A window box is really nothing more than a miniature garden. Here’s a quick guide to planting one.
1. First, make a mock planting plan. On a flat surface like the lawn or driveway, mark off the dimensions of the window box, and then arrange the plants in either two or three rows, placing the flowers where you think they will look best. Trailing vines should be planted at the front edges, taller flowers in the back, and plants with lots of flowers and leaves can be interspersed throughout.
2. Place pottery chards, small flat stones, or gravel in the bottom of the planter.
3. Fill the window box halfway with potting soil and then add several inches of compost. Mix well. If the soil is really dry you can add some water to moisten it.
4. Plant each flower or foliage plant one at a time, and about 2 to 5 inches on center. Add more soil as needed to surround the plant roots. Periodically tamp or compress the soil gently to remove air pockets. Add more soil as needed until the level is approximately one inch below the top of the window box.
5. Water plants thoroughly.