Tree identification by thorns
Thorns have evolved on trees and shrubs as a defence against herbivore grazing, particularly during the early years of growth and establishment. In upland areas of Britain, heavily grazed by sheep, Hawthorn is often the only tree to survive. Today thorny shrubs are used as hedges in the countryside to contain stock or in gardens to deter intruders but not many tree species in Britain are thorny. Botanically there are thorns and spines. They differ in the way they are formed. Thorns develop from the stem as short branches and spines develop from leaf parts. The first three photos show the thorns of the Hawthorn and the Blackthorn and the last photo shows the spine of a Berberis shrub. In practice its difficult to tell thorns from spines. Apart from Hawthorn and Blackthorn other trees that have thorns are the Cherry Plum, Purging Buckthorn, Broad-leaved Cockspur Thorn and Honey Locust. Some False Acacias have spines but often a spineless variety is chosen for city streets.
Tree identification by thorns – if you can see thorns on the tree, examine them carefully. It is a quick way of identifying one of the seven thorny trees shown here. Thorns are usually easy to see in the winter when the leaves have fallen but can be used all the year round.
The thorn of the Common Hawthorn develops in the axil between the stem and the leaf and it is always above the leaf. It starts to grow as a normal branch then quickly turns into a hard pointed thorn. Once it has switched to become a thorn it cannot develop leaves or flowers. The short thorns of the Hawthorn located along the stems are of this type. However, some branches develop leaves and flowers before they become a thorn so the thorn is on the end of a branch.