The productivity of many perennial plants, especially orchard fruit trees, relies on specialized structures called spurs. These structures are the primary mechanism for reproductive growth in spur-bearing species. Understanding their function is important for anyone involved in the cultivation or management of fruit trees. Plant spurs serve as dedicated, perennial fruiting wood, allowing a tree to consistently produce a harvest without expending energy on long, new vegetative branches.
The Anatomy and Growth Habit of Plant Spurs
A plant spur is a highly compressed lateral shoot, distinct from the long, fast-growing vegetative wood on a tree. These structures are characterized by extremely short internodes, the spaces between leaves or buds, giving them a knobby, clustered appearance. They typically develop from axillary buds on wood that is at least two years old, establishing themselves on the permanent structure of the branch.
The growth rate of a spur is slow, often extending less than one inch in length over an entire year. This minimal elongation conserves the tree’s resources, channeling energy toward flower bud development. As a spur ages and repeatedly produces fruit, it develops a rough, thickened appearance that distinguishes it from the smooth bark of younger, non-fruiting wood.
How Spurs Facilitate Flowering and Fruit Production
The spur’s function is to transition from vegetative growth to specialized reproduction. The terminal bud is programmed to differentiate into a flower bud during the late summer and fall of the preceding year. This means the flower that opens in spring was formed and waiting within the bud throughout the winter.
Once the flower blossoms and fruit sets, the spur’s leaves nourish the developing fruit. The cluster of leaves immediately supporting the fruit, often called bourse leaves, are the initial source of carbohydrates for the young fruitlet. After a heavy crop, the spur system may enter a resting phase, producing a leaf bud instead of a flower bud the following year, which prevents the tree from exhausting its reserves.
Pruning and Protecting Spurs for Maximum Yield
The long-term productivity of a tree is tied to the careful management and preservation of its spurs during pruning. When pruning, it is important to distinguish the compressed, reproductive spurs from vigorous, non-fruiting shoots, often called water sprouts. The goal is to remove the latter while protecting the former to maintain the tree’s fruit-bearing potential.
Aggressive removal of fruiting spurs eliminates potential harvests for several years, as new spurs take time to mature. Pruning should focus on thinning cuts that remove entire branches to open the tree canopy, allowing sunlight to penetrate the interior wood. Adequate light is necessary for spurs to develop the flower buds that lead to a successful fruit set. Thinning out overly crowded spurs can also be beneficial, ensuring that the remaining structures have sufficient resources and air circulation.
Common Examples and Lifespan of Fruit Spurs
Many temperate fruit trees rely almost entirely on spurs for their annual crop, including most varieties of apples and pears. Stone fruits such as sweet cherries and European plums also bear the majority of their fruit on these compact structures. The longevity of a spur varies significantly by species and the overall health of the tree.
A spur on a sweet cherry tree may remain productive for ten to twelve years, while apple and pear spurs are typically vigorous for eight to ten years before productivity declines. Eventually, older spurs become congested and less fruitful, requiring the cultivation of younger replacement spurs to maintain consistent yields.