Decomposed granite (DG) is a natural material created when solid granite rock breaks down into smaller particles and fragments. This crushed granite aggregate is popular for landscaping projects, including pathways and patios, due to its natural aesthetic and permeability. While DG is not a perfect solution, proper installation and diligent maintenance can significantly suppress weed establishment.
How Decomposed Granite Affects Weed Establishment
Decomposed granite, in its pure form, is a hostile environment for plants because it contains very little organic matter or nutrients necessary for robust growth. When newly installed and heavily compacted, the material creates a dense, hard surface that physically inhibits weed seeds from germinating and roots from penetrating the base layer. This physical property makes the initial installation relatively weed-free.
The material’s effectiveness diminishes when organic debris, such as fallen leaves and dirt, accumulates on the surface. Wind-blown weed seeds land in this newly created, nutrient-rich layer of organic matter mixed with the granite fines. Once a seed finds moisture and organic material on the surface, it can germinate and establish roots. Weeds often appear on the surface of DG rather than pushing up from the native soil underneath.
Installation Techniques for Maximum Weed Suppression
Achieving maximum weed suppression relies on meticulous preparation and specific installation techniques. Before laying down the DG, all existing vegetation, including perennial weeds and grasses, must be thoroughly removed. This often requires non-selective herbicides to kill deep root systems, preventing established weeds from growing through the DG layer.
The next mandatory step is to install a commercial-grade geotextile fabric, commonly known as a weed barrier, over the cleared and compacted sub-base. This fabric acts as a physical separation layer, blocking any weed seeds or root fragments remaining in the native soil from reaching the DG above. The DG material itself should be applied to a depth of at least 3 to 4 inches to provide sufficient volume for compaction and stability.
The most critical step involves using a plate compactor, not just a hand tamper, to achieve extreme density. This heavy compaction interlocks the small particles and fines within the granite, creating a near-concrete-like surface that is difficult for new seeds to penetrate. For high-traffic areas, using a stabilized DG product, which incorporates a polymer binder, further hardens the surface, making it significantly more resistant to weed intrusion and erosion.
Long-Term Maintenance for Continued Weed Control
Even a perfectly installed decomposed granite surface requires regular maintenance to maintain its weed-suppressing qualities over years of exposure. The primary preventative measure involves consistently removing organic debris from the surface. Leaves, pine needles, and dust should be swept or blown off regularly, as their presence is what allows wind-carried weed seeds to take hold.
Pre-emergent herbicides can be applied seasonally to the DG surface to create a chemical barrier that stops new weed seeds from germinating. This application is typically done twice a year, in the late fall and again in late winter, to target different cycles of weed seed germination. The granular material is broadcast over the surface and dissolved with water, establishing a thin barrier that prevents establishment.
Weeds that manage to break through should be addressed immediately to prevent root establishment and stop them from dropping new seeds. Spot treatment with a post-emergent herbicide is effective, or they can be hand-pulled when the DG is damp. Hand-pulling when damp allows for complete removal with minimal disturbance to the compacted surface. If the surface shows signs of erosion or low spots, new DG should be added and re-compacted to restore the uniform barrier.