Watering a window box may seem similar in theory to watering houseplants. However, you have additional variables to consider. Since window boxes are exposed to the elements, they often require more frequent watering than houseplants. They also require more frequent watering than a traditional garden, because there simply is not enough material in a window box to retain moisture as long as in a traditional garden. Much of the guesswork in window box gardening can be eliminated with the use of a moisture meter.
Install a moisture meter in each window box according to the manufacturer's instructions. Usually, you will just need to stick the probe into the soil.
Water your window boxes until you see water run out the drainage holes. Do not overwater.
Monitor your window boxes carefully, especially in dry, hot weather. Your moisture meter will tell you when the boxes need watering. In some cases, you may need to water your window boxes twice a day.