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Tree Flowers (and fruits) 
Bur-oak (Quercus macrocarpa)

Q. Hey, what's the coldest tree? A. Bur-oak (brrr-oak?).

Q. What was J.S. Bach's favorite tree? A. Bur-oak (baroque?).

O.K. Enough with the jokes already. Bur-oak is a beloved and magnificent tree, well known for being the only tree that grows in prairies. Grasslands with scattered trees are called "savannas." There is a magnificent savanna in Crawford County, the Daughmer Bur-oak Savannah where this species reigns supreme.

Daughmer bur-oak savannah

Daughmer Bur-oak Savannah, Crawford County, OH.

Other species of trees, even native ones such as ashes and maples, are considered weeds in prairies. If ever a windstorm were to sweep through Daughmer Savanah and damage trees, the instructions to tree surgeons dispatched to remedy the damage would be very clear: "If it ain't bur-oak, don't fix it!."  

Bur-oak trees are being preserved at the Ted Myers Savannah, and have been planted at the Larry R. Yoder Prairie, both located at OSU's Marion campus.

bur-oak at Marion

Bur-oak at the OSU's Marion Campus

A common pattern for some wind-pollinated species such as oaks, hickories and walnuts, is for the pollen-producing male flowers to be individually tiny, but presented abundantly in elongate drooping finger-like catkin. Nearly on the same tree there are female flowers which are slightly larger, and much fewer in number. These eventually mature into the familiar nut or nut-like fruit.

During the spring and summer of 2007 I followed the progress of flowering on a bur-oak tree. This species is monoecious, i.e., the individual flowers are either male or female, but both types are borne on the same tree. Being wind-pollinated, the flowers are not showy. 

bur-oak flowering

Flowering branch of bur-oak, May 10, 2007, Marion, OH.

Seen closer, the female flowers display pollen-receptive stigmas that seems to have a remarkably small surface area in comparsion with many other wind-pollinated trees. (Contrast this with the fairly elaborate "rabbit ears" of walnut.)  

oak female flowers

Black walnut pistillate flowers, May 10, 2007, Marion, OH.

By the following week the staminate catkins withered and soon after they fell from the tree.

bur-oak staminate

Spent staminate catkins of bur-oak, May 18, 2007, Marion, OH.

Over the next month, the baby acorns slowely got bigger.

pistillate bur-oak 18maypistillate bur-oak 25may

Bur-oak very young fruits in mid-spring, 2007  (left: May 18, and right: May 25). By this time the spent staminate catkins have fallen to the ground.

pistillate bur-oak 30maypistillate bur-oak 5jul

Bur-oak young fruits in late spring and early summer, 2007  (left: May 30, and right: July 7).  

By late summer the acrons are almost ripe, ready to be snatched by a squirrel or fall to the ground. 

bur-oak acorn

Bur-oak acorn, September 5, 2007, Marion, OH.

Oaks (genus Quercus) are neatly divisible into two sub-genera: (1) the white oak group, and (2) the red oak group. White oaks, including bur-oak, mature their acorns in one year, while red oaks take two years to develop. Acorns are nuts, and, although bitter owing to the presence of tannic acid, are edible (some moreso than others). The tannic acid can be leached out by boiling and changing the water frequently.   

bur-oak leaes and fruit

Bur-oak tree, September 16, 2007, Marion, OH.