Tall planters, defined generally as those twenty-four inches deep or more, introduce unique challenges for successful container gardening. Their significant depth can lead to issues with excessive weight, high soil volume requirements, and compromised root health if not managed correctly. Specialized planting techniques are necessary to ensure proper aeration and drainage, which are the foundations for a thriving root system in these deep containers.
Preparing the Base: Drainage and Weight Reduction
The base of tall planters requires careful attention to manage both drainage and weight. Before filling, confirm that the planter has adequate drainage holes, as stagnant water at the bottom can quickly lead to root rot. To prevent the container from becoming overly heavy and expensive to fill entirely with potting mix, the bottom one-third to one-half of the depth should be filled with lightweight, non-decomposing materials.
Filler materials, such as upside-down plastic nursery pots, empty plastic bottles, or foam packing peanuts, serve a dual function. They drastically reduce the overall weight of the container and minimize the amount of expensive potting mix required. Avoid using traditional heavy materials like gravel or rocks, despite the intuitive thought that they improve drainage. Adding a layer of coarse material raises the “perched water table,” which reduces the usable depth of well-drained soil and can waterlog the roots above the filler layer.
Selecting the Growing Medium and Plants
The material placed above the filler must be a high-quality, lightweight container potting mix, specifically formulated for the confined environment of a pot. Unlike denser garden soil, these mixes contain ingredients like peat moss, coir, and perlite that maintain airspace for oxygen exchange and promote efficient drainage while retaining moisture. A thin layer of landscape fabric should be placed directly on top of the filler materials to prevent the soil from migrating down and clogging the drainage holes.
When choosing plants, consider the ultimate root depth and the overall visual composition of the container. An attractive display often utilizes the “thriller, filler, spiller” design: a tall, upright plant (thriller) is placed centrally, medium-sized plants (fillers) surround it, and trailing plants (spillers) cascade over the edges. For deep-rooting vegetables or small shrubs, the planter’s depth is beneficial because it allows for extensive root growth and helps regulate soil temperature. Shallow-rooted annuals only require the top portion of the container, making the use of base fillers highly efficient.
Specialized Watering and Maintenance
Deep containers present a specific watering challenge because the soil surface dries out quickly while the lower section can remain saturated. To accurately gauge the moisture level, perform a finger test several inches below the surface or use a moisture meter to check the bottom half of the growing medium. Water slowly and thoroughly until water drains from the bottom, ensuring the entire soil column is moistened.
Since container potting mixes are primarily inert, they lack the sustained nutrient content of native soil. Fertilization is necessary and can be accomplished by mixing a slow-release granular fertilizer into the potting mix during planting or by regularly applying a diluted liquid fertilizer. Over time, the organic components of the mix break down and settle, leading to compaction and reduced airspace. To counteract this, the top few inches of the potting mix should be refreshed or replaced annually to maintain proper soil structure and aeration.