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Trees of Iowa: An Interactive Key

Iowa's Oaks

In 1961, the Iowa General Assembly designated the "oak" as Iowa's official state tree. Certainly, prior to that designation and since, discussion has centered around whether a single species of oak should have been Iowa's state tree. Most woodlands and all communities have one or more species of oaks as a component. It can be argued that, no other group of trees is more important to both rural and urban forests in Iowa. Twelve different species of oaks are native to Iowa, although only a single species (bur oak) is found throughout the state.  Iowa oaks are separated into either of two groups-red oaks or white oaks. 

The white oaks (white, bur, chinkapin, swamp white, post and dwarf chinkapin) have lobed leaves with rounded lobes, acorns which mature in a single growing season and germinate in the fall sending down a root system, and have plugs (tyloses) in the water conducting tissue of the wood or vessels, making oak containers such as whiskey barrels waterproof. 

The red oaks (red, pin, black, Northern pin, blackjack, and shingle) have mostly lobed leaves with bristle tips at the ends of the lobes, acorns requiring two growing seasons to mature and do not germinate until the following spring, and vessels without plugs.

Pin oak (native to the SE 1/4 of Iowa) is probably used more as a shade tree than any other oak. It is a bottomland species, tolerates wet and poorly drained soils which are acidic. Pin oak is fast growing, easy to transplant, should be used only on soils which are acidic; on basic (high pH) soils, they often exhibit iron deficiency chlorosis in which the leaves turn yellow and the veins remain green. 

Red oak (native except in the far NW counties) is slower growing, but may be a better choice on non-acidic soils. It is fairly easy to transplant, grows faster than most oaks, and is adapted to a wide range of sites. Red oak is the most valuable of the red oaks for lumber production. 

Shingle oak (S 1/3 of Iowa) has a leaf without lobes, but a bristle tip on the tip, prefers acidic soils and will tolerate tough, dry sites. Shingle oak is relatively easy to transplant and has become more common in the urban landscape.  Northern pin oak (N 1/2 of Iowa) looks like pin oak, but has more oval acorns, and may be more difficult to transplant. 

Black oak (all but far NW corner) will grow on a variety of sites from very dry upland ridges to deep rich cove sites to dry sandy bottomlands. Black oaks vary greatly in their appearance because of genetic diversity and because they hybridize with other species of red oaks. 

Blackjack oak (far SE Iowa) has leaves with three distinct lobes, is a small tree rarely exceeding a foot in diameter, and tolerates dry upland soils.

White oak (E 2/3 of Iowa) is the most valuable "white" oak for lumber production. It will grow on a variety of sites, from moist cove sites, with deep soil to the drier ridges and southern exposures. Iowa white oaks live to 400 years and the largest white oak in Iowa is more than 57 inches in diameter.

Bur oak (all) is the oak of the midwest, very slow growing, long-lived, and adapted to a wide range of sites and soils, from very dry exposures to good soils which are fertile and moist.

Swamp white oak (along streams in eastern, central, and south central Iowa) tolerates the moist low-lying sites along streams, has leaves with more shallow lobes, bark that exfoliates like a birch on small limbs, and. Swamp white oak is used frequently as a shade tree, but should not be planted on non-acidic soils because it also may suffer from iron chlorosis. 

Chinkapin (SE Iowa) is a oak, that does not look like a oak; its leaves are not lobed, but have very coarse teeth without bristle tips. Chinkapin means chestnut, and its leaves resemble the leaves of a chestnut. Chinkapin makes an excellent shade tree and it is very tolerant of dry, high pH soils; it grows naturally on ridges, hill tops and rocky southern exposures.

Dwarf chinkapin oak (E Iowa) has smaller leaves than chinkapin oak and seldom reaches small tree size; it is considered a shrub. It is native to upland sites, often growing on the same sites as chinkapin oak.

Post oak (Lee, Henry, Van Buren and Appanoose counties) is the least common of the oaks. It is a dry species, often growing on ridges or hot dry exposures. Its leaf shape resembles a cross, with two smaller lobes at the base.

Many species of oaks may be incorporated into our urban and landscape planting. Pay attention to the natural habitat of the oaks when selecting them for landscape use; matching the tree to the site will result in greater success in the landscape. Remember that many of the oak species become very large trees and they need room to develop and grow in the landscape. Avoid large plantings of single species to avoid potential problems with oak wilt. Many of the oaks are difficult to move because of their non-fibrous root habit. Moving smaller stock may improve success because less of the root system is lost.