Rectangular planters introduce structured greenery into spaces like narrow balconies, sunny patios, or along deck railings. Their linear shape naturally creates an architectural focal point, providing a defined edge or border for outdoor living areas. This type of container is popular for urban gardeners or those with limited square footage, allowing for concentrated bursts of color and texture. Mastering the technique of planting in these containers ensures that the arrangement thrives visually and biologically throughout the growing season.
Essential Preparation for Rectangular Planters
Successful container gardening begins with the vessel itself. Rectangular planters, especially long ones, require adequate drainage to prevent waterlogging and root rot. While containers must have several drainage holes, adding gravel or broken pottery is unnecessary and can impede proper water flow by raising the perched water table. Ensure the existing drainage holes are clear, and consider using a piece of screen or landscape fabric over them to prevent soil loss.
The choice of growing medium is equally important; a lightweight, sterile potting mix must be used, not dense garden soil. Potting mixes are designed to provide the necessary aeration and porosity for root systems confined within a container. Garden soil compacts too easily and restricts the oxygen flow required for healthy root growth.
The planter’s material affects its moisture retention, which requires attention when watering. Terracotta and unglazed clay pots dry out faster than non-porous plastic or glazed ceramic containers. Wooden planters often benefit from a plastic liner to protect the wood from constant moisture, which prolongs the container’s life. Select a planter depth that corresponds to the mature size of the plants intended, as shallow containers may restrict root development.
Designing with Thriller, Filler, and Spiller
The visual impact of a rectangular planter is maximized by employing the Thriller, Filler, and Spiller technique. This strategy ensures a balanced and dynamic composition by assigning specific roles to different plant types based on their growth habit. It is particularly effective in linear containers because it establishes rhythm and proportion along the length of the box.
The “Thriller” plant serves as the vertical accent and the primary focal point of the arrangement, drawing the eye upward. In a long, rectangular planter, these taller elements are typically placed near the center or spaced evenly along the length of the container to maintain a consistent visual height. Thrillers provide structure and height, often reaching two or three times the height of the container itself.
The “Filler” plants are medium-height, mounding varieties that surround the base of the Thriller, providing volume and mass. These plants are generally bushy and full, bridging the space between the tall focal point and the trailing edges of the container. Fillers often contribute the bulk of the color and texture, creating a lush, rounded form.
The “Spiller” plants are those with a trailing or cascading habit, which flow over the sides of the planter. This softens the hard, linear edges of the container, making the overall design appear more natural. Spillers should be planted along the perimeter, especially near the corners and along the long sides. For a very long planter, repeating the pattern of Thriller-Filler-Spiller multiple times creates a sense of continuity and visual flow.
Specific Plant Groupings for Light Conditions
Selecting the appropriate plants depends entirely on the amount of direct sunlight the planter receives throughout the day. Groupings must match the light exposure to ensure the plants thrive, as the light requirement is the most important factor after proper drainage and soil. Full sun is defined as six or more hours of direct light daily, while shade areas receive less than three hours of direct sun.
Full Sun
For planters situated in full sun, the plant selection should favor varieties that tolerate intense light and heat. A suitable Thriller could be a tall ornamental grass, such as fountain grass, or a dramatic canna lily, providing strong vertical lines. Excellent Filler choices include the vibrant, heat-tolerant blooms of petunias or the continuous color offered by geraniums. The perimeter benefits from Spillers like sweet potato vine, with its striking foliage, or the profusely blooming calibrachoa (million bells).
Partial to Full Shade
Planters placed in partial to full shade require plants that flourish with less intense light and often prefer consistent moisture. Thrillers in shaded areas might include tall, structured ferns, like the Japanese painted fern, or the large, textural leaves of certain hosta varieties. For the Filler role, shade-tolerant annuals such as wax begonia or the richly patterned foliage of coleus provide color without direct sun. Trailing Spillers that perform well in lower light are creeping jenny, known for its bright chartreuse leaves, or traditional English ivy.
Edible Plants
Beyond purely ornamental displays, rectangular planters are also suitable for growing certain edible plants, provided their light needs are met. Herbs such as basil, thyme, and oregano require full sun and can be used as Fillers. Leafy greens like lettuce, spinach, and radishes can be successfully grown in partial shade conditions. When combining edibles with ornamentals, ensure all plants share the same moisture and nutrient requirements for a successful growing environment.