The butterfly bush, Buddleja davidii, is a fast-growing ornamental shrub valued for its fragrant flower spikes that attract numerous pollinators. Understanding the nature of its root system is important for managing this shrub, as its vigorous subterranean structure contributes to its reputation for being aggressive and difficult to remove.
Understanding the Root Structure
The butterfly bush possesses a dense, fibrous root system rather than a single, deep taproot. This structure consists of a mass of slender, branching roots that grow outward from the central root crown. The fibrous nature allows the plant to efficiently colonize the surrounding soil and quickly absorb moisture and nutrients from the upper soil layers.
These roots are woody and robust, providing strong anchorage for the fast-growing stems above ground. The root mass sustains the shrub’s rapid annual growth, particularly in regions where the plant dies back to the ground each winter. Because the roots are concentrated near the surface, the butterfly bush requires well-drained soil and is susceptible to root rot if left in standing water.
Maximum Root Depth and Spread
The root system of Buddleja davidii is relatively shallow, with the dense root mass concentrated in the top 18 to 24 inches of soil. This depth profile reflects the plant’s reliance on surface water and oxygen. Gardeners are often advised to check the soil moisture only four or five inches down to determine if the plant requires watering.
The lateral spread of the roots is substantial, often matching or exceeding the width of the shrub’s canopy. Depending on the variety and maturity, a standard butterfly bush may spread 5 to 10 feet across, meaning its root system extends a similar distance underground. This wide, shallow network allows the shrub to outcompete nearby plants for available resources, contributing to its aggressive nature. The plant’s sensitivity to poorly drained conditions is a consequence of this shallow growth.
Managing Root-Related Concerns
The vigor of the butterfly bush root system creates several practical management issues for homeowners. While the roots are generally not large enough to cause structural damage to solid foundations or deep underground pipes, their dense, woody nature can be problematic. The plant’s aggressive growth and competitive roots can quickly choke out smaller, less established plants in a garden bed.
The biggest concern is the root system’s capacity for vigorous regrowth, known as suckering, from the root crown. If the main shrub is cut down, the dense, perennial root crown typically sends up multiple new shoots quickly. This aggressive re-sprouting contributes to the plant’s designation as an invasive or noxious weed in many regions.
Removal Methods
Complete removal of a mature butterfly bush requires more than just cutting the stems to the ground. The woody rootstock must be entirely dug out, which is a physically demanding task due to its dense, intertwined structure. Any fragments of the root crown left in the soil can quickly regenerate the plant.
A more effective method for permanent eradication involves the “cut and paint” technique, where the stems are cut close to the ground and a concentrated herbicide, such as glyphosate, is immediately applied to the exposed root crown. This treatment allows the chemical to be translocated down into the root system, killing the entire structure. It is advisable to choose planting locations away from delicate infrastructure, vegetable gardens, or areas where its vigor will crowd out less robust species.