Window Boxes for a Balcony: What Size, How to Mount, What Fits

A balcony window box should run the width of the space you want to fill, sit on brackets its maker designed for your wall or rail, and have drainage holes plus a reservoir or tray to catch runoff. Use only a load limit the maker states, and mount it the way the maker and your building allow.

Boxes that clamp or straddle a rail without separate brackets have their own guide, railing planters. Boxes with a built-in reservoir are compared in self-watering planters, and the balcony planters hub covers every other type.

What size window box do you need?

Window box length comes first. Clemson Extension says a box looks best running the entire width of the window. On a balcony, measure the stretch of wall or rail you want to plant, then check how many brackets that length needs.

Volume decides what grows. A small box holds little mix and dries out quickly, so match the crop to what the box holds. University of Maryland Extension lists ½ gallon of mix as enough for leaf lettuce and 2 to 5 gallons for basil, thyme and mint.

Check the volume a maker states for a box against that table before you buy. A box that holds a few gallons suits herbs or lettuce, not a tomato.

Window box brackets: wall or railing

Window box brackets come in two kinds. Wall brackets screw into the building, and rail brackets hook or clamp over a railing. Some boxes come with brackets and some sell them separately, so check what is in the box before you plan the mounting.

A wall bracket means drilling into a wall that belongs to the building. Ask your landlord, building manager or HOA before you drill, and get the answer in writing if you can. A rail bracket avoids holes, but it must fit your rail’s width and shape as the maker describes.

A box can also sit on the floor along the edge of a small balcony. That route needs no brackets at all and keeps the weight on the floor instead of the rail. If pigeons perch on your rail or sills, see pigeon-proofing a balcony before fitting boxes around deterrents.

Is it safe to hang a window box on a balcony?

Window box safety depends on the bracket, the load and where the box hangs. Clemson Extension says window boxes call for a strong support system and that containers are much heavier when watered.

Many maker pages list no load limit. Where no figure is stated, treat the limit as unknown, and never add weight on the strength of a guess. Mount the box exactly as the maker’s instructions say, and only where your building’s rules allow it.

Hang the box on the balcony side of the rail. A box on the outside face drips onto the street or the unit below and is hard to reach if a bracket shifts. What the balcony itself can carry is a separate question; our guide to balcony weight limits explains who to ask.

Window box drainage and the unit below

Window box drainage protects the roots. Clemson Extension says baskets and window boxes need drainage holes, and the RHS calls drainage holes essential and says to keep them clear.

Drainage also means runoff. University of Maryland Extension says a saucer helps when you garden above a neighbor’s balcony and that drained water can stain concrete and wood. A self-watering box holds water in a reservoir in the base instead. Our guide to planter saucers and drainage covers trays for boxes without one.

Wire hayrack boxes need a liner. Clemson Extension lists plastic, wood and long wire hayracks as the usual window box types; see coco liners for lining a wire frame.

What fits in a window box?

Window box plants should share the same light and water needs, because they share one box. Clemson Extension suggests a lightweight potting mix, trailing plants at the edges, and bushy or upright plants at the center or back. It also says containers may need water daily or even twice daily, and that it is time to water when the mix feels dry 1 inch below the surface.

For flowers, start with what your balcony’s light allows. A window box also makes a good spring present; see our Mother’s Day garden gifts. For edibles, University of Maryland Extension shows kale and leaf lettuce in a self-watering window box on a balcony. See growing herbs in pots for which herbs share a box well.