Quercus macrocarpa
Bur Oak, Burr Oak, Mossycup Oak
by Martin (Mort) Schmidt for Simply Living
While traveling through Missouri recently, I stopped to pay homage to the magnificent “Big Tree”, by some accounts, tied for the country’s Champion Bur Oak. I was not disappointed. This impressive tree was hundreds of years old when Lewis and Clark passed through the area. Bur Oaks are widespread in Ohio, and one of the largest ones lives in Ohio.
Bur Oak has simple leaves with an alternate arrangement. Simple leaves are attached directly to the tree, compared to compound leaves, on which 3, 5, 7, or some other more or less constant number of leaflets share a stem. Bur Oak leaves are attached to the tree in an alternate pattern – left, right, left, right, like a human’s footprints, instead of directly across from each other, like a kangaroo’s. The leaves are pinnately veined, i.e., there is one primary vein with secondary veins attached to it at various points along its length. Palmately-veined leaves have multiple primary veins that converge at the stem.
Bur Oak leaves can be very distinctive. Like other members of the White Oak group (known collectively as white oaks, herein, not capitalized), their leaves have rounded tips, whereas members of the Red Oak group (red oaks) typically have pointed tips. It might help to associate white oak leaf tips with snowballs, which also are round and white. Overall, Bur Oak leaves are egg shaped (ovate), but with the narrow end of the egg attached to the stem (obovate). The classic Bur Oak leaf looks like two different leaves joined in the middle. Near the stem, Bur Oak leaves have deep sinuses between lobes, like White Oak (Quercus alba), while the leaves’ far ends have shallow sinuses between lobes, like Swamp White Oak (Quercus bicolor). But there’s a lot of variability, and you might need to look at other traits, especially acorns, to be sure which oak you’ve got. Some variability is caused by white oaks hybridizing with other white oaks. Bur Oak leaves have short stems, and like Swamp White Oak, they often have distinctly lighter-colored undersides.
Oak flowers are wind-pollinated. Therefore, the flowers have long green tendrils that catch wind, rather than brightly-colored petals that attract insects.
Following successful pollination, oak trees produce acorns, and Bur Oak is named for its distinctive acorns. Quercus macrocarpa means “oak with large fruit”, and Bur Oak produces North America’s largest acorn. The acorn is also distinctive because of the fringe on its cap, hence the common names, “Bur”, “Burr”, and “Mossycup” Oak. Sawtooth Oaks from Asia also have a fringed cap, but they’re readily distinguished by their toothed leaves, and they mostly grow in landscaped areas, not forests. According to The Sibley Guide to Trees, Bur Oak’s acorn caps have more pronounced fringe near the southern end of the tree’s range. Finally, Q. macrocarpa caps cover approximately half of the acorn.
Bur Oaks don’t produce acorns until around 35 years, and peak around 75 to 150 years. Like other white oaks, they generally produce acorns only 4 out of 10 years. This is because the population of acorn browsers is limited by the availability of food during lean years. Then in the abundant years, the “mast” years, there are more acorns than the browsers can eat, and the acorns have a better chance of becoming oak trees. (Red oaks also have mast years, but their mast crops average around 6 out of 10 years).
Another strategy to improve the survival of acorns involves their acidity and how quickly they sprout. We distinguish between white oaks and red oaks by their rounded versus pointed leaf tips, but the botanically significant difference is their acorn growth. White oak acorns are sweet, so they reduce the risk of being eaten, by sprouting days after falling. Red oak acorns are higher in tannic acid, so they can afford to lie dormant over winter and sprout the following year when winter is over. Remember that fruits and flowers are more important than leaf shape in tree classification. But leaf shape is more obvious, so we look at leaf tips to tell white oaks from red oaks in the field.
But wait – unlike other white oaks, in the northern extremes of its range, Bur Oak acorns sometimes lie dormant over winter and germinate the following spring, like red oaks. Also unlike other oaks, white or red, Bur Oak acorns commonly contain two seeds, and occasionally as many as five. This, and their need to develop strong roots early on, before they start photosynthesizing, might explain why Bur Oaks develop such large acorns. Arguably, the tendency for some Bur Oak acorns to ride out winter before sprouting would place them in the Red Oak group. But the boundary between these Bur Oaks and those that behave like other white oaks by sprouting immediately, is gradational, so we regard them as a single species. Nature doesn’t care about our classification schemes. (Incidently, the singular and plural of biological species both end with s. Specie, without the s refers to coinage).
Many trees can be identified to genus or even species level by their bark. Bark has the advantage of being visible all year, and it can be recognized on taller trees without binoculars. While Bur Oak trunks have deeply furrowed, but otherwise indistinct bark, the smaller branches and twigs are often recognizable by their ragged surfaces and the presence of corky ridges. Bur Oak trunks have thick bark, which helps protect them from fire.
As the lead photo in this article shows, Bur Oaks can reach massive proportions, partly because members of the White Oak group are among the longest living hardwoods. Mossycup Oak typically reaches a height of 50 to 80 feet with a trunk diameter of 2 to 4 feet. Ohio’s Champion has a trunk diameter of 8.3 feet, a height of 67 feet, and a crown width of 86 feet. Missouri’s Big Tree, pictured above, is listed by Monumental Trees as the country’s second largest Bur Oak in girth at 7.6 feet in diameter, and the sixth largest in height at 89.9 feet.
ID Summary: Obovate leaf with deep sinuses near stem and shallow sinuses near tip. Large acorn with frilled cap covering approximately half the acorn. Leaves can resemble White Oak or Swamp White Oak, but Bur Oak acorn is larger with frilled cap, and Swamp White Oak acorns have unusually long stems.
Q. macrocarpa’s preference for open sun and its drought tolerance make it an important tree at the eastern edge of the American prairie. Donald Culross Peattie’s A Natural History of Trees of Eastern and Central North America relates that on their westward migration, the white settlers enjoyed the change from dense forest, to grassland punctuated by trees, and called it the “Oak Openings”. Bur Oak generally prefers alkaline soils to acidic soils, and it often thrives in areas with limestone bedrock. According to the USDA PLANTS website, it commonly grows along river bottoms with water-loving trees, such as Cottonwood (Populus deltoides), but it does not tolerate flooding. Bur Oak is native to the east-central U.S. and southeastern Canada, and is the northernmost North American oak. The detailed range map and the text in Lucy Braun’s The Woody Plants of Ohio indicate that Bur Oak occurs in most Ohio counties west of the Appalachian foothills, but is mostly absent in southeastern Ohio. The bedrock in southeastern Ohio is sandy and the soil is acidic, and Bur Oak prefers alkaline soils.
Bur Oak lumber has a Janka hardness of 1,360 lbf and a density of 0.72 g/cc, according to the Wood Database, compared to the median of my list of 73 Ohio trees of 1,075 lbf and 0.64 g/cc. Bur Oak is a quality wood and is widely used for furniture, flooring, and veneer. Q. macrocarpa is often sold as simply, “white oak”. Like other members of the White Oak group, Bur Oak is water-tight, making it suitable for tight cooperage (barrels designed to hold liquids), and it has superior rot resistance in outdoor use. Bur oak’s water resistance also makes it useful for fences, boats, and railroad ties. Oak has a high fuel value and it splits parallel to grain, making it excellent firewood.
According to the Native American Ethnobotany Database, because Bur Oaks yield large, relatively sweet acorns, a number of Indians groups used them as a food source, including the Cheyenne, Chippewa, Dakota, Lakota, Ojibwa, Omaha, Pawnee, Ponca, and Winnebago. Medicinal uses among the Natives included:
- Chippewa – inner bark decoction to treat cramps, heart, and lung problems;
- Iroquois – infusion from wood chips for diarrhea and as an aid for STDs;
- Meskwaki – compound from wood and bark for worms;
- Ojibwa – bark as an astringent and to make bandages for broken limbs.
Additionally, the Chippewa used Mossycup Oak bark to make a black dye, while the Ojibwa used it to make a mordant for setting dyes.
Rebecca Rupp’s Red Oaks and Black Birches, The Science and Lore of Trees, says that between the days of muddy ruts, and the invention of Macadam roads of crushed, tamped rock, were the plank roads, usually made of oak. Oaks of the White Oak group were preferable due to their rot resistance, and Mossycup would have been among the most available, especially at the eastern edge of the prairie in Missouri, Kansas, and Nebraska.
Peattie’s Natural History of Trees tells of the Council Oak, a Bur Oak in Sioux City, Iowa, that was probably already 150 years old when Louis and Clark held a council beneath it with the Indians. At Exira, Iowa is the Plow Oak, a Mossycup against which was laid a plow at the beginning of the Civil War. Presumably, the owner never returned, because the plow was engulfed by the tree, leaving only its handles exposed. Yet another notable Bur Oak is the Mark Twain Cave Bur Oak, which stands near the opening of the Mark Twain Cave. This is the cave that Twain visited often in childhood, and which was used by a local physician to experiment with preserving corpses, including his daughter’s. One of Twain’s misadventures surrounding the corpse is related in The Adventures of Tom Sawyer.
Adele Nozedar’s The Tree Forager discusses how oak galls were mixed with iron oxide and lemon juice to make the jet-black permanent ink used to write the Magna Carta and the Declaration of Independence. A recipe is included. I haven’t used oak galls, but I have dissolved steel wool in vinegar to make a liquid that turns black seconds after contact with oak. A drop of dish detergent helps the liquid soak into the wood’s pores and form an attractive matte finish that can be left plain, or covered with a clear finish.
The white oaks are exceptionally beautiful as trees, and they make beautiful and useful lumber. Bur Oak is one of the finest white oaks. What more need I say?
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SAVE THE DATE: August 24, 1:00 to 5:00. The inaugural Arborfest will be held at Whetstone/Park of Roses. Arborfest will host a number of garden, nature, and tree experts (including yours truly) who will sell plants, answer questions, give talks, and lead walks. There will also be food trucks, so save your appetites! Admission is free.
You can also listen to my talks on Ohio Broadleaf Tree Identification on YouTube, Part 1 and Part 2.
