When deciding on a planting scheme, take into account the shape and height of a plant, both in relation to the scale and size of the area and to other plants. For example, a small garden would feel and look overcrowded if it contained a large tree such as a horse chestnut (Aesculus); a smaller-growing species, such as rowan (Sorbus aucuparia), would be more in proportion.

Successful plant associations often depend on harmony of scale: a scheme that combines many different plants, all of greatly different heights, may simply look uncoordinated and unplanned. Having said that, you should always try to introduce a fair amount of variation so that the scheme does not look regimented. Think of your garden not as a flat landscape but as three dimensional; whilst planting in overlapping layers takes more planning and effort, it is far more effective.

Even on a small scale, such as in a rock garden, cushion- and carpet-forming rock plants may be combined with conifers and dwarf shrubs to create interest and variety through height and scale relationships, whilst still appearing in well-balanced proportion with each other. If grouped with larger species, these plants often appear insignificant and are easily overlooked.

Perennial and shrub borders are often arranged by height, with the tallest plants at the back and the lowest in the front, so that they may all be clearly seen. However, this style of planting leads the eye straight down, and much planting interest may be missed on the way. Instead, break the regularity in one or two places by introducing an occasional tall plant midway or at the front, especially one that allows a view beyond them, such as the airy 'tall verbena' (Verbena bonariensis).