Close up of a scarlet oak leaf in fall in the Arboretum (Photo by Niall Dunne)
BY THEODORE HOSS
Autumn is here, and we all know what that means: It’s time for our local deciduous trees to begin their annual process of chlorophyll degradation and leaf abscission!
Okay, so perhaps that isn’t the first thing that jumps to mind when thinking of fall, but most people in our region do associate it with the appearance of colorful tree foliage—a bright spot in a season usually otherwise defined by the return of rainy skies and cold weather.
As the green pigment chlorophyll breaks down and is reabsorbed by the trees for winter, a multitude of other leaf pigments are suddenly unmasked. These include the yellows and oranges of flavone, xanthophyll and carotene; the red of lycopene; and the purple of anthocyanin. Some tree species—such as the maples, sassafras and stewartias profiled elsewhere in this issue—produce a more spectacular display of these pigments than others.
Our fall “Hidden Treasure” is a tree whose brilliant, deep-red autumnal display has inspired both its common name, the scarlet oak, and its scientific name, Quercus coccinea (the species epithet is derived from “kokkinos,” the Greek word for “scarlet”). An adaptable landscape tree—with a rounded, open habit and lovely, deeply cut foliage—the scarlet oak is well worth getting to know, and the Arboretum has a number of large specimens to visit, most of them dating back to the early days of the plant collection.
Scarlet fall color on the specimen at the north end of Azalea Way. (Photo by Niall Dunne)
Range and Ecology
The scarlet oak is a fast-growing, medium-to-large tree native to the central and eastern U.S., including a large swath of the Appalachian Mountains. The leaves range from three to seven inches long and are arranged alternately on branches. Flowers appear in early spring as the tree leafs out and consist of slender, drooping, yellow-green male catkins and relatively nondescript female flowers. Scarlet oak is a common co-dominant tree species in eastern broadleaf and mixed forests, preferring somewhat dry soils. It serves as the official tree of the District of Columbia.
Like other oaks, this species produces acorns as its fruit. These little nuts, with a fuzzy cap known as a cupule, are perhaps the most distinctive identification feature for the genus Quercus and are of immense value to wildlife, such as wild turkeys, black bears and squirrels. The leaves also are a vital food source in the tree’s native range: Scarlet oak is a host plant for the larvae of many species of butterflies and moths, including the Banded Hairstreak and the Imperial Moth.
Scarlet oak acorns are an excellent source of food for humans as well (after first being leached of their bitter tannins), providing high concentrations of carbohydrates, amino acids and proteins. The acorns take about 18 months to develop, becoming ripe in fall. You might expect the leaves to drop around this time of the year, however, such is not always the case with Q. coccinea. The tree can exhibit a trait known as marcescence—retaining dead leaves on branches until their removal via a mechanical force, such as wind, or the opening of new leaf buds the following spring. Do not be alarmed, then, if you find a scarlet oak still clad in a multitude of brown leaves deep into the winter. Research shows that marcescent leaves can offer winter benefits to trees, such as the protection of tender, young branches from munching herbivores like deer.
Close up of the spring/summer leaves. (Photo by Niall Dunne)
Scarlet Oak ID Tips
The scarlet oak belongs to a grouping within the genus Quercus known as the red oaks, which typically display red fall color, darker-colored bark, and leaves with sharply pointed lobes. (They are distinct from the white oaks, which bear more smoothly rounded leaf lobes and lighter-colored bark.) Two other eastern U.S. members of the red oaks—northern red oak (Quercus rubra) and pin oak (Quercus palustris)—are commonly planted in the Pacific Northwest and look pretty similar to scarlet oak. (The three species can actually hybridize to create plants with intermediate traits, further confusing matters.) Following are some quick ways to tell them apart.
Like scarlet oak, northern red oak has large leaves with sharp points, however, these leaves lack the deeply cut “C”-shaped lobes of Q. coccinea. The pin oak is a little trickier because it does share the trait of deep lobes, but these are generally more “U”-shaped in their appearance. Additionally, pin oaks often have horizontal and descending branches, while those of the scarlet oak are often ascending.
In nature, each of these trees may be identi-fied in part based on their preferred niche within a forest. (Pin oak, for example, likes its feet wet!) But, in a planted urban landscape setting, some practice might be needed to identify them. Again, the Arboretum—which features a dozen pin oaks and more than 20 northern red oaks in the collection—is a great place to cut your identi-fication teeth.
Acorns on a scarlet oak in the Arboretum. (Photo by Niall Dunne)
An Urban Adaptor
With its fast growth, adaptability and ornamental qualities, Quercus coccinea makes an excellent specimen tree for parks, streets and yards. In open conditions, especially, it forms a well-developed, rounded crown that provides cooling shade for adjacent homes and sidewalks, while still allowing sunlight to filter through to under-story plantings. Scarlet oak is considered highly intolerant of shade, and thus will not thrive if planted beneath an existing dense canopy. That said, it can hold its own growing up alongside other trees.
When young, Q. coccinea often develops a drooping leader (the topmost branch of the main trunk), giving the tree a graceful, almost wilting appearance. As the tree ages, it becomes more erect. Scarlet oak seedlings develop a strong taproot that grows down deep into the soil with few lateral roots. This makes it tough to transplant but also less liable to damage nearby sidewalks and roads compared to trees with more shallow, horizontal, fibrous root systems. Scarlet oak usually tops out at less than 80 feet tall, with a two-foot-wide diameter at maturity. The expected lifespan of an established tree ranges between 80 and 300 years.
In its native range, scarlet oak exhibits a greater ability to thrive in drier soils than some of its Quercus compatriots, including on arid ridges and southern aspects. This is an excellent characteristic for an urban tree, especially in the context of climate change! Urban environments come with a wide array of constraints on arboreal life. Artificial structures and roads soak up heat in summer, causing temperatures to stay higher than surrounding natural areas. Roads, foundations, pipes and disturbed soil limit root growth, and impermeable surfaces reduce the amount of moisture that can seep into the ground. A drought-tolerant tree adapted to life on a rocky ridgeline makes excellent sense for planting in such an altered environment!
Arboretum specimen in fall color. (Photo by Niall Dunne)
Quercus coccinea has few cultivars, but the selection ‘Splendens’, introduced by an English nursery in the late 19th century, has received an Award of Merit from the Royal Horticultural Society. Its leaves are bigger than those of the straight species and develop richer fall color; however, the tree is less hardy and is also very rare in cultivation.
In the Arboretum
The Arboretum has eight specimens of scarlet oak, most of them planted between 1938 and 1940. The easiest ones to find are clustered near the east side of the Willcox Bridge on the south end of the main Oak Collection, as well as by the small parking lot just north of the bridge.
My favorite is a tree from 1940 that stands sentinel-like along the north end of Azalea Way, facing a large grouping of flowering cherry trees across the promenade. (A side trail leading to the nearby bridge intersects Azalea Way at the foot of the tree.) In general, mid-October is best time of the year to see the foliage in all its scarlet glory, though peak color can vary each year depending on weather conditions. ❦
Theodore Hoss is a nature educator, writer, and member of the “Bulletin” Editorial Board.
Subscribe to Our Bulletin Magazine!
This article is reprinted from the fall 2024 issue of the Arboretum Bulletin, our quarterly botanical and horticultural magazine. Published since 1937, the Bulletin is a benefit of Arboretum Foundation membership and features in-depth articles on the Arboretum collections and history, as well as general information of interest to plant lovers in the Pacific Northwest and beyond. Join/subscribe today.
