Espalier is the art of training shrubs and trees, usually fruit trees, to grow flat against a wall, on a trellis, or between support wires.
Training trees on a sturdy trellis can provide a fruiting wall in a narrow space, either free standing with space on both sides (preferable) or against a fence or house. Apples, pears, and Asian pears are especially well adapted to espalier training, but other species may work with extra effort. There are many ways to train the branches, including fan shapes, but typically three to four horizontal wires are spaced about 1½ - 2 feet apart vertically, and the lateral shoots are tied (trained) along the wire running in either direction. For best results, shoots can be initially trained upward at about a 45° angle on a bamboo stake to keep them growing vigorously. Once they reach the desired length, shoots can be lowered to the wire. Vigorous shoots should be cut back during the growing season to encourage spur growth. Lateral growth from the branches should be kept short to prevent shading of lower branches. If shoot growth is excessive, provide more space by extending the trellis outward or upward.
By selective pruning, the branches of citrus trees can be espaliered, that is, trained to grow flat against a wall or a framework, allowing you to grow fruit in a confined space such as a narrow bed or side yard. According to the Sunset book Citrus (1996), some of the best choices for espalier are Eureka lemon, Nagami kumquat, Eustis limequat, Tarocco blood orange, and Chandler pummelo.
Start with a young tree because it will be easier to train. You can design an informal espalier in which you plant the tree directly in front of a structure, allow it to branch naturally, and prune (cut away) any branches that stick out too far. Or you can design a formal espalier in which you train the tree into a precise geometric pattern (see diagram).
For a beginner, the informal design is easiest. In a climate at the edge of ideal for growing citrus, citrus espalier may have a distinct advantage. Training a citrus tree against a sunny, south-facing wall may supply enough heat that fruit ripening and winter tree survival will be more likely.