October Nature Almanac: Don’t Underrate the Understory
By Ruth Carol Cushman with Stephen R. Jones
October 2025
Golden aspens steal the show in September, and then in October they decay into spicy-smelling brown humus. But when the aspen show fades, you can revel in the often-overlooked understory plants.
Poison ivy turns wine red, cherry red, and lemon yellow. Photo by Kit Basom.
Smooth sumacs transmute into a translucent ruby-red by early October. Three-leaf sumacs and wild roses look like scarlet, salmon and orange fires. Virginia creepers and golden currants turn crimson. Chokecherries and wild plums become pinkish orange; dogbanes become an aspeny yellow; and poison ivy invites the unwary to pick its brilliant multi-colored leaves. To a westerner, these plants and shrubs rival New England’s sugar maples.
Smooth sumac along the South Boulder Creek trail. Photo by Glenn Cushman.
What magic wand creates these glorious colors? Some Native American stories say that one autumn long ago three hunters and a dog chased a bear that led them faster and faster in all directions. Finally the bear ran into the sky, where the hunters killed and butchered him on a stack of maple and sumac branches. The leaves, stained by his blood, still turn blood-red every fall. The Great Bear, followed by the three hunters and their dog, can be seen in the sky all year long. These eight stars (also called the Big Dipper) can never rest until the hunters and the dog again catch the bear.
The scientific explanation for fall color change is less romantic and more complex. Shortening days and cool nights trigger the growth of a corky membrane at the base of the leaf stem, preventing the flow of nutrients into the leaf. As photosynthesis and chlorophyll production shut down, the green pigments fade while the yellow pigments, carotene and xanthophyll, remain.
Virginia creeper. Photo by Ruth Carol Cushman.
Reds are caused by anthocyanins--chemical substances that form when high sugar concentrations react with proteins in the sap. It’s thought that the red color may protect the leaves from sunburn. So, tender young spring leaves are often red as well as the autumnal leaves. If the sap is acidic, leaves become bright red; if the sap is less acidic, leaves become purplish. The most vivid colors occur when dry, sunny days are followed by cool nights.
Golden currant leaf. Photo by Glenn Cushman.
Some hardy wildflowers—asters, bottle gentians, gumweeds, gaillardias, rabbit brush, and gayfeather--continue to bloom in the understory. You might even find a late butterfly getting a final sugar fix. Common buckeyes, and clouded and orange sulphurs add to the color show. Western pygmy blues, the smallest butterfly in North America, have been seen at Walden Ponds and Queen butterflies, a Monarch relative, have been seen at Pella Crossing. You might even see a stray monarch since some don’t leave for Michoaca`n until early October.
Other October Events
Bear scat with half-digested garter snake. Photo by Kit Basom.
Black bears fatten up on wild fruits, raid garbage cans, and climb trees to nap. Last year we encountered one sauntering up the Saddle Rock Trail munching on fallen apples. And this year Kit Basom found bear scat containing a partially digested garter snake in late September.
Garter snakes, bullsnakes, and rattlesnakes converge on communal hibernating sites in hillside hollows and prairie dog burrows.
Elk bulls bugle to attract harems along Plain View Road south of Boulder, at Caribou Ranch, and in Golden Gate State Park as well as in Rocky Mountain National Park.
Migration gets into full swing as sandhill cranes, ducks, and many songbirds head south. Listen for their calls in the night sky and turn off lights to prevent fatal collisions with buildings.
Smooth sumac along the Bluebird-Baird trail. Photo by Glenn Cushman.
Nature Almanac is a monthly series by Stephen R. Jones and R. Carol Cushman, along with other guest contributors. Ruth Carol Cushman and Stephen Jones are authors of A Field Guide to The North American Prairie (Peterson Field Guides) and Wild Boulder County: A Seasonal Guide to the Natural World.