The shape of trees

When leaves are on trees, we often look at the trees without actually observing the individual character each provides. After the leaves have fallen, however, trees take on a new look. The structure of trees becomes more evident as we are able to view the size of their branches and twigs. The density or openness of branching that we were not able to see in summer suddenly appears. And in winter, the shape of trees becomes more obvious.

Shape is usually the most dominant feature of a tree any time of year. But as you observe winter landscapes or drive along highways, the tree's shape alone is a major clue to its identification.

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The growth habit of trees, such as spruce (Picea sp.) and many other evergreens, is usually quite precise. Most deciduous trees have less dominant shapes, but each is distinctive in its own way. The basic growth habits of trees are columnar, weeping, pyramidal, horizontal branching, vase shaped, round headed or globe shaped.

In selecting trees for your own landscape, consider both summer and winter impact. Try to envision the forms that best fulfill landscape needs. The forms themselves can provide different feelings.

* Pyramidal. Pyramidal trees give a feeling of formality or organization.

* Columnar. Columnar trees provide a stately feeling. These are more often used in groups or in lines as a series of tall sentries or sometimes as single accents.

* Weeping. Weeping trees add grace and movement to most landscapes and are normally used very sparingly as specimen trees.

* Horizontal branching. Large, broad horizontal-branching trees are strong and durable and provide many uses without becoming dominant.

* Round headed. Round-headed trees provide a similar feel but with more formality.

* Globe shaped. Globe-shaped trees also give a formal feel. Globe-shaped usually applies to smaller trees.

* Vase shaped. Vase-shaped refers to trees that are narrow at the base and broaden at the top. The American elm (Ulmus americana) is the prime example of a vase-shaped tree. It often lined streets to form a natural gothic arch. Unfortunately, Dutch elm disease has made this a relatively rare site.

Many trees fit into each category. With some experience at observing trees, you can identify some trees by shape and branch structure during winter. Among more common pyramidal trees-other than evergreens-is the pin oak (Quercus palustris). Sweet gum (Liquidambar sp.) generally also have a pyramidal shape as does the smaller-growing Bradford pear (Pyrus calleryana 'Bradford'). Pyramidal trees, as they mature, often begin to broaden at the top, but they hold their pyramidal shape for many years during their development.

A popular small tree with horizontal branching is the flowering dogwood (Cornus florida). Horizontal branching is also a characteristic of the black gum, which is known for its striking fall color. Many pines (Pinus sp.) as well as the deciduous trees, such as Kentucky coffeetrees (Gymnocladus dioicus), feature this structure.

Round-headed varieties include many maples (Acer sp.) and flowering crabapples (Malus sp.). Green ash (Fraxinus pennsylvanica) develops into a basically rounded form. One of the best known globe-shaped trees is the globe locust (Robinia pseudoacacia 'Rehderi').

Weeping trees are usually selections of common trees that also have a normal growth form. This is also true of most columnar trees. Each has a special purpose. The species you select depends on need and availability. The weeping willow (Salix sp.) is the best-known weeping tree, and the Lombardy poplar (Populus nigras 'Italica') is a familiar columnar tree. Each has problems, however, and better trees with these forms are available.

When selecting tree forms, keep winter in mind and envision the impact of graceful or stately shapes in your landscape.

Technical credit: University of Missouri, University Extension and Agricultural Information, Columbia, Mo.

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