Grow bags are porous fabric containers that offer gardeners unique benefits, primarily portability and superior root health. Unlike solid plastic pots, the breathable material encourages a natural process called air pruning. This process prevents roots from circling and becoming root-bound, promoting a dense, fibrous root structure that efficiently absorbs water and nutrients. Because the fabric is porous, excess water continually drains out the bottom and sides. Determining the correct base layer is essential, as placing the container directly onto a surface can seal the base, leading to poor drainage and potential damage from moisture and soil staining.
Solutions for Hard Surfaces
When grow bags are situated on patios, balconies, decks, or concrete, the primary concern is managing water runoff and protecting the underlying material. Saturation of a wooden deck can accelerate rot, while concrete can develop permanent mineral and soil stains. Containment devices capture the excess water that flows freely from the fabric base after watering.
Drip trays and saucers provide an effective containment method for smaller bags. These should be large enough to catch all runoff water, but they require diligent monitoring. The grow bag must not be allowed to sit in standing water for extended periods, as this defeats the purpose of the fabric’s superior drainage and can lead to waterlogging. A rubber mat or a continuous, low-sided plastic tray can be used to contain runoff and protect the surface area.
If a larger, continuous liner is used, it should be slightly sloped or checked frequently to ensure water drains away or is evaporated quickly. This approach focuses on preventing staining and controlling runoff, but it still requires a separate step to lift the bag slightly out of the collected water. The goal is surface protection and containment, which is achieved by trapping the initial water flow.
Ensuring Optimal Drainage and Airflow
To maximize the biological advantage of air pruning, the grow bag must be elevated off the surface to allow air to circulate beneath the bag. When the bag sits flat, the bottom layer of soil remains saturated, and the air-pruning effect is nullified at the base, potentially leading to waterlogging. This elevation needs to be at least half an inch to be effective for air circulation.
Specialized pot feet, small terracotta blocks, or wooden risers can be placed beneath the bag to create a gap. These supports lift the bag’s weight, allowing root tips that reach the bottom fabric to be exposed to air. Exposure to air causes the root tip to dehydrate and stop growing, signaling the plant to produce new lateral roots further up the root ball.
For a more permanent and uniform solution, a wire rack or a section of plastic shelving placed on the ground provides a continuous, open platform. This structure ensures that no part of the grow bag base is sealed off, promoting uniform air circulation across the bottom surface. Proper elevation maximizes the root density within the bag, leading to a healthier and more vigorous plant overall.
Preparing the Ground for Bags on Soil or Grass
When grow bags are placed directly on a permeable surface like a lawn or garden soil, the challenges shift from surface protection to biological barriers. The porous fabric allows plant roots to grow through the material. This root penetration can make the bag impossible to move without damaging the plant and allows soil-borne diseases to enter the container mix.
A heavy-duty landscape fabric or a woven weed mat should be placed under the bags to act as a root barrier. This material prevents the plant roots from growing into the native soil while still allowing water to pass freely through. For areas with burrowing pests like gophers or voles, a layer of galvanized hardware cloth with a small mesh size, such as half-inch, can be placed under the weed barrier.
This wire mesh acts as a physical shield, preventing pests from chewing their way up into the soil of the grow bag from below. For very large bags, creating a small, level pad of sand or gravel ensures the base remains stable and prevents the weight of the bag from settling unevenly into soft soil. The primary function of these layers is to maintain the integrity of the container system by separating it from the surrounding environment.