A planter box provides a self-contained, portable environment where plants can thrive almost anywhere, from a small balcony to a spacious patio. Success in this confined space relies entirely on the quality of the container setup and the components placed within it. This guide provides advice on preparing your container, selecting the optimal growing medium, choosing the right plants, and maintaining their health throughout the season.
Preparing the Container for Success
The physical structure of the planter box dictates the health of the root system, making proper preparation a foundational step. Every container must possess adequate drainage holes to prevent the soil from becoming waterlogged, which suffocates roots and encourages rot. For large wooden planters, drainage holes should be drilled to a diameter of about 1/2 to 3/4 inch, with multiple holes spaced across the bottom surface for efficient water exit.
Lining the container is beneficial, especially for boxes made of wood or metal. A liner protects wooden boxes from constant moisture contact, slowing decay and extending their lifespan. For metal containers, a liner helps insulate the interior, shielding roots from excessive heat buildup.
Gardeners should avoid the outdated practice of placing a layer of gravel or broken pottery at the bottom of the container. This layering does not improve drainage; instead, it raises the perched water table, creating a perpetually saturated zone of medium just above the layer, which inhibits root respiration.
Selecting the Ideal Planting Medium
The growing material inside a planter box must balance aeration and water retention, requiring a specialized potting mix. Garden soil or topsoil is too dense; when confined and repeatedly watered, it compacts heavily. This compaction squeezes out the air pockets roots need for gas exchange, leading to poor drainage, lack of oxygen, and root decay.
Commercial potting mixes are often referred to as “soilless” because they consist primarily of lightweight components like peat moss, composted bark, and water-retaining materials. Amendments such as perlite, a volcanic glass that adds permanent air space, or coco coir, a sustainable fiber that holds moisture effectively, are often included to optimize the mix.
For planters with great depth, you can save on cost and weight by filling the bottom third with lightweight, non-degradable materials. Clean, empty plastic containers or crushed aluminum cans can be used as filler, but they must be placed well below the intended root zone.
Matching Plants to Location and Box Size
Selecting plants that align with the box’s dimensions and the available light is paramount to long-term success. Full sun, defined as receiving at least six hours of direct sunlight, is suitable for choices like tomatoes, peppers, and ornamental grasses. Areas with partial sun or shade (three to six hours of light) are well-suited for plants such as coleus, begonias, and ferns. For deep shade, impatiens and hostas are excellent options.
The volume of the box directly affects the frequency of watering and the overall size a plant can reach. Small containers dry out faster, requiring drought-tolerant plants or daily watering. Larger boxes provide a greater buffer against temperature fluctuations and moisture loss, supporting deeper root systems and allowing for more complex arrangements.
A common design strategy uses the concept of “thriller, filler, and spiller.” This involves a tall, upright plant as the focal point (thriller), mounding plants to add volume (filler), and trailing plants that cascade over the edge (spiller).
Essential Ongoing Container Care
The isolated nature of a planter box makes watering and feeding different from in-ground gardening. Because the potting mix volume is limited and exposed to the air, it dries out faster, often requiring daily watering in hot weather. Apply water slowly until it flows freely from the drainage holes, ensuring the entire root ball is saturated. To check moisture, insert a finger two inches into the mix; if it feels dry at that depth, it is time to water.
Fertilization is necessary because the frequent watering that saturates the mix also causes nutrients to leach out of the container quickly. Most potting mixes contain low nutrient levels that are rapidly depleted by actively growing plants.
Gardeners can choose a slow-release granular fertilizer, incorporated into the top few inches of the mix, which provides a steady food supply over several months. Alternatively, a liquid fertilizer can be applied every two to four weeks, offering immediate nutrient boosts useful for heavy feeders like flowering annuals and fruiting vegetables. Regularly checking the underside of leaves and stems for pests like aphids or spider mites is also recommended, as they thrive in the contained environment.