Black Cottonwood (Populus trichocarpa) Willow Family (Salicaceae)

It is the largest broadleaf tree in western North America. It is a fast growing tree that can reach 100’ within 20 years. Cottonwoods rarely grow far from water, thus you will find them all along the river out at the arboretum.

Bark

As they mature, their bark becomes thick and fissured. I have peeled back the bark on fallen trees to find that underneath the woody outer bark, the inner bark is soft and consists of papery-thin layers that you can peel off.

Stems

The small branches at the tops of the trees had an interesting structure. They were knobby from what appeared to be old scars created from where the previous years’ leaf stems were attached. With the new leaves sprouting out of the top, it reminded me of a palm tree. The stems also looked like they had been stacked like rock cairns—beautiful formations.

Where the stems attached to the larger branch, there were thick, bowl-shaped nodes. Many of the larger branches were bumpy with scars where these smaller stems had broken off.

Buds

The buds are coated with a sticky resin that has a sweet fragrance that permeates the air. The combustion releases a syrupy aroma that flows and eddies in air currents along the river. Its swirling fragrance is a calming dizziness. As the buds are warmed in the sun in the spring, the sticky resin starts to ooze. Some of the buds are completely coated with resin as seen in the third photo and turn a beautiful red.

Leaves

In the spring, the cottonwood leaves are awakening from the quiet repose of winter. The warmth of the sun kindles their dreams, bursting their flaming sheaths. The leaves erupt into an untamed bouquet and ripple outwards like the water they so love. Their bodies will uncurl into ovate blades to scoop up the sun. They will dance and collide in the wind, circulating a song of water in the blue sky of summer. I always look forward to luminous July days and sitting in the shade of the cottonwood tree. I will uncurl my toes in the water and watch the waxwings swing between the trees over the river.

The leaves are 3 - 6” long. They are glossy and fairly dark green above and a light gray-green below. They are very noticeable when blowing in the wind because of the bold contrast between the top and bottom of the leaf. The leaf’s edge has small rounded teeth or the margin can be smooth. They can form two distinct leaf shapes. They can be long and pointed similar to the tip of a spear, or they can be triangular in nature. In the autumn they turn yellow.

Catkins

Black cottonwoods are dioecious, meaning the male and female flowers are on separate trees. They are wind-pollinated.

Male Flowers
The males have 40 - 60 stamens. As the catkin forms, the stamens are small, red pods. The catkin will continue to grow and elongate. Upon maturity, the red pods split open and release small dust clouds of yellow pollen.

Female Flowers
The female flower is interesting. I took some close-up photos that you can see below. The most obvious feature is the curvy, fleshy, yellow part on top. I interpreted the yellow part projecting out of the top as the stigma that receives the pollen. Since taking these photos the stigmas have started to dry out, turn brown, and their shape has become more clear (see photo below). Underneath this is a green, fuzzy capsule that is firm to the touch. This will become brown, split open, and release small seeds with white, fuzzy parachutes. When the seeds are being let go it looks like snow blowing around through the air. I cut one of the capsules in half. The walls were thick and a bundle of seeds were nestled inside.

Seeds

The tiny, cream-colored seeds are nestled in a downy fluff that can easily be carried off in the slightest breeze. They are encased in small, woody capsules that are clustered along a woody stem. In the spring, the capsule will split 3 ways to open and release the seeds. It is a magical sight to see the seeds glinting in the sunlight as they drift through the air. It looks like snow. I remember huge, old cottonwoods in Arizona that would literally blanket the ground white with fluff. I am always amazed that inside the tiniest of seeds is the blue print to grow such towering, majestic trees.

Animal Life

Beavers are North America’s largest rodent. One of their notable features, besides their long flat tail, is their large chisel-like incisors which are each about a quarter inch wide. The beaver has an extensive list of foods that it eats, and one of its favorites is the cottonwood tree shown here. Out at the arboretum, I can usually find these small branches cut off at an angle, and I often see branches that it has stripped lying around the bank of the river. Beavers eat the inner layer of bark (cambium), buds, leaves and twigs. Beavers are a keystone species and through their dam construction, they create a diverse ecosystem. I highly recommend reading more about these remarkable creatures.

Sprouting

Poking around the river, I saw this small log lying on the ground and decided to turn it over to see if anything cool was hiding underneath. To my surprise, I saw a branch from a fallen cottonwood tree that had started to sprout roots. The log had been deposited there from the river swollen from recent rains. The moist, dark conditions under the log were perfect for sprouting new roots. I imagine that most water-loving trees and shrubs that grow along streams, ponds, marshes, etc., have evolved to sprout new roots from limbs easily.