened, 1-inch needles cover the branches. The yellowish green needles have pointed tips. The small red flowers are male, and the green blossoms are female. The Douglas fir cones are 3 to 4 inches long and reddish brown. The Douglas fir is one of the tallest trees in the world. The thick bark of the Douglas fir allows it to survive forest fires when other trees perish.
Pacific madrone (Arbutus menziesii) is a broadleaf evergreen with reddish-brown, thin bark that flakes and peels once it is mature. New bark growth is yellow-green and satin smooth. The Pacific madrone develops crooked branches and oblong leaves that are 3 to 5 inches long. The thick, shiny green leaves turn red in the summer as new leaves grow in. Fragrant, white-pink, urn-shaped flowers appear in March through June. Bright orange, small berries develop after the flowers fade. The Pacific madrone grows more than 120 feet tall in poor types of soil, though it is adaptable to nearly any soil type. Pacific madrone is susceptalbe to Phytophthora ramorum, which gradually causes tree death with purple leaf spots and twig blight.
Quaking aspen (Populus tremuloides) grows straight and tends to shed its lower limbs as it gains height. This tree prefers sandy soil and full sun exposure. Quaking aspen has no shade tolerance. The oval leaves have small teeth along the edges and shiver in the slightest breeze. The tops are shiny, dark green and duller below. In the fall the leaves turn a golden yellow before being shed. Male and female catkins are produced on separate trees. Quaking aspens grow 25 to 60 feet tall with shallow roots. Disease and insect pests are common aspects of a quaking aspen, which unusually does not live much beyond 20 years.
Western hemlock (Tsuga heterophylla) is an upright evergreen with a natural pyramid shape. This hemlock enjoys damp to wet soils and partial to full shade. It suffers in the heat of full sunshine. Soft, flattened needles cover the branches. New needles appear light green and yellow, turning dark green as they age. Drooping cones are small and abundant. The Western hemlock rapidly grows 40 to 60 feet tall. It is resistant to attacks by the hemlock woolly adelgid, which leaves white eggs masses on its host trees.
Western larch (Larix occidentalis) is a deciduous conifer that competes with nearby trees for full sunshine. The flattened needles are yellow-green in the spring. In the fall the needles turn bright yellow and then are shed. The red-brown cones are elongated. Western larch grows up to 150 feet tall and is fire-resistant. Western larch does not suffer from attacking pests and insects.