How to Build a Window Flower Box for an Instant Boost of Curb Appeal

Your first step in learning how to build a window flower box is to transform all of the wood you bought into the appropriately sized planks. 

For experienced DIYers, you’ll need to break out the table or miter saw to accomplish this task. Before you start, wear head, ear, and eye protection, along with chainsaw chaps, heavy pants, and boots. 

For newbies, talk to a tech at your local hardware store and have them make the cuts for you. You just need the dimensions listed below and the length of your window. Use a tape measure to get this information. 

Here are the plank dimensions you need for this job:

Cedar or Cypress Boards 

Start by making three planks 3 inches wide and the same length as your window. Next, make two planks 1-1/4 inches wide and the same length as your window. Finally, make three more planks 1-1/4 inches wide and 5-3/4 inches long. 

Pressure-Treated Lumber

Split your two-by-four planks up into three 8-inch-long pieces.

When you fill your window box with flowers, it will hold a lot of soil and plants. In other words, it’ll be heavy and require secure mounting. The best way to do this is to use simple cleats or clasps. 

Buy the appropriate cleats at the hardware store if you don’t have prior experience, but advanced DIYers can make their own by cutting three more 8-inch pieces out of the two-by-fours. 

• No matter which option you choose, affix a cleat to the wall by holding it steady as you push it flush directly against the exterior wall under the windowsill on the right or left side. 

• Drill two pilot holes on each side of the cleat and drive two concrete screws through these holes to secure the cleat to the wall. 

• Repeat the process for the other side and the remaining cleat.

Use one of the 3-inch-wide wood planks you cut and place it flat on a work surface. This is going to be the bottom of your DIY window box planter. 

• Place a similar plank directly on top of this bottom plank to act as the front (so put it on the front-facing edge). 

• Hold it in place and drill pilot holes every 8 inches. 

• Insert 1-1/4-inch screws into each pilot hole and tighten them up. 

• Repeat this process with the window-facing side and the remaining 3-inch plank.

Now it’s time to move on to the sides of the window flower box. This step requires the two smaller planks you made with a width of 1-1/4-inch wide. You will notice that these planks are a bit oversized, but this is what you want. 

• When affixing these planks to each side, allow the excess to overhang on the back side to cover up the exposed cleat attached to the house (this amounts to an excess of 1-1/2 inches).

• Now, hold each piece tightly to the window box and use a drill to insert the 1-1/4-inch wood screws.

Your final step in learning how to build a window flower box is to create the bones to attach the trim. 

• Grab the final cypress or cedar pieces that measure 1-1/4-inches wide and 5-3/4-inches long. Make sure the first trim piece is exactly flush with the top and bottom of the front-facing panel. 

• Next, tap the trim into place with simple nails. 

• Repeat this process on each side for the remaining pieces. 

• For most window boxes, one trim piece goes on the center, one on the left side of the box, and one on the right side of the box.

Your flower box is basically built, though you still have work to do. It’s time to paint and prime your new creation.

• Break out the wood filler and fill any nail and screw holes to ensure a smooth finish. 

• Start with the sash brush to apply primer to the box. 

• Allow the primer to dry for a couple of hours, and then apply one to two coats of exterior semi-gloss paint with the same brush. 

• Allow everything to dry, and then finish it off with medium-grit sandpaper to get a smooth finish. Since this is an outdoor window box that is subject to environmental conditions, this step is optional.

Now that you’ve successfully learned how to build a window flower box, it’s time to affix it to your windowsill.

• Hold the box in place against the cleats you attached earlier. 

• Drive four screws directly through the back panel, the side touching the cleats. 

• Test the box to ensure it is properly supported and, if not, attach some brackets where the box hits the cleats. As an added bonus, many of these brackets are decorative.

Without proper drainage, the box will eventually succumb to the added water weight from rainstorms in your area. 

To create drainage, lay underneath the window box and drill a few small holes through the bottom panel. Go for one hole every 6 square inches or so.

Now the fun part: filling up your brand new DIY window box planter. Whether you go with perennials or annuals (or both) or want to plant flowers that attract bees, follow these three steps:

• Start with 1 inch of gravel at the bottom to assist with water drainage. 

• Cover the gravel with an appropriate level of potting soil. Stop at around 1 inch from the top. 

• Finish it up with whatever plants strike your fancy, from daisies and dahlias to a variety of veggies.  

Contact a local gardener for advice if you need help deciding on the best plants for your region and climate.