Borders are a necessity for a formal garden and are usually made of dense evergreens. Borders are placed in a geometric configuration and run around and through sections of the garden. Boxwood is a likely candidate for borders because it is evergreen and grows into a dense hedge that can be clipped to any size. Boxwood can be clipped short into a knot configuration for a formal knot garden or it can encircle a series of flower beds planted with herbs and flowers.
Some formal gardens include trees that are thin and pointed called columnar trees. Many types of cypress trees grow in columnar fashion and can be clipped back to a few feet or higher and will create a lovely screen or backdrop for the formal garden.
Topiaries are shrubs and other plants trained into different shapes. Evergreens are particularly suited to be made into a topiary. Shapes can be round, cone shaped, geometric shapes or animal shapes.
Roses are a favorite plant in a formal garden and can be used as individual plants or planted so they ramble over an arbor. Hybrid tea roses are great for a formal garden because they are tidy and neat and produce flowers in a rainbow of colors. Old English roses are also popular because of the large flowers and wonderful scent. Even miniature roses can grace a formal garden.