Of Thorns, Spines and Prickles

Published: January 3, 2013

One of the interesting aspects of enjoying plants is how our perception (and appreciation) of them changes with the seasons. Most avid gardeners probably would list spring, summer or fall as the season(s) plants are most attractive to them, and for obvious reasons. There are, however, virtues of some deciduous (often woody) plants that are most obvious during winter, after leaves have been shed. It is at this time special plant features that were hidden during the growing season are clearly visible, adding interest to the landscape.

One example of the above is the whitish-colored bark on the upper branches of the American sycamore. It is much more evident after its leaves have fallen than during summer. The same also could be said of trees and shrubs that bear colorful fruit, such as flowering crab apple, winterberry holly and cranberry-bush viburnam. Although less visually dominant than the previous examples, thorns, spines and prickles are unique appendages some plants produce that make them more interesting (if not attractive) during the winter.

Most people consider any sharp projection from a plant to be a thorn. This is understandable, since most people are familiar with roses and the sharp (sometimes painful) “thorns” they bear. However, roses don’t bear true thorns; instead they produce prickles. The proper (botanical) classification of these sharp projections differs depending on their origin in the plant that bears them.

Accurately used, the term “thorn” is applied only to a sharp-pointed structure that is a modified branch. Thorns often arise from the main stem at leaf axils. Landscape plants with true thorns include firethorn (Pyracantha), hawthorn and Japanese flowering quince.

A sharp projection developed from a leaf, stipule or leaf part (rather than from a branch) is called a “spine”. Honey locust probably is the most notorious woody plant that bears spines, which often are compound in their occurrence. The spines of this species are so threatening that a spineless botanical variety is used for landscaping purposes. Other familiar landscape plants bearing spines include barberry and black locust.

In cacti, the entire leaf of the plant is transformed into a spine. In addition to reducing water loss by restricting leaf surface area, this unique adaptation protects the succulent stem of the plant from animals that would use it for food or a source of water.

Other plants bear spines only around the margins of their leaves. American holly, English holly and Oregon grape-holly are good examples of the latter.

The third type of sharp projection found on plants is called a “prickle”. Prickles arise from stem tissue and are extensions of its cortex and epidermis. Perhaps because it nearly rhymes with the word “tickle”, a prickle sounds must less threatening than a thorn or a spine. Such is not always the case. Undoubtedly, the most popular garden plant that bears prickles is rose and most avid gardeners have had more than one painful, unfortunate encounter with its sharp appendages. Other examples of plants bearing prickles include prickly ash (Zanthoxylum americanum) and Aralia spinosa, also is known as devil’s walking stick.

In nature, the purpose of thorns, spines and prickles is to protect plants from would be predators. Any herbivore with average intelligence will not likely try to nibble at a species such as barberry more than once. Unfortunately, growing plants with thorns, spines and prickles in the landscape can pose a certain amount of danger, depending upon the species.

When used as background plants, border plants or when tree branches are trimmed well above head height, most plant bearing thorns, spines or prickles are harmless. Additionally, they add a unique character to the winter landscape can serve as an impenetrable barrier to make our home and its contents more secure. However, when placed in the landscape where people often walk or drive by they can present a safety problem, especially to children and pets. Proper care in selecting, placing and maintaining them will help to avoid unfortunate encounters and add aesthetic value to the landscape.