A planter box is a contained, often elevated garden structure, offering a versatile and accessible way to grow plants. This container gardening method allows for precise control over the growing environment, including soil quality and moisture levels. Its portability is a key advantage, offering flexibility to maximize sunlight exposure throughout the seasons.
Preparing the Planter Box for Use
Placement is important and directly impacts plant health. Select a location that provides the necessary amount of sunlight for your chosen plants, typically six to eight hours of direct sun daily for most vegetables and flowers. Accessibility for watering and harvesting is also a practical factor to consider when finalizing the position.
Proper drainage is necessary for container gardening, as waterlogging causes root rot. If the box lacks drainage holes, create several openings, at least one-half to three-quarters of an inch in diameter, across the bottom surface. Water must flow freely to prevent the soil from becoming saturated.
For wooden planter boxes, lining the interior with a material like heavy-duty plastic sheeting or landscape fabric can significantly extend the box’s lifespan by preventing direct contact between the moist soil and the wood. If using a plastic liner, poke corresponding holes in the liner over the drainage holes to ensure water can still escape. Contrary to popular belief, adding a layer of gravel or broken pottery at the bottom is not helpful for drainage and can actually increase the water saturation level in the soil directly above it.
Selecting and Filling with the Growing Medium
The growing medium used in a planter box differs distinctly from native ground soil. Standard garden soil is too heavy and dense for container use, leading to rapid compaction that restricts air circulation and inhibits healthy root growth. Compaction also makes it difficult for water to permeate the soil, which stresses the plants.
Instead, select a high-quality, lightweight potting mix, often called potting soil, which is a soilless blend designed for containers. These mixes typically contain a balance of organic matter, such as peat moss or coir, and amendments like perlite or vermiculite. Perlite is a puffed volcanic glass that promotes rapid drainage and aeration, while vermiculite, a soft mineral, excels at retaining moisture for plants that require consistent hydration.
Incorporate high-quality compost into the potting mix to introduce beneficial microorganisms and slow-releasing nutrients. The ideal composition remains loose, fluffy, and well-draining, preventing dense, anaerobic conditions that suffocate roots. When filling the box, leave one to two inches of space between the medium surface and the rim. This “headspace” prevents water from running over the sides before it soaks into the growing medium.
Planting and Spacing Techniques
When it is time to plant, select varieties that are appropriate for the depth and volume of your specific planter box. Shallow-rooted plants like lettuce, radishes, and herbs can thrive in relatively shallow containers, but deep-rooted vegetables such as full-sized tomatoes and squash require greater depths, ideally at least 12 to 18 inches, to accommodate their root systems. Choosing compact or “bush” varieties of typically large plants is often advisable for container growing.
Spacing requirements in a planter box are typically different from those for in-ground gardening, allowing for plants to be placed closer together. While you can space plants more tightly to achieve an immediate, full appearance, a slightly more traditional spacing that allows for adequate air circulation reduces the risk of disease and promotes long-term plant health. A useful guideline is to space transplants approximately one-third closer than the recommendations for in-ground planting.
Before placing a transplant into the prepared medium, gently loosen any tightly bound or circling roots to encourage them to grow outward into the new soil. After the plants are set into the mix, immediately water the entire box thoroughly until you see water flowing out of the drainage holes at the bottom. This initial watering settles the medium around the roots and eliminates any air pockets.
Essential Ongoing Care
Ongoing maintenance differs from caring for an in-ground garden, particularly concerning water and nutrients. Container media heats up and dries out faster than garden soil, requiring more frequent watering, sometimes daily during hot weather. To check if watering is needed, insert your finger one to two inches deep into the mix; water only when the medium feels dry at that depth.
Because water moves through the medium quickly, essential plant nutrients are easily washed out, a process called leaching. Plants in a planter box require regular fertilization to replenish these lost elements. Even if the initial potting mix contained a slow-release fertilizer, a regular feeding schedule is necessary, typically every one to three weeks with a water-soluble, all-purpose fertilizer.
Using a complete fertilizer that includes nitrogen, phosphorus, and potassium (in a balanced ratio like 1:1:1 or 2:1:2) supports plant health and productivity. Routinely inspect the foliage for pests or disease, and remove spent flowers and yellowing leaves to encourage continuous growth.