A raised garden bed is a self-contained structure, typically made of wood, metal, or stone, that sits above the native ground level and is filled with a specialized soil mix. These beds are popular because they allow gardeners to control soil quality, improve drainage, and offer easier access for planting and harvesting. When setting up a new bed, a common question is whether a barrier material like landscape fabric should be placed underneath. Gardeners usually aim to prevent unwanted growth or separate the new soil from the ground below. This article explores the motivations for using fabric, its long-term consequences, and preferred alternatives for creating a healthy, productive growing environment.
Why Gardeners Consider Fabric Under Raised Beds
The primary motivation for placing landscape fabric beneath a raised bed is to establish a physical barrier against encroaching weeds or aggressive grasses. This synthetic material suppresses growth by blocking light, which is appealing when building a bed over an existing lawn or areas known to have persistent weeds like Bermuda grass.
Another perceived benefit is separating the expensive, high-quality soil mix from the native soil below. Gardeners often use this technique when the existing ground is poor, rocky, or potentially contaminated, such as in older urban areas. The fabric acts as a defined boundary, helping to retain the specialized soil and prevent it from mixing with the underlying ground. In some instances, the fabric is also considered a partial defense against certain burrowing pests.
The Long-Term Problems with Landscape Fabric
While landscape fabric offers immediate convenience, its long-term impact on soil health often outweighs the initial benefits. Over time, the permeable fabric becomes clogged as fine soil particles and organic matter filter down from the bed above. This clogging significantly impedes proper water flow, leading to poor drainage and potentially causing soggy spots directly above the barrier.
The fabric also acts as an impenetrable barrier for beneficial soil organisms like earthworms and microbes, preventing their natural movement between the native ground and the raised bed soil. These organisms are responsible for aerating the soil, breaking down organic matter, and cycling nutrients essential for long-term garden fertility. By restricting their movement, the fabric limits the development of a healthy, living soil ecosystem.
For deep-rooted plants like tomatoes, the fabric prevents roots from growing down and accessing moisture and nutrients in the subsoil beneath the bed. When roots hit the synthetic barrier, they are forced to circle within the confines of the bed, which restricts the plant’s growth and overall productivity. Furthermore, the fabric eventually degrades, making removal a messy and difficult process as it tears into small pieces mixed with soil and roots.
Effective Alternatives to Landscape Fabric
Instead of using synthetic landscape fabric, gardeners can achieve similar goals with materials that promote soil health. For initial weed suppression, laying down several layers of plain cardboard or newspaper is highly effective. This biodegradable material blocks sunlight, smothers existing weeds, and naturally breaks down over one or two seasons, adding carbon to the soil and allowing earthworms to pass through.
If the primary concern is protection from burrowing animals like gophers or voles, use galvanized hardware cloth or wire mesh. This metal mesh, typically a quarter-inch size, is laid across the bottom of the bed and secured to the frame. It provides a permanent physical barrier that pests cannot chew through, while still allowing for excellent drainage and permitting plant roots to grow down into the native soil below.
For raised beds built over healthy native soil, the most straightforward approach is to use no barrier at all. Simply filling the bed with a quality soil mix and maintaining a thick layer of organic mulch on the surface will effectively suppress most annual weeds. This allows the soil in the bed to connect with the ground beneath, encouraging a thriving soil food web and giving roots unrestricted access to moisture and nutrients.