The burr oak, Quercus macrocarpa Michx., is a common tree found primarily in Southern Michigan but occasionally in Northern Michigan and the Upper Peninsula. Burr oak is a member of the white oak genus Quercus, as are all the oaks we will cover. Its acorns are very much desired for food by several wildlife species, especially deer. We are blessed to have a large quantity of these trees, of all ages, occurring on both of our farms, in Ingham and Barry Counties, about an hour’s drive apart.
Our experience is that burr oak is more precocious than the other oak species found on our property and can produce acorns every year but, on average, seldom do. I have an area with about 40 of these trees growing in a two-acre plot on low, organic soil, ranging in size from three inches in trunk diameter up to two feet or more. I have an elevated shack in the center of this area, and in certain years the deer seldom leave the plot around the middle of September as they compete for every acorn they hear drop!
These trees often start bearing acorns at a young age. Some produce them in most years, while others will go a few years in between. The larger, older burr oaks seem to have less productive years than younger, smaller trees. The trees growing in non-shaded areas, like fence rows or southern exposed woodlot edges, bear more frequently than those in wooded or shaded areas.
I keep good records of the frequency of bearing years for certain trees and gather acorns each reproducing period to make a concerted effort to duplicate them. I hope some will demonstrate a genetic trait for frequent or annual reproduction. I presently have about 200 oak seedlings featuring over six species of white oak growing in my garden at this time.
Burr oak has a long, strong tap root that is adaptable to all areas of our farms. The above grove is in a fairly low area of an often wet, mucky bottomland sharing the area with soft maple, white ash and other moisture-tolerating trees and shrubs. Few trees in our latitude can survive “wet feet” for very long! Some of our largest trees are growing in fence rows and forest edges in close proximity to tillable land.
Burr oak trees are easily transplanted at a young age but require a tree guard to protect them from deer at any age, as most species do. Deer seem to enjoy rubbing and raking their antlers on younger oak trees as well as other species growing in wooded or edge areas. We have had two protective wire cages disappear, probably the result of deer catching them in their antlers. All young trees, even four to six inches in diameter or more, can be girdled or severely damaged by deer and other critters. Fruit trees and conifers are especially vulnerable, often damaged by the same wildlife we are trying to encourage!
The older burr oaks, when grown in the open on good, rich fertile soil, can become very large trees reaching heights of 80 feet or more. They are supported by a deep tap root that makes them very stable. The width of the canopy is often wider than the height, and they make a very nice yard tree if space permits. In the open, the lower limbs droop towards the ground while the top branches grow out and upward, forming the crown. When the tree is grown in a wooded environment, the top or crown is small, and the trunk is straight and absent of limbs, making the tree very desirable for lumber purposes that have many uses. The trunk is recognizable on older trees by deep vertical ridges that are parallel and gray. The trunks in the open often develop a number of short, gnarly horizontal branches in the lower areas, especially viewable in the winter months.
All white oak species in my latitude seemed to have produced an abundant crop of acorns in the fall of 2021, and I collected seeds from several of them. Burr oak was no exception and even our very large trees produced pails full of acorns. When the burr oak and some hybrids first emerge from the ground, they are recognized as a stiff, pinkish, fast-growing shoots. The acorns germinate well if placed in a damp plastic bag, then stratified by keeping them in the refrigerator’s crisper drawer for the winter. If viable, they will start to grow a root in the spring and can be planted in a garden. Inspect the acorns for little holes to determine if a small white larva has exited the nut, ruining the chance for germination. Before storing the acorns, place them in a container with a few inches of water and discard all that float.
The true white oak is an exception and must be planted as soon as it hits the ground as the radical or root emerges from the acorn’s lower point immediately. The planted acorns are vulnerable to both squirrels and other wildlife during the early growing stages. They grow fast and can be transplanted in about two years if cared for properly. I place a plastic container, such as a two-liter pop bottle, with the neck and bottom removed over the growing seedlings as soon as they emerge. After they outgrow them, they are moved to another area and protected with a cage.
The burr oak has several unique characteristics that separate them botanically from other white oak trees. Their leaf is the largest size of the Michigan white oak genus and has a very unique structure. The leaf, as well as the acorn cap, sets it apart from all others. The leaves can reach a length of 8 to 10 inches. The leaf margins are made up of several lobes that are separated by deep cuts or sinuses. The lobes are rounded on the ends, and the two top ones are nearly squared in that area. The distal half of the leaf is the widest and has a spatula shape. The two pairs of central indentations are cut almost to the midrib on most leaves and serve as the species’ best, consistent identifying characteristic. The margin of the leaf from these sinuses tapers down to the petiole or stem end.
Both sexes occur on the same tree (monoecious), the male flowers forming on hanging catkins and the inconspicuous female flower originating at the base of new leaves.
The acorns subsequently develop at this location and are either sessile or with a very short peduncle or stalk. They are usually nearly round, about the size of a nickel, but are often variable, normally occurring in pairs. The acorns are covered by a thin shell called a cap that encloses about half to all of the nut. Rows of thin scale cover the cup-like material on the stem end and have a burr-like, hairy or fuzzy covering more prevalent near the nut’s outer margin, forming a ridge. This leads to the specie’s name; burr oak. They are sweet, edible and preferred by deer! The burr oak species, as with all white oaks, can produce acorns that develop in one year.
I find that all white oaks are very susceptible to being crowded out during the early stages of growth by more aggressive and spreading species, such as beech and bitternut hickory. After learning that several year-old oak was dying from being shaded by nearby trees with less value, we removed most of those that threatened our acorn bearers! So far, this has reduced most of the tree loss, but we learned that even burr oak could be crowded out by their own species if too thick of a stand occurs.
We use every effort available to protect our oaks. When we recently logged out one of our woodlots with a considerable amount of white oaks, I sprayed a wide, green band of paint around the circumference of each tree, about face height. I instructed the cutters not to harvest trees with a green band, nor should they damage them with machinery or other falling trees! No damage occurred to the white oaks. In the spring, we place a quantity of 19-19-19 fertilizer under the canopy of each white oak.
I have attempted to illustrate in both photos and words the different characteristics of the burr oak species the best I could. I tried to select the botanical type or most common features occurring on burr oak trees. However, it must be taken into account that there is a high degree of variability to the appearance of the leaves, the size of the acorns and the pattern of the bark on the trunks of all oak species. There can be a substantial variation between the leaves on the same tree. We must consider the size and age of the tree, the range, the month of the growing season, the wind, and the insect and animal damage affecting the leaf. We must also appraise the effects of seasonal climate changes when collecting or viewing specimens for identification purposes. Even some hybridization between oak species can occur. I learned that even foresters struggle with identifying some trees, shrubs and plants they encounter!
Burr oak and its close relatives of the white oak genus are not as susceptible to the “oak wilt” disease as red oaks. The MDNR tells us the disease is occurring close to western Barry County now!
We hope you find this article both interesting and helpful. Until next time enjoy our natural resources.