How Long Can Thorns Grow to Be?

A Thorn By Any Other Name

• Thorns are defined as stiff projections that end in a point and protrude from any part of a plant. They can be straight or curved, but they can't be soft -- even if they are sharp on the tip. Thorns are sometimes called spines, but the generally accepted botanical definition of a thorn is that it is a "very short, hard, leafless branch or stem with a sharp point," according to "Webster's New World College Dictionary." The thorns of a rose are actually prickles, which arise from the inner wood of stems.

A Thorny Situation

• Thorns -- along with foul smells and bitter flavors -- are a means of defense for many plants, although it's up for debate whether the thorns actually deter hungry predators. For example, giraffes of the African Savanna use their long, dexterous tongues to lick away the leaves that grow on thorny trees. What thorny plants might deter, however, are small animals like rabbits or even larger animals such as human trespassers. If this is the case, the home gardener will likely want to choose plants that have the longest, sharpest thorns possible.

A Matter of Size

• The honeylocust (Gleditsia triacanthos) has black bark and interestingly zigzagged twigs, but by far its most distinctive feature is its long thorns. The thorns, which can grow up to 20 inches long according to Floridata, protrude from the branches and trunk. The primary reason this tree's thorns are so long is because they are branched, which means they sort of stack up on each other. Most other thorny plants have single or double-pronged thorns. The honeylocust is hardy in U.S. Department of Agriculture plant hardiness zones 4 through 9 and grow rapidly in full sun and in moist soil.

A Prickly Plant

• Among plants that have single or double thorns, the acacia (Acacia spp.) trees stand out as having the longest. The umbrella thorn acacia (Acacia tortilis) in particular has the longest, which grow straight and hooked up to 3 inches in length. The branches of the tree are used to create cages and barriers in Africa, according to Purdue University. Umbrella thorn trees grow in dry, hot, arid regions of the Middle East and Africa. Many of acacias, however, grow in the United States. The whitethorn acacia (Acacia constricta) is named for its inch-long, white thorns. The shrub displays bright yellow flowers and thrives in dry, arid conditions in USDA zones 7 through 11. Many acacias have significant thorns and many of the approximate 800 species grow in the United States. Another tree with single, 3-inch-long thorns is the cockspur thorn (Crataegus crus-galli). This tree is hardy in USDA zones 3 through 7 and thrives in full sun and in average, well-draining soils.