Scientific name
Populus balsamifera
Also Known As
Black cottonwood, Populus trichocarpa
Balsam poplar trees (also known as cottonwoods) have rounded, lance-shaped leaves with short stems and fruits that are capsules which split in 2 parts.

Scientific name
Populus grandidentata
Leaves somewhat round in shape, with a  pointed tip and 5-15 coarse teeth on each side of the leaf. Leaves are densely silvery hairy when young, hairless when mature, and flutter in the wind.

Scientific name
Populus trichocarpa
Also Known As
California poplar
At one time considered by botanists to be a subspecies of P. balsamifera, this closely related species differs in having a 3-valved fruit capsule covered in short hairs (balsam poplar has a 2-valved capsule with no hairs).

Scientific name
Salix exigua
This willow shrub has long, thin leaves and its catkins (the flower clusters) come out at about the same time as the leaves. Watch it closely!

Scientific name
Salix lasiandra
Also Known As
Golden willow, Western black willow, Western shining willow, Whiplash willow
Pacific willow is a small deciduous tree or large shrub that appears tall and slender. The shiny leaves on this fast-growing willow are long and thin with finely toothed edges. The fruits are thick catkins that are hairless, light reddish-brown. The bark is furrowed with broad flat scaly plates. Source: USDA Plants Database

Scientific name
Populus deltoides
Also Known As
Common cottonwood, Eastern cottonwood, Plains poplar
This tree has triangular shaped leaves, which is where the "deltoid" in the Species name comes from. The leaves have coarse teeth along their edges.

Scientific name
Populus tremuloides
Also Known As
Common aspen
Has smooth rounded leaves which flitter in the slightest breeze. It also has bright white or cream colored bark that looks similar to birch bark except that it does not peel.