Railing Planters and Window Boxes: Sizes, Brackets and Drainage

Railing planters attach in one of three ways: they straddle the rail, clamp onto it, or hang from adjustable arms. Each fits only the rail widths its maker lists, so measure your rail first. Hang boxes on the balcony side, choose models with drainage holes and a way to catch runoff, and use only maker-stated load limits.

This guide is part of our balcony planters section. For containers that keep the rail clear, see vertical planters and hanging planters.

How do I attach a planter to a railing?

Most railing planters fit without tools or drilling, which suits renters. They differ in how they grip the rail and which rails they fit.

Clemson Extension lists plastic, wood and long wire “hayrack” baskets as the usual window box types. Wire hayracks need a liner of sphagnum moss or coir to hold the mix.

Measure your railing first

Measure three things before you shop: the width across the top of the rail, its shape, and what sits below it.

• Width. Use a tape across the top rail at the spot you want the box. Compare it with the maker’s stated range, not the box’s length.
• Shape. Flat, square and round rails grip differently. Gardener’s Supply says its adjustable planter is not suitable for round railings.
• Glass panels. Many glass railings have a narrow cap or no top rail to grip. Buy only a model whose maker says it fits that cap.

Hang the box on the balcony side of the rail. A box on the outside face drips onto the street or the balcony below, is hard to water and is out of reach if it shifts. Building rules on anything fixed to the railing vary, so check your lease or HOA rules before you hang anything.

How much weight can a railing planter hold?

Use only the load limit a maker states. Bloem’s Classic Deck Rail page gives the rail widths it fits but no load limit, and the Gardener’s Supply pages above give sizes and reservoir capacity but no load limit either. Where no figure is stated, treat the limit as unknown.

The weight is mostly wet potting mix and water. The Gardener’s Supply adjustable box weighs 2 pounds empty but holds 12 quarts of potting mix and a 1-quart reservoir. Our guide to balcony weight limits explains how to add up what you put on a balcony and who can tell you what it carries: building management, your landlord or HOA, or a licensed structural engineer.

Drainage, liners and the unit below

Clemson Extension says baskets and window boxes need drainage holes so the mix does not become waterlogged. Bloem’s deck rail planter has knockout drainage holes, so you choose whether to open them. Gardener’s Supply says the drainage holes on its adjustable box protect against overwatering in heavy rain.

Drainage holes create runoff. University of Maryland Extension notes that self-watering containers and saucers keep drained water from staining surfaces, and that a saucer helps when you garden above a neighbor’s balcony. A self-watering railing box holds water in a reservoir rather than letting each watering run through.

Skip the gravel layer. Oregon State University Extension calls the advice to put coarse material in the bottom for drainage wrong, because it slows drainage instead.

What are the pros and cons of railing planters?

The main advantage is space: a railing box uses the rail and leaves the floor for a chair. It also sits where the light is on many balconies and is easy to reach for watering and picking.

The main drawback is volume. The Gardener’s Supply adjustable box has a 6-inch planting depth and holds 12 quarts of potting mix, and small containers dry out quickly. Oregon State University Extension says a 10-inch container may need water every 1 to 2 days, and smaller ones more often. In hot weather, expect to water a railing box daily. A self-watering model or our self-watering planters guide can help.

What are the best plants for railing planters?

Pick plants that share the same light and moisture needs, because they share one box. Clemson Extension’s tables of basket and window box plants include these:

• Sun: petunia, verbena, French marigold, geranium, moss rose, fan flower, sweet alyssum and trailing lantana.
• Part shade or shade: pansy, impatiens, tuberous begonia, caladium, polka dot plant and fuchsia.
• Herbs: thyme, oregano and rosemary, which like the soil to dry out between waterings.

For what will grow in your balcony’s sun, shade and wind, see what to grow on a balcony and growing herbs in pots.