Burr oak tree acorn

Bur Oak

Bur oak (Quercus macrocarpa) is a member of the broad white oak group (white, bur, chinkapin, swamp white, and post oaks). This group is characterized by having rounded lobes on the leaves and acorns which mature in a single growing season and sprout soon after they fall in the autumn.

Habitat: Grows on dry uplands and slopes. Found throughout the state.

Hardiness: varies with the species of oak tree ranging from zone 3 to 9

Bur Oak Tree – Photo by Paul Wray, Iowa State University

Growth Rate: Slow to Moderate

Mature Shape: Broad, rounded

Height: Varies with species. Often maturing between 50 to 75 feet tall. Capable of growing upwards of 100 feet.

Width: 40 to 70 feet. Varies with species

Site Requirements: Best growth in moist, well-drained soils. Adaptable to adverse soil conditions.

Leaves: Alternate, simple, lobed. Lobes have rounded tips

Flowering Dates: April – May

Seed Dispersal Dates: September – October

Seed Bearing Age: 35 years

Seed Bearing Frequency: Every 2-3 years

Seed Stratification: Prechill for 1-2 months at 34°F to 40°F

Bur Oak Leaves – Photo by Paul Wray, Iowa State University

Bur oak exhibits more variation in its vegetative characteristics than any other oak species. The leaves are simple and arranged alternately on twigs. Single leaves have rounded lobes with a deep sinuses near the center of the leaf which appears to split the leaf in two. The leaves are dark green above and lighter green to gray below. Fall color varies from dull yellow, yellow green to yellow brown. The fall buds are clustered, 1/8-1/4 inch long and covered with pale, gray, fine hairs. The twigs are usually stout and yellowish brown and smooth at first and usually developing corky ridges with age. The bark of bur oak is dark gray, rough, and deeply ridged on older trees. The acorn is 3/4 to 1 1/2 inch long, enclosed half or more in a deep cup conspicuously fringed on the margin.

Bur oak is native throughout the state. It grows on a wide range of sites from stream terraces and floodplains to the driest of uplands. It is the most drought resistant of all the oak species mainly because of its extensive root system. In western Iowa it is the most abundant tree often forming pure stands; in the rest of the state is a less abundant member of the forest community growing with a variety of other species including other oaks, hickories, and aspen.

Bur oak is the prairie tree species because it was often associated with the prairie-forest border. Because of its relatively thick, fire-resistant bark and natural resistance to drought, it could compete successfully with the prairie grasses. These “oak groves” were favorite home sites for early settlers in Iowa.

Because of its slow growth rate and poor fall color, bur oak is not used extensively for landscape purposes. Because of its wide tolerance of site conditions and its tolerance to city environments, it could be used with success in landscape plantings.

Bur oak lumber has the same strength, hardness and durability characteristics as white oak but is often of less value because the trees develop more limbs and often are open grown. With good management, bur oak can produce high quality oak lumber and veneer.

Diseases that Can Affect Bur Oak

  • Pin Oak Chlorosis
  • Oak Leaf Blister
  • Oak Tatters
  • Oak Wilt
  • Anthracnose

Insects that Can Affect Bur Oak

  • Oak Sawfly
  • Giant Bark Aphid
  • Lace Bugs
  • Acorn Weevil
  • Walking Stick
  • Yellownecked Caterpillar

Bur Oak Fruit – Photo by Paul Wray, Iowa State University

Bur Oak Twig – Photo by Paul Wray, Iowa State University

Bur Oak Flowers – Photo by Paul Wray, Iowa State University

Bur Oak Bark – Photo by Paul Wray, Iowa State University

Bur oak

Full sun in well drained soil, but adaptable to many soils. Drought tolerant once established.
Prune oaks in the dormant season to avoid attracting beetles that may carry oak wilt.

Disease, pests, and problems

Can be affected by pests such as the leaf galls and kermes scale.
Anthracnose, bacterial leaf scorch, and powdery mildew. Oak wilt is a serious disease of oaks.
Bur oak blight has been found in isolated areas in Illinois.

Native geographic location and habitat

C-Value: 5

Commonly found in upland savannas.

Bark color and texture

Mature bark is dark gray to brown with deep furrows.

Bur oak (Quercus macrocarpa) photo: John Hagstrom Stems are stout and smooth but young twigs can be develop corky ridges.

Leaf or needle arrangement, size, shape, and texture

Alternate leaf arrangement.
Large, 4 to 10 inch long leaves with 5 to 7 rounded lobes. The terminal lobe can be fiddle-shaped.
Leaves are lustrous, dark green above with lighter silvery green beneath.
Fall color is yellow-brown.

Flower arrangement, shape, and size

Male flowers hang down in drooping catkins, female flowers are small spikes in leaf axils. Not ornamentally important.

Fruit, cone, nut, and seed descriptions

Large, 2 inch diameter, fringed acorns. The conspicuous fringe covers most of the nut. Fruit ripens in the fall.

Cultivars and their differences

“This plant is a cultivar of a species that is native to the Chicago Region according to Swink and Wilhelm’s Plants of the Chicago Region, with updates made according to current research. Cultivars are plants produced in cultivation by selective breeding or via vegetative propagation from wild plants identified to have desirable traits.”

Urban Pinnacle® (Quercus macrocarpa ‘JFS-KW3): Form is narrow and pyramidal (25 feet wide). Summer foliage is glossy dark green, changing to yellow in fall. Acorns are much smaller than typical for bur oak. Resistant to powdery mildew and anthracnose.

Related hybrids

Heritage® Macdaniel’s oak (Quercus x macdaniellii ‘Clemson’): This is a hybrid between Bur oak (Quercus macrocarpa) and English oak (Quercus robur). The dark green foliage of this hybrid is resistant to powdery mildew. Fall color is yellow.

Below: Bur Oak, Quercus macrocarpa Michx.: Ovate, broadest at the middle, (Fiddle-shaped) with many rounded lobes and a broad rounded tip that may resemble a crown as the lobes appear more as large teeth instead of lobes. There is considerable variety on how deep the lobes cut to the central rib. On some the indentations are slight, on others the two middle lobes are divided almost to the central rib. Color is dark green and slightly shiny on top and gray-green with fine hair under. The leaf stalk is short. Bases are rounded to pointed. Fall color is a rusty brown.

Below: Swamp White Oak, Quercus bicolor Willd.: Ovate with a triangular or narrow wedge shape base and with a rounded point at the tip. The edges are not deeply cut but with broad large teeth, either on the entire leaf or just the top half. There can be many variations. The upper leaf is a dark shiny green and the underside much lighter with dense fine hair. Fall color is yellow to rusty brown – not considered striking.

Below: White Oak, Quercus alba L.: Oblong to ovate in shape, with a base that is a narrow wedge shape to triangular with the point forming the short stalk. There will be 7 to 10 (5 to 9) ascending lobes looking like fingers, and a rounded apex at the top end of the leaf. The lateral lobes indent to the mid-vein by 1/3 to 7/8ths of the distance and these lobes too have a somewhat rounded tip. The upper surface is a bright gray-green and underside is more whitish with erect hairs that disappear as the leaf matures. The secondary leaf veins are arched. Fall color can be yellow to yellowish-red. Leaves from the juvenile part of the tree can resemble those of Swamp White Oak and Bur Oak.

Oak leaves with pointed lobes – with bristles (awns). Oaks of this group tend to hybridize with each other.

Below: Black Oak, Quercus velutina Lam.: Elliptical, long stalked, usually with 5 to 9 (5 usually) lobes that are either shallow or deep and narrow. Lobes end with a few bristle-tipped teeth (awns) of which there can be 15 to 50. The leaf base can be obtuse to truncate in shape and is usually a bit unequal from side to side. The upper surface is shiny dark green and the underside a pale green with some brown hairs along the main veins. The secondary veins appear raised on both surfaces. Fall color is brown to dull red.

Below: Northern Red Oak, Quercus rubra L.: Elliptical in shape, on a slender stalk, divided into 7 to 11 shallow, wavy lobes cleft 1/3 to 1/2 of the distance to the mid-vein. The lobes have bristly tips (awns) and are rounded near the mid-vein. The upper surface is a duller green, the underside a dull lighter green with tufts of hair along the mid-vein. Fall color is reddish-brown. A deeply lobed example (like the 1st photo) will resemble Northern Pin Oak shown below, but in viewing several examples note that the section along the mid-vein will generally have more leaf. Nor are they as shiny as Black Oak shown above.

Below: Northern Pin Oak, Quercus ellipsoidalis E. J. Hill: Elliptical, broadest across the lobes above the middle, and can be divided over half way to the mid-vein with 5 to 7 deep lobes (sometimes 9). Each lobe ends in several bristle-tipped teeth (awns). Between the lobes (the sinuses) the area is generally round. There is a short leaf stalk, a shiny green upper surface and lighter under with tufts of hair along the mid-vein. The branching secondary veins appear raised from the surface. Fall color is brown to a deep red.

Below: Pin Oak, Quercus palustris Münchh.: Of the 8 oaks listed here, this one is fairly consistent in looks except for the base which can be truncate (1st photo) or obtuse (2nd photo), otherwise, the leaf is elliptic to oblong in outline with 5 to 7 deep lobes, cut nearly to the mid-vein (much deeper than the Northern Pin Oak shown above). The sinuses of the lobes are irregular to rounded with the larger lobes forming a “U” shape. The lobes are tipped with 10 to 30 pointed bristles (awns). The upper leaf surface is bright shiny green, the lower surface paler. Both surfaces are free of hair except for tufts of brownish hair on the underside at the veins. If the base is truncate in shape the base pair of lobes is frequently recurved. Fall color is red to reddish brown.

Below: Scarlet Oak, Quercus coccinea Münchh.: Elliptical to oval in outline, deeply divided, nearly to the mid-vein or at least half way, with 7 to 9 lobes, each lobe ending in bristle tipped teeth (awns) that number 18 to 50. The sinuses between the lobes are wide and round forming more than a half circle. Stalks are long and slender. Secondary veins of the leaf appear raised on both surfaces. The upper surface is a shiny green, the underside pale green with tufts of hair along the mid-veins. The leaf tip is pointed, the base is blunt (truncate) to slightly pointed (obtuse). Fall color is scarlet.

Additional information: The species name, macrocarpa, is derived from the Greek words macros (large) and carpos (fruit), referring to the bur oak’s large acorn. The bur oak acquired its common name because of the resemblance of the fringed acorn cap to the bur of the chestnut fruit.

The massive root system of the bur oak is said to be a mirror reflection of the trunk and branch system above ground. With this extensive root system it is not surprising that the bur oak is the dominant tree of midwestern prairies, often considered the advancing pioneer species. Remaining trees are considered the most fire-resistant, having withstood the ravages of periodic prairie fires. The root system of bur oak successfully competes with prairie grass roots as well as neighboring bur oaks. Intense root competition keeps bur oaks scattered apart from each other throughout the savanna.

Bur oak seedlings cannot survive in the shade of a dense forest. Because the species is not reseeding itself as fast as individuals are dying, planting bur oak in the landscape is encouraged. In Kentucky, bur oak was once common in the Bluegrass region and some stately trees remain standing.

The national champion bur oak (96 feet tall, 103-foot spread) is in Paris, Ky.

Tree Species Profiles: Top Rated Shade Trees, Bur Oak (Quercus macrocarpa)

Bur Oaks get their common name from the shaggy, fringed caps that contain the acorn. All photos by Bill Seaman.

Published April 19, 2013 By Steve Houser

When French botanist André Michaux (1746-1802) garnered the botanical naming rights for Bur Oaks, he went for the obvious: Quercus macrocarpa, the Oak with the large fruit. Even his description is a bit of an understatement considering this Oak can grow to exceed 80 feet in height, have leaves longer than 10 inches, and grow acorns the size of lemons. Bur Oak, which is also known as Mossycup Oak and is sometimes spelled with an extra “r,” gets its common name from the distinctively rough and shaggy acorn cap that can often enclose much of the acorn itself. The acorns are a highly desirable food source for wildlife, but their size alone provides a challenge for squirrels anxious to dig a hole large enough to bury them or to find the strength to hoist them to their cache. To a squirrel or other wildlife, Bur Oak acorns are like a huge steak dinner.

With a native range that stretches from the Texas Coastal Plains to Manitoba, Bur Oaks are adapted to most Texas landscapes. In North Central Texas, they can be found growing in the rich-soil bottomlands along creeks and rivers, where moisture is available during periods of drought. One of the state’s largest Bur Oaks grows along Rowlett Creek in Plano’s Bob Woodruff Park. Recognized as a Bicentennial Tree in 1976, this landmark is believed to be more than 400 years old, and requires at least four children to give it a hand-in-hand hug. Remember what it was like to be a barefoot little tree hugger?

A little “tree hugger” at the Bicentennial Bur Oak in Plano’s Bob Woodruff Park.

Bur Oaks are a relative newcomer to the retail nursery trade in Texas. Few trees were grown commercially before 1980. Many Bur Oaks planted in landscapes and gardens before that time were homegrown by industrious gardeners who had access to the easily germinated acorns and were eager to pass them along to other gardeners. Bur Oak acorns require no cooling period to sprout, so acorns gathered in the fall can be planted in containers immediately. They grow fast enough to transplant into the garden the following year.

Bur Oak acorns are easy to plant and will germinate in a few weeks. They can be transplanted into the yard the following fall.

Bur Oaks have a number of characteristics that gardeners can appreciate. First, the tree has a moderate growth rate. Once fully established, a well-cared-for tree can generate more than 20 inches of shade-producing growth a year. As the tree ages, the stout limbs are resistant to wind and ice damage, and the corky bark protects it from sunscald as well as fire, to some degree. Our hottest Texas summers pose no challenge to its drought tolerance. While Bur Oaks are not known for fall color, occasionally they delight us with a golden display before leaf drop reveals a coarse and rugged winter branching form. Strong wood and a tendency to grow some horizontal lower branches can make bur oaks good climbing trees and great candidates for swings. What may be the most valuable trait is their resistance to Oak Wilt, making them a good Oak substitution or replacement for often over-planted Live Oaks and Red Oaks.

The Bicentennial Bur Oak is thought to be more than 400 years old.

Bur Oaks are suitable trees for large urban landscapes, since the canopy widths can exceed 75 feet as they mature. Keep the mature size in mind when determining the proper location for planting, avoiding future conflicts with structures, as well as utility lines. Bur Oaks can be planted to shade sidewalks, driveways, and patios. Even though the acorns can be both large and prolific, they are manageable. Some years, the squirrel patrol will be happy to manage them for you.

Native to much of Texas and adapted to more, the Bur Oak ranks high on almost all top shade tree lists. Plant them knowing they will be enjoyed for the next 100 years. Future generations of children (as well as squirrels) will be eternally grateful!

To continue reading the Top Rated Trees article series, .

Posted by Mr. Steve Houser

About the author

Mr. Steve Houser

Mr. Houser is a Dallas native with almost 40 years of experience as a consulting arborist and expert tree climber. He is the president and owner of Arborilogical Services, “The Experts Your Trees Deserve.”®

What Is A Bur Oak Tree: Learn About Bur Oak Care In Landscapes

Mighty and majestic, the bur oak (Quercus macrocarpa) is a survivor. Its massive trunk and rough bark help it exist in a very broad natural range in a variety of habitats – from bottomlands to dry uplands. What is a bur oak? Read on for bur oak information and tips on bur oak care.

What is a Bur Oak?

Bur oaks, also called mossycup oak, are decidedly impressive oak trees native to North America. They grow in the wild in central and eastern sections of the continent. The common names comes from a mossy scale, or bur, on the acorn cup rim.

Bur Oak Information

Bur oak trees are medium to large sized trees. They are deciduous members of the white oak group and grow to heights between 60 and 150 feet tall (18 to 46 m.). If you are thinking of planting a bur oak, you’ll want to take height into account when selecting a site. Keep in mind that the trees also have broad, rounded crowns.

Bur oak trees produce yellow catkin flowers in springtime, but they are not particularly showy. The acorns are oval with fringed cups, and offer a good food source for wildlife, including both birds and mammals.

Don’t expect brilliant fall color in bur oak tree leaves. The green leaves turn a dull yellow-brown before they fall.

Planting a Bur Oak

Planting a bur oak is only a good idea for homeowners with very large backyards, given the size of the trees. The massive oak grows best in U.S Department of Agriculture zones 3 through 8. Be sure you site the tree with enough room to grow and in a permanent location. Bur oak information says that these native trees can live up to 300 years.

If you do decide to start planting a bur oak, site the tree in full direct sun. Be sure the tree gets at least six hours of unfiltered sunlight every day.

For best bur oak care, plant the tree in soil that is well drained and loamy. It will grow in either acidic or alkaline soil, and tolerates sandy, wet and clay soils too.

And speaking of bur oak care, don’t forget to water the tree regularly, especially during its first year in your garden. Bur oak trees have some drought tolerance, but they will grow faster and healthier with moderate moisture.

Note that bur oak trees tolerate city smoke and other air pollutants as well as compacted soil. They are often used as shade tree on U.S. city streets.

How to Plant & Care for a Bur Oak Tree

oak’s leaf image by Marcin WyÅ‚uda from Fotolia.com

Bur oak (Quercus macrocarpa) is a large, deciduous tree that is native to most of North America. Bur oak grows best in USDA hardiness zones 3 through 8. These trees mature into large, crown-shaped trees, reaching heights from 70 to 90 feet with spreads ranging from 60 to 80 feet. Planting bur oak trees entails careful planning, to ensure room for its massive full-grown size. Caring for bur oaks begins right after planting.

Select an area providing full sun and well-draining soil. Locate the site for planting the bur oak tree well away from other foliage, structures and overhead wires. Pick a planting site that accommodates the tree’s mature size.

Cultivate an area from three to five times wider than the size of the bur oak tree’s rootball (container) to allow the roots to spread. Create a planting hole in the center that is the same depth of the container.

Remove the bur oak from its container and check the roots. Cut off any damaged or broke and loosen any tangled. Place the tree in the hole at the same height or slightly higher than it was in the container to allow for settling. Add or remove dirt from the hole if needed.

Backfill the hole with the same soil removed. Drench the area with water to get rid of any air around the transplanted bur oak tree and to settle the tree. Add more dirt, if the rootball is exposed.

Place a layer 2 to 3 inches thick of mulch extending out over the planting area. Do not put any of the mulch within 6 inches of the bur oak’s tree trunk.

Supply 15 gallons of water slowly to the bur oak two times a week for the first month. Water the tree weekly for the next two months, every other week for the next three months and then once a month after that.

Seeds for the Future: How to Plant an Acorn

Bur Oaks produce large acorns that are easy to gather and plant in containers. Before planting, remove the caps and soak in water over night.

Published March 26, 2013 By STEVE HOUSER

Why should we plant and care for trees? Research shows that trees clean our air, our water, and our soil. Trees make significant contributions to improve our health, sense of well being, quality of life, and our economic future. According to USDA Forest Service research, the benefits provided by large mature trees are 70 times greater than small, recently planted trees. In essence, caring for our existing mature trees provides significant benefits today, while planting young trees benefits future generations.

Many green industry professionals agree that homeowners get the most “bang for the buck” by planting a two to four inch diameter (or caliper) tree. There is an immediate effect without spending a fortune. For those with funds in their budget, hiring a professional to plant this size of tree works well.

For a less expensive approach to tree planting, and a way to teach children about the many joys of growing a tree, consider planting an acorn for the future. Planting acorns can be a more successful method of establishing a tree than planting a small seedling (or sapling) “gimme tree”. Planting acorns has the following advantages:

  • Transporting acorns are convenient. They are not sensitive to wind and temperature and do not have to be watered in transit.
  • Locally collected acorns produce trees that are adapted to the same climate and soils.
  • An acorn can be planted in a container filled with the same soils it will grow in once it is planted in the landscape, making final establishment quicker.
  • Sprouting acorns produce growth that is immediately acclimated and less likely to be “sunburned”.
  • Planting acorns in an appropriately sized container allows for the growth of a root system that successfully transplants into the landscape and requires less frequent watering.

All of these factors reinforce the idea that a properly planted acorn from a local tree is likely to grow into a healthy mature tree.

How to Plant an Acorn

Step 1: Select an acorn. Discard any acorns that may be cracked or with holes in the shell. Place the acorn in water and let it soak for 24 hours. If it floats in the water, it will not grow and another acorn is required.

Step 2: Find a one or two gallon container that has holes in the bottom for proper drainage (drill or poke holes if necessary). Fill the container with soil from the same location the tree will eventually be planted, leaving about one inch between the top of the soil and the rim of the container. The soil can be amended with a small amount of finished compost or potting soil, but this is not required.

Fill a one to two gallon container with native soil and place the acorn on its side. Cover the acorn with one to two inches of soil.

Step 3: Plant the acorn in the soil at a depth of one and a half times the diameter of the acorn. For example, a one-inch diameter acorn should be planted around one and a half inches deep.

Step 4: Place the container outside where it receives only morning sun and it is shaded in the afternoon. Water often enough to prevent the soil from pulling away from the sides of the container. If you are in doubt, use a screwdriver or your finger to check the soil moisture. (Gardeners prefer to let their fingers do the walking.)

Place the container where it will receive direct sun from morning until noon, and water it as needed to keep the soil moist. The acorn will germinate in four to six weeks.

Step 5: Stand back and watch your acorn sprout into an oak. Continue to water and fertilize your new tree as needed. Let it grow in its container until fall.

In October, plant the “oak from an acorn” in a sunny location with plenty of room to grow, and away from overhead utility lines. Dig the planting hole twice the width of the container to reduce conflicts with any nearby plants and to provide loose soil for new roots to develop. Do not plant too deep. The soil level in the container should match the soil level of the existing grade once planted. Planting tip: if you let the soil in the container dry slightly before planting, the root ball will slide out of the container easily, eliminating the need to cut the container.

These Bur Oak seedlings have grown all summer long, and are ready for fall planting in the landscape.

Add a ring of soil to the outside of the planting hole to help retain rainfall and to aid in watering. A half-inch of native tree chip mulch or another type of mulch can be added to aid in water retention as well as reduce evaporation.

The newly planted tree will become fully established in about three years. Until that time, water and fertilize it as needed. As it grows, we all benefit.

To properly plant an acorn is good…to ensure it survives is golden.

A special note to those guests that receive a “seed for the future” acorn when visiting our booth at Earth Day Texas 2014:

Plant your acorn immediately and keep it moist. It should sprout in a few weeks. If it does not send a shoot above the soil by June or July, and you live in the North Central Texas area, please contact us for a replacement acorn. The idea of providing “seeds for the future” is sustainable only if the acorns survive. The benefits of these trees are for all to enjoy–but only if your acorn grows. Our ability to replace acorns that do not germinate will be based on seasonal availability. We are hopeful that our local oaks will grow acorns each year to harvest and pass along to others. However, acorn availability in the fall is dependent on the weather conditions during the spring flowering period. Once you contact us, you will be placed on our replacement list and we will mail you an acorn as soon as they are available.

If you received a large acorn, it is most likely to be a Bur Oak acorn, Quercus macrocarpa. When transplanted from the container into the landscape, remember that your new tree needs full sun and will become very large. Because the acorns are so large, you may want to plant it away from pools, driveways, and other areas where the acorns might be a nuisance. If the acorn is less than an inch in diameter, it is likely a Shumard Red Oak acorn, Quercus shumardii. It too, needs full sun and a large growing area. Some trees can produce large numbers of acorns, so plant the tree in a location where dropping acorns will not be a problematic.

Posted by Mr. Steve Houser

About the author

Mr. Steve Houser

Mr. Houser is a Dallas native with almost 40 years of experience as a consulting arborist and expert tree climber. He is the president and owner of Arborilogical Services, “The Experts Your Trees Deserve.”®

Brooke Byerley Best, Ph.D.

“What is this thing???”

We often hear this question from friends and family in relation to natural “treasures” found in the landscape. Sticks, leaves, flowers, fruit, fungi, lichens, moss. You name it, somebody has likely brought it to BRIT for identification at some point (or emailed us a photo).

This time we feature the crazy, gargantuan, monster acorn caps from the bur oak tree (Quercus macrocarpa).

RAWR! Monster caps!Bur oak distribution in Texas. Adapted from digital version of “Atlas of United States Trees” by Elbert L. Little, Jr. U.S. Geological Survey.

Bur oak is native to the central and eastern US, including most of the middle swath of Texas, top to bottom. This fast-grower typically likes an open, limestone or chalky clay habitat and is adapted not only to fire and drought but also to extreme cold and flooding. You can find it in the prairies and savannas as well as along waterways. So basically it’s a super champ of trees, which is why it’s common in cultivation.

And as you can see above, THE CAPS OF ITS ACORNS ARE WEIRD, mostly because they’re just so big. Bigger than we’re used to seeing relative to what we think of as lil’ ol’ acorns. But they’re also weird because of the shaggy ornamentation encircling the cap. This is a feature that can vary over the tree’s range. Caps in the southern portion have long fringe hairs while others at the far north of its range are much smaller and barely have any shag at all. The caps seen above have what one might call “average shag.” The shagginess and the size of the cap are the reason for one of the tree’s other common names: mossy-cup oak. To me, because the size of the cap often dwarfs the little acorn attached, it resembles one of those big Russian fur hats (which Google tells me is called a “ushanka”…think George Costanza’s “leave behind” hat).

Just your average megafamous hip-hop mogul sporting an acorn cap (coined it!). (Credit: everyskyline, Wikimedia)

There’s another oak, Quercus lyrata (overcup oak), that the bur oak can sometimes be confused with—their ranges overlap a bit in East Texas, they can hybridize, and overcup oak also has an acorn cap that wraps much of the way around the acorn (hence its common name).

Bur oak (Q. macrocarpa) vs overcup oak (Q. lyrata). Adapted from Flora of North America. Bur oak (Quercus macrocarpa) acorn caps. The size can be pretty variable within a single tree, but they’re all pretty bulky compared to other species. Bur oak (Quercus macrocarpa) acorns. Bur oak (Q. macrocarpa) vs. English oak (Q. robur). Bur oak (Q. macrocarpa) vs. Texas live oak (Q. fusiformis).

I mean, LOOK at the size of these things! You know that scene in Honey, I Shrunk the Kids where the kids are tiny and running around in the yard, and they happen upon a crumb from an oatmeal creme pie, and it’s as big as a house? That must be what it’s like for a squirrel to eat these acorns! But they’re not just a treat for the wee animals. American black bears also consider them worthy snacks, breaking off branches to get to those still attached to the tree.

This is my dream, y’all. Bur oak leaves and acorns (Illustrated Flora of North Central Texas). Ornament made from a bur oak acorn cap. (Thank you, littles!)

So why the big cap? Why the big fruit? The Latin name macrocarpa translates to “large fruit.” The acorns of bur oak are the largest of all the native oaks. But making fruit this big is energy-intensive; it’s expensive to the tree. So bur oaks use a strategy called masting (=”synchronous and intermittent reproduction”) where large nut crops are produced only every few years in order to overwhelm seed predators, making more than could be eaten in a single season and hoping that some will survive to become new trees. Many oaks do this. Serendipitously, the MinuteEarth channel on YouTube just posted a new video on this very topic (When Trees Go Nuts – YouTube).

This doesn’t exactly explain why individual bur oak acorns are BIGGER than acorns from other oak species, but the evolutionary advantages of any large-fruited plant are somewhat related to the idea of masting: fruit/seed predators are given more than they can possibly consume in total, hopefully leaving some fruit and seeds intact to germinate and contribute to the next generation. Think of the strategy of making few big watermelons versus hundreds of little grapes. That may be what’s going on here. Why do you think the acorns are so big?

Excerpt from Shinners & Mahler’s Illustrated Flora of North Central Texas (1999):
Quercus macrocarpa Michx., (large-fruited), BUR OAK, MOSSY-CUP OAK, PRAIRIE OAK, MOSSYOVERCUP OAK. Large tree; nuts and cups (3-6 cm wide) largest of all nc TX species. Stream bottoms, lower slopes, upland woods; usually in at least moderately drained places; in or near limestone areas; se and e TX w to West Cross Timbers and Edwards Plateau. This species is well known for its large acorns and thick, fire-resistant bark.

Links to more info on the species…

So keep bringing us your weird and wonderful treasures! We’ll do our best to turn them into teaching moments that benefit us all. Happy botanizing!

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