A window box is a specialized container garden positioned outside a window, used to display flowers or grow herbs. The materials placed at the bottom serve distinct structural and biological functions. Getting the bottom layer right influences the health of plant roots, prevents soil compaction, and manages water flow. Understanding these principles ensures the container remains an optimal environment for plant growth.
Laying the Foundation for Effective Drainage
The most important element in the bottom of a window box is an exit point for excess water. Most window boxes have drainage holes, but if yours does not, drilling several small holes (typically between a quarter and a half-inch in diameter) is necessary to prevent root suffocation. Without proper drainage, water accumulates at the base, displacing oxygen and leading to root rot.
Once drainage holes are present, ensure the potting mix does not wash out or clog the openings. Placing a small piece of fiberglass window screen, landscape fabric, or a paper coffee filter over each hole holds the soil back. These permeable barriers allow water to pass through freely while keeping the fine particles of the potting mix contained.
A common but flawed practice is adding a thick layer of gravel, broken pottery shards, or heavy rocks to the bottom of the box. The intuitive belief is that this layer will improve drainage, but in reality, it often has the opposite effect. Adding a coarse layer beneath the potting mix creates what is known as a “perched water table.”
This phenomenon occurs because water moving downward through the fine-textured potting soil resists crossing the barrier into the large air spaces of the gravel layer. Instead of draining, the water collects and saturates the soil immediately above the gravel, raising the saturation level into the root zone. To maximize the usable volume and aeration for the roots, the potting mix should be placed directly over the screened drainage holes without any intervening thick layer of coarse material.
Lightweight Fillers for Deep Window Boxes
For deep window boxes, especially those mounted on railings or high windows, reducing overall weight is a primary consideration. Potting mix, particularly when saturated with water, becomes exceptionally heavy, which can strain mounting brackets and make the container difficult to move. Using a lightweight filler material to occupy the non-essential space at the bottom of the box is an effective solution.
Approved lightweight materials include empty plastic water bottles, non-biodegradable foam packing peanuts, or blocks of rigid styrofoam. When using plastic bottles, keep the caps screwed on tightly to prevent collapse and displace the maximum space. These materials will not decompose or compress over time, maintaining the reduced weight of the container for multiple growing seasons.
The filler should only occupy the bottom half or, ideally, the bottom one-third of the window box depth. This ensures a sufficient volume of potting mix remains for the plants to thrive. A minimum of six to eight inches of actual potting soil is needed to support healthy root development and provide adequate moisture retention for most common annuals and herbs.
To maintain separation between the lightweight filler and the potting mix, a permeable barrier is necessary. A layer of landscape fabric, a piece of old window screen, or permeable plastic mesh should be placed directly on top of the filler. This barrier prevents the potting soil from settling into the gaps, which would negate the weight-saving benefit and complicate future replanting.