Window Box Calculator

This is the most widely recommended window box design formula from university extension services. Thrillers are tall, upright plants that provide height and drama (ornamental grasses, upright petunias, spiky cordyline). Fillers are mounding plants that fill the middle (petunias, marigolds, geraniums). Spillers are trailing plants that cascade over the edge (bacopa, sweet potato vine, calibrachoa). Aim for 1 thriller, 3 fillers, 2 spillers per 24-inch box.

Use a high-quality commercial potting mix, not garden soil. Potting mix drains well and provides the air pockets roots need. For window boxes that dry out quickly, add 20 to 25% perlite for improved drainage, or use a mix with a moisture-retaining polymer. Avoid mixes with too much peat, which can become hydrophobic when very dry.

Window boxes dry out quickly, especially in hot or windy conditions. In summer, watering once daily or even twice daily (morning and evening) may be needed. A simple test: stick your finger 1 inch into the soil. If dry, water thoroughly until water drains from the drainage holes. Consider self-watering window boxes with a reservoir for convenience.

Most flowering annuals and herbs grow well in 6 to 8 inches of soil depth. Deeper boxes (10 to 12 inches) suit vegetables like lettuce, radishes, and shallow-rooted tomato varieties. Standard window boxes are 6 to 8 inches deep and 6 to 12 inches wide. Ensure adequate drainage holes, at least one hole per 6 inches of length.

Window box brackets must support the weight of the box plus soil and water, which can exceed 50 lb for a large filled box. Use heavy-duty metal brackets screwed into wall studs or using proper masonry anchors. Never rely on lightweight plastic hooks or the window ledge alone. Check bracket load ratings against the expected filled weight of your box.