White Oak is the Illinois State Tree. Photo: Alan Branhagen

October is OAK Awareness Month in Illinois: “OAKtober”

By Alan Branhagen, Executive Director, Natural Land Institute

Oaks are the most important tree to the forests, woodlands and savannas of our area (and almost every place they are found). Their mast of acorns they produce is the number one food for wildlife in terms of the number of species that utilize the acorns. Maybe even more importantly they are the most important tree hosting the widest diversity of insects – more species of caterpillars feast on oak leaves than on any other group of trees. Caterpillars don’t mean tree damage they mean bird food and the building block of the web of life.

Oaks come in two groups: white oaks with rounded lobes and acorns that mature in one growing season and red oaks with pointed lobed leaves whose acorns take two years to mature. Their wood also differs, white oak can hold water so is where wine and spirits are white oak barrel aged. How many oak species grow locally?  It’s seven but a couple more have now naturalized from plantings by us or spread by their primary disperser and planter: Blue Jays!

Here are the 7 indigenous species:

Bur Oak Quercus macrocarpa: the most common oak at settlement by far (as noted by the original land surveyors). Always a living sculpture, no two are alike and with a distinctive mossy cup to the acorn and fall color that is at best an olive-golden-tan. The twigs on young plants are often corky, an adaptation to discourage browse (the hungry elk are gone but the deer are still here!) and protect from fire. White oak group

White Oak Quercus alba: State tree of Illinois and the second most common at settlement. White oak has very fine fall color ranging from orange to almost purple-red. See my recent article in Fine Gardening magazine if you could plant only one tree, this is my recommendation. White oak group

Northern Red Oak Quercus rubra: A magnificent woodland and forest oak, becoming the largest of our oaks at maturity with a tall clear trunk of striated bark and wood that is highly desirable for flooring and wood trim. The acorns are large with a shallow cap.  Fall color can be golden to a vivid red. Red oak group

Black Oak Quercus velutina: Formerly mainly a scrub oak at settlement but now a common second growth tree of drier situations especially sandy savannas.  The buds are large and gray-furry on velvety twigs. Fall color is drab and usually an olive-golden more like a bur oak. Red oak group

Northern Pin Oak Quercus ellipsoidalis: An uncommon oak but prevalent along the Kishwaukee River gorge and other hilly areas like Lind- McGeachie Preserve and Burr Oak Valley Nature Preserve. Similar to black oak, always with deeply lobed leaves and hairless buds on smooth twigs. Fall color is always a dazzling red, the showiest fall color of any of our oaks. Red oak group

Swamp White Oak Quercus bicolor is limited to the wild along the Sugar River and Raccoon Creek with a few trees along the Pecatonica River.  It is the most commonly planted oak. Its acorns are distinctive on their long stems and twigs and young branches have exfoliating bark to rid them of climbing vines in their bottomland home.  Fall color is a golden tan. White oak group

Chinkapin Oak Quercus muehlenbergii is our least common oak found only on rocky outcrops – it is easiest to find in the Kishwaukee Gorge Forest Preserve, but also grows along the Rock River at NLI’s William & Gayle Keefer Nature Reserve, Klehm Arboretum (the part east of Main its native, planted in the parking lots) and Cedar Cliff Forest Preserves, as well as Alpine and Aldeen Parks and NLI’s Howard D. Colman Dells Nature Preserve. Leaves are toothed along the edge without lobes and the fall color is usually a burnt orange. White oak group

Editor’s Note:
We invite you to celebrate OAKtober with us at NLI’s annual OAKtober Hike on Sat., Oct. 28, 2023. The guided hike starts at 1:30 p.m. See details and please register here.

Additional Photos by Alan Branhagen

Bur Oak Quercus macrocarpa










Northern Red Oak Quercus rubra









Black Oak Quercus velutina










Northern Pin Oak Quercus ellipsoidalis











Swamp White Oak Quercus bicolor acorns









Chinkapin Oak Quercus muehlenbergii










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