How to Build a Flower Box for Windows, Railings and Porches

A flower box looks light, but once it is filled with wet soil it can weigh 60 pounds or more, which is why so many store-bought boxes sag, rot or pull away from the wall within a couple of seasons. Learning how to build a flower box yourself lets you size it exactly to the window, choose wood that will not rot, and hang it on brackets strong enough for the load. The build itself is simple: five boards, a handful of screws and a few drainage holes.

Here is the core method. Measure the window or railing, cut a bottom, two long sides and two ends from 1 by 8 or similar rot-resistant boards, screw and glue them into a box, drill drainage holes in the bottom, and hang it on heavy-duty brackets fastened into framing, not just siding. Line the inside or use a removable plastic insert, and the box will last years longer.

The same approach works for a window box, a railing planter on a deck or a porch-front box. Only the mounting changes.

How to Build a Flower Box: Sizing It to the Window

A window box usually looks best when it matches the full width of the window, including trim, or runs a couple of inches wider on each side. A box narrower than the window tends to look like an afterthought. For depth, 7 to 8 inches front to back gives roots room and holds enough water to survive a hot afternoon. Height is typically 7 to 10 inches, which is roughly a quarter of the window height on most standard double-hung windows.

Keep long boxes manageable. Anything over about 4 feet should have a center divider or a third bracket, because the bottom board will sag under soil weight. If your window is very wide, two matching boxes side by side are easier to build, hang and remove for cleaning than one giant one.

For railings, measure the rail width and the span between posts. A box that sits on top of a 2 by 6 rail needs a cleat or saddle on the bottom, while one that hangs on the side needs brackets made for the rail thickness.

Choosing Rot-Resistant Wood

Cedar is the popular choice for a cedar flower box because it is light, resists decay and takes paint or stain well. Redwood and cypress are similar. Untreated pine is cheap but will rot quickly in constant contact with wet soil unless you line the box thoroughly and paint the outside. Cellular PVC trim boards are another rot-proof option that cuts and screws like wood and paints well.

Use exterior wood glue and coated or stainless screws so fasteners do not rust and streak the finish. Inside the box, a liner keeps soil from touching the wood: heavy plastic stapled in place with slits at the drain holes, a coco coir liner, or a plastic planter insert that you can lift out at the end of the season.

Tools and Materials

• 1 by 8 cedar or similar boards (a 4-foot box takes about 12 linear feet)
• Exterior wood glue and 1 5/8 inch coated deck screws
• Cordless drill driver kit with a 1/2 inch spade or twist bit for drain holes
• Circular saw, miter saw or handsaw
• Tape measure, speed square and clamps
• Heavy-duty steel window box brackets or railing brackets rated for the load
• Exterior paint or stain, plus a liner or plastic insert
• Safety glasses and hearing protection when cutting and drilling

Step-by-Step Build

1. Cut the bottom. Cut the bottom board to the finished length minus twice the thickness of the end boards, since the ends will cap it.
2. Cut the front and back. Cut these to the same length as the bottom. They will attach to the bottom’s edges.
3. Cut the ends. Cut two end pieces to the full depth of the box (bottom width plus front and back thickness) and the full height.
4. Drill drainage holes. Drill 1/2 inch holes every 4 to 6 inches along the bottom board, staggered in two rows. Drainage is the single biggest factor in keeping plants and wood healthy.
5. Assemble the long sides. Run a bead of glue along one edge of the bottom, clamp the front board in place and drive screws every 6 inches. Repeat for the back. Pre-drill near ends to avoid splitting.
6. Attach the ends. Glue and screw the end pieces over the box ends, driving screws into the bottom and side boards.
7. Add trim if you like. A 1 by 2 cap around the top rim or a decorative face board stiffens the box and hides end grain.
8. Finish the outside. Paint or stain all exterior faces and the bottom. Leave the inside bare or seal it if you are using a liner.
9. Line it. Staple in plastic or set in an insert, and cut holes that line up with your drainage holes.

Mounting Securely Under Windows or on Railings

The mounting is where safety comes in. A filled 4-foot box is heavy, and it hangs over a walkway or lawn. Fasten brackets into wall studs, the window framing or solid blocking, not into vinyl or fiber cement siding alone. On brick or stone, use masonry anchors sized for the bracket holes. Space brackets no more than about 24 inches apart, and set the box an inch or two below the windowsill so water does not splash against the window and so the sash can still open.

Mount the box empty, then fill it in place. Working from a ladder with a soil-filled box is how people get hurt. Set the ladder on firm, level ground, keep three points of contact, and do not lean out to the side; move the ladder instead. For second-story windows or any box you cannot reach comfortably from a short stepladder, hire a handyman or install from inside if the window allows it safely. Anything at two stories or higher is a job for a pro with proper equipment.

On a deck or porch railing, use brackets made for the rail size, or build a saddle from two cleats screwed to the bottom so the box straddles the rail. Through-bolt or screw the brackets so the box cannot be knocked off by a bump or high wind.

Filling, Planting and Seasonal Care

Use a quality potting mix, not garden soil. Garden soil compacts in a box, drains poorly and adds a lot of weight. A light potting mix with some compost holds moisture without turning to mud, and it keeps the total load on your brackets lower. Fill to about an inch below the rim so water does not spill over the front edge every time you water.

Plant the back row with taller, upright plants, the middle with mounding fillers and the front edge with trailing plants that spill over the face of the box. Match plants to the light: a north-facing window wants shade lovers, while a south or west window in full sun calls for heat-tolerant annuals or herbs. Feed with a diluted liquid fertilizer every couple of weeks during the growing season, since frequent watering flushes nutrients out of a small container fast.

At the end of the season, pull spent plants, lift out the insert or liner if you used one, and let the wood dry out over winter. Check the screws and brackets each spring, touch up chipped paint or stain, and replace any liner that has torn. In cold climates, emptying the box before hard freezes keeps expanding wet soil from pushing the joints apart.

Troubleshooting

• Bottom sagging: add a center bracket or a 1 by 2 cleat screwed under the bottom board.
• Soil staying soggy: add more drainage holes and mix perlite or bark into the potting soil.
• Water staining the siding: leave a 1/2 inch gap behind the box with spacers so it drains and dries.
• Box splitting at the corners: pre-drill every screw and use glue so the joint shares the load.
• Plants drying out fast: use a self-watering insert or a liner with a shallow reservoir, and water daily in heat.

Cost and When to Call a Pro

A DIY cedar box usually costs modestly in lumber, plus brackets, which vary from basic steel to decorative wrought styles. Finished cedar or PVC boxes from a store or custom builder cost more, and installation on upper floors adds labor. Hiring a handyman makes sense for second-story windows, masonry walls where you are not comfortable drilling anchors, or when you are not sure where the studs are behind the siding.

Built well, a cedar flower box can last a decade with a fresh coat of finish every few years and a liner swap when it wears out.

Once you have learned how to build a flower box for one window, the same cut list scales easily to matching boxes for the rest of the house, a porch rail or a fence line.