Answer: For an east exposure, impatiens should do well as should coleus. Fora south exposure, you must select plants that can take heat as well as full sun. These might include zinnias such as "Profusion", red or blue lobelia and small dahlias. The flowering vinca should also do well. These are not frost tolerant plants so they should not be planted out until all danger of frost is past.
You should start with fresh soil each year. It becomes compacted and exhausted in one season and replacing it also reduces the chance of carrying over pests and diseases. The boxes should also be scrubbed out, sprayed with a bleach and water solution to disinfect them and resealed if needed.
You will need to fertilize regularly according to the label instructions. Use a complete water soluble fertilizer for flowering plants. Overfertilizing is no better than underfeeding. A top dressing of compost and a dose of compost tea can be beneficial, as can using a fertilizer with "minors" as well as the big three nutrients, N-P-K.
You will need to keep the soil moist but not sopping wet. In the beginning of the season you will be watering less than at the height of summer when it is hot and the plants are relatively large in relation to the container.
The biggest problem with window boxes is usually the small volume of soil in relation to the plants. This means watering is critical. You might try using the waterholding polymer in the soil mix and see if that helps; use your finger stuck into the soil to see if you need to water or not. The south boxes will need more water than the eastern ones.
At the same time, the boxes must have drainage holes in the bottom to allow excess water to drain out. Only a thinlayer of gravel is needed, if any. Most potting soils for containers are well drained, especially if you use a soilless mix.
The soil in window boxes also tends to heat up during the day and can cook the plants roots, smaller boxes offer less protection than larger ones. It may be that your boxes are simply not deep enough or are not wide enough.
Finally, you might want to take a look at a book or two about container gardening since you have had so much trouble with it in the past. One I like is called "Container Gardening for Dummies".
I hope this helps you trouble shoot.
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