What is the horticultural term for a plant trained to grow in a formal two-dimensional form?

The term "espalier" refers to a method of growing plants, particularly trees or shrubs, in a flat, two-dimensional format against a support structure, such as a wall or fence. This technique is commonly used to create aesthetic designs in gardens and allows for efficient use of space. The practice can enhance light exposure and air circulation for the plant while making it easier to manage and harvest fruits if applicable. Espalier requires an understanding of pruning and training plants to achieve the desired shape and is often employed in both ornamental and productive horticulture.

In contrast to this, fumigation relates to a method of pest control and soil sterilization, not a form of plant training. Grafting involves joining two plant tissues together to grow as one, which affects growth forms but does not specifically pertain to a two-dimensional shape. The term "genus" refers to a classification in the biological taxonomy of plants but does not address the structural training of individual plants. Thus, the only choice that accurately describes a formal two-dimensional training method is espalier.