Tree of the Month: Bur Oak (Quercus macrocarpa)

   Bur Oak, also known as Mossycup Oak, is a native tree in the entire state of Indiana. The most common image of a Bur Oak is the lone tree in a fence row in a farmer’s field. Germans would refer to such a tree as a “wolf.” Bur Oak has thick, fire-resistant bark which protected it from the fires which were common in oak savanna and prairie habitats.

   Bur Oak trees can develop an enormous taproot, even by oak tree standards, sometimes as deep as 10 feet. This adaption allowed it to thrive in prairie habitats, where it was most commonly found. The tree’s root structure allows it to thrive well in a city environment, where urban soils are easily susceptible to drought.

Farm fields act as a substitute for the prairies, now mostly gone, in which Bur Oak used to occur.

Farm fields act as a substitute for the prairies, now mostly gone, in which Bur Oak used to occur.

   Bur Oaks produce the largest acorns of any oak tree in North America. Most oaks, including Bur Oak, produce a large crop of acorns only every few years. This strategy is designed to overwhelm seed predators, ensuring that at least some acorns are not consumed. If oak trees produced a steady quantity of acorns each year, seed predators such as squirrels or turkeys would adapt their numbers to consume almost the entire crop.

The Bur Oak has the largest acorns of any North American oak tree -- the hairy cup aids in identification.

The Bur Oak has the largest acorns of any North American oak tree -- the hairy cup aids in identification.

   Bur Oaks are moderately susceptible to Oak Wilt, a potentially fatal fungal disease. Oak Wilt fungus infects the water-conducting tissues (xylem) that run lengthwise up and down the trunk of the tree. In an attempt to block the fungus, oak trees will form a barrier in these tissues. This blocks the fungus from advancing, but it also cuts off the supply off water from the roots to the leaves.

   Bur Oaks and White Oaks are much less susceptible to Oak Wilt than Red Oaks and Pin Oaks, where the fungal disease is almost certainly fatal and spreads through grafted root systems of neighboring trees. This is because Bur and White Oaks have tyloses, an additional cell in the xylem designed to block the flow of water during drought conditions. It turns out tyloses are also excellent at blocking fungus. Tyloses make wood of the white oak group water tight, which is why is it used for shipbuilding and casks for wine and bourbon. The presence of tyloses also means that Oak Wilt is usually not fatal in trees of the white oak group.

Bur Oak does well along city streets, and just as well in a back yard.

Bur Oak does well along city streets, and just as well in a back yard.

   Oak trees should never be pruned or wounded in any way during the growing season (April-November). Even the smallest nick exposing tissue under the bark will attract picnic beetles, which carry oak wilt spores on their body. This is one way Oak Wilt is transmitted from one stand of trees to another. Pruning tools can also transmit the disease. If an oak tree must be pruned during the growing season, you should apply pruning paint to the wound immediately. This is the only time you should ever use pruning paint on any tree.

   Bur Oaks are very long lived, hardy trees. Their adaption to the drought conditions of less-forested areas such as prairies translates well to an urban environment. They occur naturally in open areas, so require little training (pruning) to acquire a desirable shape. Their bark is unique among oak trees, especially the cork-like bark of the youngest twigs resembling the wings found on Euonymous (burning bush). This makes them interesting all year round.

   You can expect to see many young Bur Oak trees in tree lawns around LaPorte in the next few years.

3 Responses to “Tree of the Month: Bur Oak (Quercus macrocarpa)”

  1. Liz Kaminski

    Apr 20. 2009

    Keith,
    Another great tree article. Wish I had more acreage…you make me want to plant, even the Hackberry!
    Thanks for sharing your knowledge.
    Liz

    Reply to this comment
  2. Michael Mikeska

    Mar 05. 2010

    Just what I needed to know.
    Thanks,
    Mike

    Reply to this comment
  3. Tim

    Jun 11. 2014

    A very informative article. I live in Edmonton, Alberta and have planted a bur oak on front lawn about 3 years ago. The tree was purchased from a reputable nursury and when purchased, it was about 15 years old. Tree has adapted well to planting, however, in the past year, has not produced any acorns. I have searched online for answers, but no luck until I came across this article. Thanks

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