A window planter is placed outside of your home underneath a window. Another word for window planter is a flower box, because many people are using rectangular box shaped planters to hold flowers. It is extremely easy to water a window planter (or window box) because most of the time it is suited just under a window an you can reach them easily from the inside of the window.
You can use broken pottery pieces to allow for drainage when you are watering your plants or flowers. The disadvantage of these kinds of materials is that they are too heavy for a window planter. You would not be held responsible when someone is passing by and is hit by a fallen window planter. A better solution is to use lightweight packing materials (peanuts) as a drainage system for a window planter. Also watch out for plastic window planters, especially when they are hanging the whole day in the sun. Plastic can be weakened by the sun and crack resulting in the same problems as described before. The best thing you can do is using wood or metal window planters, they give a great atmosphere and character to your house with the right flowers and plants. You can paint the wooden window planters according to the style and color of your house.