There is an old saying that states “every majestic oak tree was once a nut who stood his ground." For centuries oaks have stood their ground in forests throughout the United States. Indiana is home to almost two dozen different species of these trees that contribute to the beauty and biodiversity of our natural lands.

The Mighty Oak

There are so many different kinds of oaks that it is hard to classify them by appearance alone. Some are grand and found in large stands while others are surprisingly stunted and have to fight with other tree species for room to grow. Regardless of these differences, there are characteristics that make then unique to oaks alone.

What most oaks have in common is how their leaves are found, large and alternating on the stem. The shape of the leaf is also very similar though some are smooth while others have serrated leaves. They all produce catkins – a spike of unisexual flowers with no petals – in the spring, and all bear fruit called acorns. Oaks are the only tree specie that produces these nut-like fruits.

Reproduction and Acorns

The Oak is unisexual, meaning it has both male and female flowers. The male flowers, which produce pollen, grow on long stiff bunches of catkins. The female flowers, which produce eggs, are the spiky part of the flower. Pollen is normally transferred to the female through wind dispersion. The seeds, known as acorns, grow out of the female flower, in a woody brown cap. This brown cap is not part of the seed, although it is usually associated with the acorn.

Oak trees can start producing acorns at 20 years old, but it is common for the tree to be 40 – 50 years old before a successful seed crop occurs. Acorn production varies, sometimes generating very little in any given year. On mast years – when, in every three to five years, large numbers of acorns are produced – a mature oak tree can turn out tens of thousands of acorns.  

Acorns fall before the leaves do, germinating quickly so the leaf litter can provide protection from frost and seed-eating animals. Despite the precautions, very few of the acorns produced by a single oak will successfully grow into a mature tree. In fact, it is thought that only one in 10,000 acorns will pass the seedling stage. It is not that oaks are slow to grow, but the fact that other species of plants can crowd the area and hinder the acorn, or seedling, from maturing to an adult tree.

The acorns that do not mature are not all lost to rot or pest infestations. Acorns are an important wildlife food for numerous birds, mammals and insects. Deer, squirrels and various birds get their much-needed nutrients and energy from the nut-like fruit during the winter months. Forest creatures also depend on the cavities of oaks for nesting and protection from the winter elements.

What's Wrong with that Tree?
Oak Diseases and Pests

Just how mighty is the oak? Let’s just say that oak trees have gone through several possible specie-killing threats, and have been able to survive thus far. This doesn’t mean we shouldn’t be concerned about the diseases and pests that affect the oak species. It means we should consider protecting them even more.

The following is a short list of diseases and pests that plague the oak tree…

Oak Decline: Environmental stresses – like waterlogging or frost– weaken trees, making them more susceptible to disease and pests that would not have much of an effect if healthy. Oak decline is slow, occurring for several years before the tree succumbs to the eventual root killing and other effects of disease and pests. (Read more about oak decline from the USDA’s Forest Service.

Oak Tatters: This disease affects the foliage, creating a tattered appearance due to the loss of tissue between the leaf veins. Commonly found in white oaks, but also occurs in other oaks.

Oak Wilt: a fungal disease that can quickly kill oaks. The oak dies by it’s own defenses; reacting to the presence of fungus, the tree with plug its own cambial tissue – which is crucial for nutrients and water delivery to the rest of the tree – which eventually will kill the tree. 

Sudden Oak Death: caused by a “water mold,” or Phytophthora ramorum, it is a complicated disease as it causes varying symptoms on the various oak species. This disease is capable of easily killing a live oak.

Gypsy moth: The gypsy moth, or Lymantria dispar, is a notorious pest of hardwoods. Since 1980, this moth has defoliated almost a million acres of forests each year. Although it will not always kill the tree, or kill it immediately, the gypsy moth is a pest you do not want to have to deal with. 

Oak leaftier: a bud miner and defoliator that may kill oaks (red oak seems to be more susceptible than others).

Oak leafroller: like the oak leaftier, the leafroller will also severely defoliate a tree, causing its death. These pests are very similar therefore hard to distinguish. Check out USDA’s Forest Service’s pamphlet on how to differentiate these pests. 

For more information as well as help for diagnosing and treating oak disease and pests, check out Purdue University’s Plant & Pest Diagnostic Lab’s information on Oak Troubles, the University of California’s cooperative extension’s Landscape Notes or Clemson’s Extension information on Oaks. 

Facts, Folklores, Uses and Cultural Significances

• The average oak tree can live to be more than 200 years old. Some are as old as 1,000 years.  
• Many ancient oaks are found with rotten trunks. It is believed that this leads to a hollow structure that is actually stronger than a solid trunk would be. 
• A mature oak tree can draw up to 50 gallons of water through their root systems a day.
• The Oak is struck by lightening more often than any other tree.
• Fire, land development and the overharvesting of oaks for production destroy oak populations faster than any disease or pest.
• The wood of oaks is used to make barrels for wine, bourbon and other liquors.
• Furniture and cabinets made from oak have always been very popular.
• Cork oak is used to make wine stoppers.
• White oak bark is dried and used in medical preparations.
• Tanner use oak bark for tanning leather as it is rich in tannins.
• Oak leaves and acorns are highly toxic to horses due to tannins.
• Acorns can be roasted to make acorn coffee. Yum!
• The oak is a common symbol of strength and survival.
• The oak is the national tree of England, France, Germany and the United States.
• It is thought that sleeping under an oak tree will bring good luck.
• In Celtic mythology, oak doors are believed to be the gateway between worlds.
• In Greek mythology, the oak was a symbol of Zeus – the god of thunder and king of all gods.

For More Information

Arbor Day Tree Guide - for information on various species of oaks and other trees.

The Nature Conservancy & Indiana's Forests

Purdue University's Cooperative Extension guide to Planting and Care of Fine Harwood Seedlings

Nature picture credits (top to bottom, left to right): Photo © TNC (oak tree); Photo © Wabash Valley Audubon Society (oak trunk); Photo © TNC (acorns).