Window Boxes vs Rail Planters
Window boxes mount directly below windows — on sills, brackets, or wall anchors. Rail planters hook over deck, balcony, or fence railings. The functional difference is weight bearing: window boxes need secure wall anchors rated for the wet soil load (a 24-inch box with saturated soil weighs 30–50 lbs), while rail planters distribute weight across the railing and need brackets matched to the railing width.
Material Options
Cedar and white oak are the premium wood choices — naturally rot-resistant, attractive, and long-lasting when the interior is lined or given a penetrating oil finish. Raw interior wood in constant contact with wet soil will eventually rot regardless of species; a plastic liner extends lifespan significantly.
Fiberglass is lightweight, doesn't rot, and can be molded to resemble wood or terracotta. Good choice when weight is a concern. Galvanized steel is modern, durable, and pairs well with contemporary decks. Plastic is the cheapest option — functional but less attractive, and prone to cracking in hard freezes.
For larger deck planting setups, also see our guide to deck planters for small outdoor spaces.
Sizing: The Rules
Standard window boxes are 24, 36, and 48 inches long. Most plants need 6–8 inches of depth; herbs and annuals thrive at 6 inches, but perennials and vegetables need at least 8–10 inches to develop proper root systems. Width matters too: 8 inches wide is the minimum for adequate soil volume; 10–12 inches provides better moisture retention.
Drainage: Non-Negotiable
Every window box needs drainage holes — at minimum one 1/2-inch hole per 6 inches of length. Without drainage, roots suffocate and rot in standing water within days of heavy rain. Add a layer of gravel or coarse perlite at the bottom, then a well-draining potting mix. Never use straight garden soil in containers; it compacts and drains poorly.
Best Plants for Window Boxes and Rail Planters
Full sun: Petunias, geraniums, calibrachoa, herbs (basil, thyme, oregano), trailing nasturtiums. Part shade: Impatiens, begonias, fuchsia, ferns, coleus. Edibles: Lettuce, arugula, radishes, herbs — all work well in 6–8 inch depth. Cherry tomatoes can work in extra-deep (10-inch) rail planters with support.
FAQ
How much weight can a window box bracket hold?
Standard iron brackets rated for outdoor use hold 50–75 lbs per pair. Always use wall anchors rated for masonry or stud attachment — toggle bolts for drywall are not sufficient for the wet weight of a planted box.
Do window boxes damage siding?
Yes, if water wicks between the box and the wall. Leave a 1–2 inch gap between the box back and any painted or wood surface, and use stand-off brackets. This allows air circulation and prevents rot and paint damage.
How often do window boxes need watering?
In summer, most window boxes need daily watering — they dry out faster than in-ground beds due to small soil volume and wind exposure. Self-watering boxes with a reservoir can extend intervals to every 2–3 days.
Can I overwinter a window box?
Wood boxes should be emptied, cleaned, and stored in a dry location over winter. Freeze-thaw cycles crack wood joints and split plastic liners. Metal and fiberglass boxes are more tolerant of outdoor winter storage.
What is the best liner for a wood window box?
Coco coir liners are biodegradable and breathable. Heavy-gauge plastic liner bags (at least 4 mil) work well and are cheap to replace. Avoid thin plastic sheeting — it tears when removing plants.