The lilac (Syringa species) is an ornamental shrub or small tree cherished for its dense clusters of fragrant spring blossoms. Unlike trees with deep taproots, the lilac’s root structure possesses specific characteristics that necessitate informed planting and maintenance practices. Understanding this root system is crucial for successfully incorporating the shrub into the residential garden.
Anatomy of the Lilac Root System
The lilac generally develops a shallow, fibrous root system that is dense and spreads widely near the soil surface. This root mass typically remains in the top 18 to 24 inches of soil, focusing on the nutrient-rich topsoil layers. The lateral spread of the roots can extend about one and a half times the width of the shrub’s canopy.
A defining trait of many common lilac varieties is their strong tendency toward suckering, which is the production of adventitious shoots from the root system. These suckers are clones of the parent plant that emerge from the ground, often several feet away from the main trunk. This habit is a natural mechanism for renewal and colonization, leading to the characteristic thicket-like growth pattern of older lilacs. If the main plant is a grafted cultivar, suckers arising below the graft point will grow from the rootstock, potentially overtaking the desired variety.
Risks to Structures and Landscaping
The shallow and expansive nature of the lilac root system can present issues when planted too close to built structures. While roots are unlikely to break through a modern, sound foundation, they can cause problems with older, shallow foundations. Damage typically occurs indirectly, particularly in heavy clay soils that swell when wet and shrink dramatically during dry periods.
During periods of drought, the dense feeder roots near the foundation can draw significant moisture from the soil. This causes the clay to shrink, potentially leading to foundation movement and cracking.
A concern involves underground utility lines; roots are unlikely to damage intact pipes. However, lilac roots aggressively seek out water and nutrients, and may penetrate and clog pre-existing cracks or leaking joints in older sewer or water lines. In the garden, the wide-ranging, fibrous roots create intense competition for water and nutrients, making it difficult to maintain a healthy lawn or garden beds directly around the mature shrub.
Safe Planting Distances and Root Barriers
To mitigate potential issues, determine safe planting distances based on the mature size of the lilac variety. A general guideline suggests planting the center of a large lilac at least 10 to 12 feet away from the house foundation. This distance helps ensure the main root mass remains far enough away to reduce the risk of soil moisture depletion near the footing. For underground water and sewer lines, a distance of 8 to 10 feet is recommended to minimize the risk of root intrusion.
If a lilac must be planted closer to a sidewalk, patio, or utility line, a root barrier can be an effective preventative measure. These barriers consist of non-porous, interlocking plastic panels installed vertically in a trench to physically redirect root growth downward. For shallow-rooted shrubs like lilacs, a barrier depth of 12 to 24 inches is sufficient to intercept the lateral spread of feeder roots. When installing, the barrier should be placed with the top edge just below the soil surface and angled slightly away from the structure to encourage roots to grow in the opposite direction.
Controlling Aggressive Root Suckers
Managing root suckers is a regular maintenance requirement to maintain a clean, tree-like form or prevent unwanted spread. Suckers often appear in greater numbers following stress, injury to the root system, or heavy pruning. Consistent removal is the most effective control method to prevent the plant from forming a dense thicket.
The best technique is to remove the suckers by tracing them back to their point of origin on the root and severing them completely below the soil line. Simply cutting the shoots at ground level is less effective, as this often stimulates the root to produce an even greater number of new shoots.
For persistent problems, an approved growth inhibitor containing naphthalene acetate (NAA) can be selectively applied to the cut surface of the removed sucker to suppress re-sprouting. Applying non-selective herbicides to suckers should be avoided, as the vascular connection to the parent plant can allow the chemical to travel back and injure or kill the entire lilac shrub.