Ever wonder what the technical or botanical differences are between trees and shrubs? Is a shrub really just a shorter tree? We’ve got you covered with answers to these questions and more with this passage from The Complete Encyclopedia of Trees and Shrubs, a fantastic resource guide worth having in your landscaping resource library. While the differences are often debated, we’ll help you to dive deeper into a simple topic that is often taken for granted. In this post, you’ll also learn about other plant classifications such as monocotyledons that would botanically not be defined as trees or shrubs. Read on to learn more…
Trees and shrubs are the mainstay of the garden, providing both a framework and background for other plantings. They offer shelter, shade and protection, and attract birds. As there is such a variety of shapes, sizes, and textures and colors to choose from, with adequate research gardeners can usually find the right tree or shrub to suit their particular situation.
What Are Trees And Shrubs?
The distinction between trees and shrubs is often debated, but to begin with we will look at what sets them apart from other plants.
Firstly, trees and shrubs are perennials in the botanical sense, that is plants that live more than two years, in contrast to annuals and biennials. But they are not herbaceous perennials, which lack permanent woody tissues in their stems. In gardening parlance, “perennial” is usually taken to mean herbaceous perennial. So, a second distinctive feature of trees and shrubs is that they are woody, with stems containing woody tissue that persists and increase for years, supporting the twigs and foliage. A third, less obvious, feature of trees and shrubs is that the buds renew their growth and are above ground, so that in cold climates where the ground becomes hard they do have have to put up new aerial growths from a rootstock every spring but can sprout again from the previous year’s branches.
Trees are generally defined as all plants, rising well above human height and with a distinct trunk. When it comes to distinguishing a tree from a shrub, we can try to devise a test based on our own use of the words, though often we find that different people have slightly different ideas. As far as height alone goes, most of us would agree that a plant taller than about 15 ft (4.5 m) is undoubtedly a tree, even though it may have more than one trunk. But what if it is, say, only 10 ft (3 m) or a bit less, but has a single thick trunk? Orange or fig trees may be examples of this. Many of us would intuitively regard these as trees. This passage does not attempt to lay down any strict test.
Of course at the other end of the scale, trees can reach quite gigantic sizes. The California redwood (Sequoia sempervirens) has been measured at 368 ft (112 m) with trunk up to 17 ft (5.2 m) in diameter, while its slightly shorter cousin the giant sequoia (Sequoiadendron giganteum) can have a trunk diameter of almost 30 ft (9 m). The Australian mountain ash (Eucalyptus regnans) reaches around 330 ft (100 m) though trees felled in the nineteenth century were said to be almost 400 ft (122 m). They almost certainly are included in the world’s largest living things.
A shrub is by default any woody plant that meets the above test but is not big enough to a tree. At the lower extreme a shrub may be a plant no more than 1 in (25 mm) high whose branches creep along the ground and take root – as long as its new growths (or some of them at least) arise from last year’s branches and not from below ground. It should be noted, though, that many shrubs do increase in width if not in height by “sucker” growths from ground level or below, at the same time as they renew growth on older branches. This results in the typical shrub form of, for example, an azalea or a berberis.
Larger shrubs with very numerous stems may form broad mounds, and it may not be obvious that the stems in the center are quite tall, perhaps well over 15 ft (4.5 m) – but we would still be inclined to call such specimens shrubs rather than trees. Of course there are many instances of plants that remain shrubs in most situations, or are normally maintained as shrubs in the garden, but when growing undisturbed in a favorable environment become small or even medium-sized trees-although for some this may take decades or even centuries. Camellias and some rhododendrons are good examples. It is worth remembering that sizes given in books for most shrubs are not absolute maximums, and you may have have to look far to find a taller specimen.
Climbing plants, or at least those with long-lived woody stems, are also included in the botanical definition of shrub, but in horticulture climbers are treated as separate category. Climbers that do not develop a woody framework are not included here.
Some of the plants that are included here would be be classed as trees or shrubs by botanists, but in the context of garden plants they certainly function as trees and shrubs. Most of these are monocotyledons that form long-lived aboveground stems; they include the palms, yuccas, dracaenas, cordylines and strelitzias. Some of these are able to lay down new, somewhat woody tissues in their stems year after year. The palms, however, cannot do this, despite the very large size attained by some of them. Instead, palms lay down hard, densely fibrous tissues behind the terminal growing point and later may open up air spaces in the lower trunk so that it increases in diameter.
Cycads are another group that not all would accept as trees or shrubs, but their stems are so long lived and often quite thick that they function as shrubs in cultivation.
There is a final class of plants that is traditionally kept separate in horticulture, although many species are undoubtedly trees and shrubs in the botanical sense. These are the succulents (including the cacti). Although their fleshy branches may have little in the way of woody tissues, they often develop thick woody trunks with age. Some of the larger cacti may reach 40 ft (12 m) or more in height, with massive trunks – likewise the tree aloes of southern Africa. And there is no clear dividing line between succulent and non-succulent shrubs. Some species, such as the crown of thorns (Euphorbia milii), have thin-textured leaves just like many of the non-succulent euphorbias, and its fiercely prickly stems are hardly more succulent than those of, say, the poinsettia (E. pulcherrima), but succulent enthusiasts have always claimed it as one of theirs. Many of these “succulent” shrubs and trees make very useful landscaping subjects.