Planting shrubs with the correct spacing is the most important decision for their long-term health, appearance, and landscape design. Ignoring the mature size of a shrub is a common mistake that leads to overcrowding, poor air circulation, and the eventual need for costly removal or heavy pruning years later. Understanding the fundamental rules of horticultural spacing ensures your shrubs develop their full, natural form and thrive.
The Core Rule: Determining Spacing Based on Mature Growth
The fundamental calculation for shrub spacing centers on the plant’s mature width, not its height. A shrub’s label or a reliable plant database will provide an estimate of its ultimate spread, which is the figure you should use for spacing. The industry standard for determining this distance is the “center-to-center” measurement.
The basic rule for planting individual shrubs is to place them apart by the distance of their full mature width. For instance, a shrub with a 6-foot mature spread should be planted 6 feet away from the next shrub, measured from the center of one trunk to the center of the next. This spacing ensures that when the plants reach full size, their outermost foliage will just barely touch, creating a continuous, natural look without severe crowding.
When planting two different species, the spacing is based on the combined radii of their mature widths. For example, if one shrub has a 4-foot spread and the adjacent one has a 6-foot spread, the distance from trunk to trunk should be 5 feet (half of 4 feet plus half of 6 feet). This calculation guarantees both plants have the necessary room to expand without encroaching on each other’s territory, preventing the need for constant corrective pruning.
Adjusting Spacing for Specific Landscape Goals
The core rule of spacing by mature width is often modified depending on the specific visual effect you wish to achieve in the landscape. Planting shrubs for individual display, known as specimen planting, requires the full mature width spacing to allow the plant to achieve its perfect, unconstrained form. This spacing prevents the foliage from touching, ensuring that each shrub remains a distinct, sculptural element in the garden.
For creating a dense, continuous formal hedge or privacy screen, closer spacing is deliberately used to promote quick interlocking and dense coverage. Plants are often spaced at one-half to two-thirds of their mature width. A shrub with a 6-foot mature spread might be planted 3 to 4 feet apart, forcing the branches to grow into one another to form a solid wall of foliage. This aggressive spacing requires a commitment to regular, formal pruning to maintain the hedge’s shape and health.
Informal grouping or mass planting, which aims for a continuous, naturalistic flow, uses a distance slightly less than the full mature width. Planting shrubs in staggered rows or triangular patterns, rather than a single straight line, helps create a continuous look without the severe crowding of a formal hedge. This allows the plants to slightly overlap, merging into a unified mass. The goal is a lush, filled-in appearance that avoids the gaps of specimen planting and the rigidity of formal hedging.
Special Considerations for Shrub Growth Habits
Beyond the mature size, a shrub’s biological traits and growth habit can necessitate further spacing adjustments. Air circulation is a major factor, as stagnant air traps moisture on foliage, creating an ideal environment for fungal diseases such as powdery mildew or rust. Shrubs that are naturally dense or those highly susceptible to disease, like boxwoods and roses, benefit from being planted slightly wider than the standard mature width, even when used in a hedge.
Allowing extra space encourages faster drying of the leaves after rain or irrigation, which minimizes the opportunity for fungal spores to germinate. The plant’s natural growth rate also influences initial spacing decisions. While fast-growing shrubs can be planted a little closer, requiring heavy, routine pruning later, slow-growing shrubs should be spaced for their mature size immediately.
Root system competition is another consideration, particularly near structures or other plants. Shrubs with aggressive or shallow root systems, such as certain types of bamboo or sumac, need more distance from foundations and other garden elements to avoid competing excessively for water and nutrients in the upper soil layers.