You'll get the best results by building your indoor window planter as wide as the outside dimension of the window casing but no wider than the interior windowsill. The planter should be at least deep enough and high enough to accommodate a 6-inch diameter flowerpot. We suggest that you use 1'' x 8'' lumber for the ends and faces and 1'' x 6'' lumber for the bottom, at a minimum.
Build Indoor Window Planters
Flower boxes aren't just for the outdoors — they add life and color to any room in your house. Building an indoor planter box yourself is a fun weekend project and you will enjoy the results for years to come.
Plan Your Indoor Window Planter
The first step in building indoor window boxes is deciding what kind of wood to use. First, you should consider the look of the finished product. You can choose a wood and finish to complement your interior woodwork — or for a more personalized touch, try your hand at a decorative painting technique like sponging or stenciling to match your décor.
If you do plan on painting the planter box, build it with any select (knot-free) grade of wood such as pine or poplar. If you prefer a natural finish, consider a fine hardwood such as oak or maple.
Waterproofing the interior of the planter will allow you to put soil directly into it. However, not all plants have the same watering needs. If you have a range of plants in the planter, it is best to pot them individually. This will make it easier to change out plantings, if need be, and maintain the planter.
How to Build the Indoor Window Planter
Step 1: Determine the Dimensions
Step 2: Cut and Mill the Parts
Cut the pieces for the front, back and ends from your "good" lumber. You can use a lesser-grade lumber, such as No. 2 pine, for the bottom. Use a circular saw or table saw when cutting the pieces for your planter.
For the front and back, cut or plane a 5-degree bevel along the bottom edge of each piece.
To make the ends, cut (with the grain) two pieces of 1'' x 8'' lumber to be 6'' wide. This will correspond to the height of the end. Then cut the sides of those pieces at an 85-degree angle to create a 5'' and a 7'' base on these two trapezoidal pieces.
To create the bottom, set the saw's bevel adjustment to 5 degrees, and cut (with the grain), the piece of 1'' x 6'' lumber so that it is 5'' wide. Use a plane to bevel a 5-degree angle on the two lengthwise edges. Next, cut the bottom piece 3/4'' shorter than the length of the front and back pieces so it will be recessed 3/8'' and fit properly with the front and back when assembled.
Step 3: Sand Parts Smooth
Use a rubber sanding block or finishing sander and sand all the pieces smooth. Start with a coarse sandpaper, such as an 80-grit paper, then use a finer 120-grit and finishing with 220-grit paper. The coarser paper removes rough edges and splinters, while the finer paper prepares the wood for finish. Always sand along the length of the boards with the grain.
Step 4: Assemble the Indoor Planter
Using a small brush with a 1/2'' tip, apply waterproof wood glue on all edges of the bottom and rest it on 1/2'' thick spacers. Clamp the two ends onto the bottom and pre-drill 3/32'' pilot holes for the fasteners. Secure the ends to the bottom with 6d galvanized finishing nails.
Glue the front and back edges of the end pieces; similarly clamp and attach the faces to the bottom and the ends. Wipe off excess glue with a damp cloth and touch up with your sander as needed.
Pre-drilling prevents the fasteners from splitting the wood (especially hardwoods) and makes it a lot easier to accurately drive the nails or finishing screws.
Step 5: Waterproof the Interior
Mask the top edges of the planter with masking tape and apply a synthetic rubber coating to the interior surfaces. Synthetic rubber coating is available in spray cans or as a brush-on liquid. It will form a flexible, waterproof surface when it dries.
Step 6: Apply a Finish
When the interior surface dries, reposition the masking tape over the top inside edge and finish the sides and top with any interior paint, stain or polyurethane finish.
Step 7: Install the Planter Under a Window
Once you've put the planter together, it's time to put it in place. To mount the planter on shelf brackets or on a shelf supported by brackets, you'll have to install those brackets into solid wall framing with screws. If you mount the planter to the brackets, glue 1/2'' wood spacers to the bottom at the bracket locations.
Step 8: Setting Your Plants and Flowers
If you plan to place soil directly into your planter, don't forget to provide drainage. Put at least 1-1/2'' of crushed stone in the bottom of the planter, covering it with a layer of landscape fabric before you add soil and plants. If you prefer to keep plants in their pots, simply put the pots in the planter and fill around them with sphagnum moss, pine bark or similar mulch.
Set smaller pots on blocks of wood or on some mulch so their tops will be even with those of larger pots.
Great job! Now you know how to build indoor planter boxes of your own and you've given your windows a little extra life with a floral flourish.
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