Planting the Window Box
Use a soilless bagged mix, which is usually a mix of peat or coir, perlite, and/or vermiculite. Provide some nutrients and help retain moisture by adding screened compost to a soilless mix—it can be up to 20 percent of the mix. Moisten the soil mix with warm water before planting. Set the plants into the mix to the same depth they were in their containers. Firm them in and water again. Allow about an inch of space between the soil and the top of the box.
Keep Them Looking Good
Water. If a window box sits in direct sun for much of the day, you may need to water both morning and evening. Water until the water runs out of the drainage holes, and don't assume that because it's rained, your plants have enough moisture. If your plants droop in the sunlight but recover in the shade or after sundown, they need more water.
Try not to let the soil completely dry out, especially if you use a soilless peat-based mix. Once peat dries, it no longer absorbs water but actually repels it, becoming almost impossible to rewet.
If the peat does dry out, try this remedy from John Begeman, agriculture and natural resources agent at the University of Arizona: Add a few drops of liquid dishwashing detergent to a quart of water; then water slowly with this solution. The soap breaks the barrier to the soil and allows the water to be absorbed. The results should last a long time, so you shouldn't need to use the soap solution more than once or twice in a season.
Mulch. Mulching the soil in a window box helps hold moisture in and gives a finished look to the box. Whatever you use, be sure to thoroughly water the plants before adding the mulch, or the mulch could act as a barrier to the water. Periodically check to be sure that water is getting through to the soil and isn't running off the top of the mulch. Use cocoa shells, finely shredded bark, or moss.
Fertilize. Soil mixes don't provide enough nutrients for the plants to thrive the entire season, and what nutrients there are can wash out the drainage holes when you water. So to keep your plants healthy and growing, fertilize every two weeks with seaweed or fish emulsion, diluted to half the recommended garden strength.
Deadhead. Dead and decaying flowers, stems, and leaves can be sources of fungal growth, so be sure to pinch them off when you see them. Regular deadheading keeps annuals flowering instead of going to seed, and keeps plants looking their best.