The lilac bush (Syringa) is a classic garden staple valued for its dense foliage and sweet-smelling spring blooms. Proper spacing is essential for the long-term health and flowering performance of these shrubs. Planting lilacs too close restricts air movement, encouraging fungal diseases like powdery mildew. Spacing them too far apart diminishes their visual impact. The correct distance depends on the specific variety chosen and the desired landscaping outcome.
Determining Mature Size and Variety
The initial step in determining spacing is identifying the mature spread of the lilac variety, as this dimension varies widely. Lilacs generally fall into three size categories.
Common lilacs (Syringa vulgaris and French Hybrids) are the largest, typically reaching 8 to 12 feet wide, sometimes up to 15 feet. These require generous spacing to fully develop their traditional, multi-stemmed form.
Dwarf lilacs (e.g., Syringa meyeri ‘Palibin’) are compact, maturing to a spread of only 4 to 6 feet wide. Their smaller size suits tighter spaces, hedges, or foundation plantings.
Hybrid lilacs (Syringa x prestoniae varieties) often fall in a mid-range, growing about 6 to 10 feet wide. Always consult the plant tag for the specific mature width of your chosen cultivar before planting.
Calculating Spacing Based on Goal
The distance between plants relates directly to your landscaping goal, whether you want a single specimen or a continuous, dense screen.
For a specimen planting, the goal is a fully developed, free-standing shrub, so spacing should equal the lilac’s full mature width. For example, a common lilac with a 10-foot spread should be planted 10 feet away from any other large plant or structure. This generous spacing ensures the shrub receives full sun, promotes air circulation, and allows its natural shape to form unobstructed.
When planting a hedge or screen, the goal is for mature shrubs to interlock, creating a solid wall of foliage. For this purpose, reduce the spacing to approximately two-thirds to three-quarters of the mature width. A dwarf lilac with a 6-foot spread should be planted 4 to 4.5 feet apart, center-to-center, to achieve density. This closer planting requires future pruning to maintain proper airflow but quickly establishes the visual barrier.
Site Preparation and Planting Technique
Successful lilac growth requires proper site preparation, including full sun exposure for at least six hours daily to ensure abundant flowering. Lilacs are intolerant of consistently wet conditions, so the soil must be well-draining to prevent root rot.
To test drainage, dig a hole 8 inches wide and 12 inches deep and fill it with water. If the water has not drained within an hour, the location is unsuitable or requires amendment.
When planting, dig a hole two to three times wider than the root ball but only as deep as the root ball itself. Plant lilacs so the top of the root ball is level with or slightly above the surrounding grade. This prevents the crown from sitting in a depression that collects water. Backfill the hole with native soil, tamping gently to eliminate air pockets, and water thoroughly to settle the soil.
Maintaining Proper Spacing Through Pruning
Periodic pruning is necessary to maintain health and the intended landscape design. Lilacs bloom on old wood, meaning the flower buds for the next year are set shortly after the current season’s bloom finishes. For this reason, the correct time to prune is immediately after the flowers fade in late spring or early summer. Pruning at any other time risks removing the next year’s flower buds.
Thinning out the shrub is the primary pruning technique used to manage density and improve air circulation, especially for closely spaced hedge plantings. This involves removing approximately one-third of the oldest, thickest canes right down to the ground level each year. Selective thinning prevents the interior of the shrub from becoming overly crowded. This helps deter fungal diseases like powdery mildew and ensures sunlight can penetrate to encourage new growth from the base.