The family Salicaceae includes cottonwoods (Populus) and willows (Salix), both indicators of the riparian woodland plant community. Populus are differentiated from Salix by leaf shape, bract segments, and number of stamens on male plant (8-60 in Populus vs. 2-10 in Salix).

Cottonwoods are large trees while willows range from medium sized trees to shrubs.  Leaves are deciduous, simple and alternate. All are dioecious with male and female flowers appearing on different plants as elongate catkins, aggregates of pollen or seed-bearing flowers.  The staminate (male) flowers have 2-10 stamens;  pistillate (female) flowers have two fused pistils; neither have a calyx or corolla. Cottonwood seeds have larger more spherical capsules which open by 3 valves while willow capsules are small and slender, splitting into two valves.  The minute seeds have long silky tails that help in wind dispersal. Some species shed large amounts of cottony seed masses.  Salix can also be insect pollinated.

There are five native willow species in Orange County. To quickly distinguish these in the field: First, is the leaf glaucous (whitish or grey) on both the top and bottom? If so, it will also be a small and very thin leaf. It is Narrow-leaved Willow (S. exigua).

The next separation should be based upon the underside of the leaf. Black Willow can distinguished, since both leaf surfaces are shiny green.

The three remaining species all show a noticeable light colored (glaucous) leaf underside. These are Red (S. laevigata), Arroyo (S. lasiolepis) and Pacific Willow (S. l. lasiandra).
Now observe the stipules (leafy growths) at the nodes of the stem. If the plant either shows no stipules or very small, rudimentary stipules it is Arroyo Willow. Red and Pacific Willow (as well as Black Willow) always show noticeable stipules.

To accurately separate Red and Pacific Willow will require examining the dormant winter buds at the leaf axil. If these buds are noticably pointed you have Red Willow. If the bud tips are rounded you have Pacific Willow.

Other factors involving lower leaf hairs and leaf shape can confirm your willow identification. Complicating identification further is that many willow species freely and commonly hybridize, showing intermediate characteristics.