A raised garden bed is a framed structure, typically made of wood or metal, that sits directly on the ground or a hard surface and is filled with imported soil. This method of gardening is popular for offering better control over soil quality and improving drainage compared to in-ground planting. A widely shared piece of advice suggests placing a layer of rocks, gravel, or broken pottery at the bottom of the bed before adding soil to improve water flow. This practice, borrowed from older container gardening traditions, is frequently adopted by new gardeners.
The Drainage Myth Explained
The belief that adding a layer of coarse material at the base of a raised bed improves drainage is counterintuitive to soil science and should be avoided. This practice, intended to help water escape quickly, actually has the opposite effect in a confined environment. The presence of a coarse layer reduces the total volume of usable soil available for root growth and nutrient uptake. It also encourages water to accumulate directly above the rocky layer, creating a saturated zone. This saturated environment restricts oxygen supply to the root system, leading to root suffocation and decay, commonly known as root rot.
Understanding the Perched Water Table
The phenomenon that explains why rocks fail to improve drainage is known as the perched water table. Water moves through soil not solely by gravity, but also by capillary action—the force of water molecules clinging to the fine particles of the soil medium. When downward-moving water encounters a dramatic change in material texture, such as the transition from fine soil to coarser gravel, the capillary forces become stronger than the gravitational pull. Water will not move into the air gaps between the rocks until the soil immediately above the gravel is completely saturated. This saturated zone, where the water is “perched” atop the coarse material, moves the wettest area closer to the roots and reduces the total depth of well-aerated soil available for root development.
Optimal Alternatives for the Raised Bed Base
Since the raised bed structure naturally offers improved drainage, base preparation should focus on structural support and pest management, not on creating an artificial drainage layer. For beds placed directly on the ground, a barrier is recommended to prevent weeds or deter burrowing pests. A layer of cardboard or landscape fabric can suppress weeds, while hardware cloth (a fine wire mesh) is effective for keeping out voles and gophers. The primary component of a raised bed should be a high-quality, continuous growing medium from top to bottom, such as a blend of topsoil, compost, and coarse amendments. For very deep beds, gardeners may use large organic fillers, such as logs, branches, or straw bales, in the lowest third of the bed to save on soil cost and slowly enrich the soil over time.