Europe is famous for it's window boxes. Walk the streets of France, Italy, and Germany and you'll see a charm that's unlike any other country. They're very popular on city streets and homes where little to no yard is present. As you walk the streets you'll them hanging from buildings on every window. If they can put them on their windows then they'll litter them on their steps, and I mean every single one of their steps!
Wrought iron window box cages have been made in the United States to simulate that feel. Although many European style window boxes are made of wood, I have found that metal ones with scrolls or designs look most authentic. If you are looking to emulate the look then find a metal cage where you can put a liner or sit potted plants inside the cage. There are a few companies that make European style window boxes and can even custom make them sized to your windows.
The advantage of potted plants is that planting is super easy and switching out plants takes very little effort. You'll want to speak to the manufacturer of the cages before you order them to measure the inside depth that way you can make sure it will accommodate whatever pots you plan to put in there. Sometimes a metal or plastic liner can be used in place of potted plants as well which can make planting easier for you rather than having to hang out a window.
Hayrack window boxes are a cheap and common way as well, though most hayrack window boxes are made of thinner metal so these types of cages are usually small and only work for two foot and three foot windows only. They can also twist and sag over time if they're made from cheaper, thinner metal. One of the disadvantages to the hayracks is that the coco fiber usually has to be replaced every year and dirt and water can fall through them resulting in drier soil and more watering maintenance.
Some companies even make self-watering European window box cages, so if you're really looking less maintenance it's easier now than it's ever been.
About the Author
Matthew Buquoi is the owner of Flower Window Boxes, a manufacturer of wrought iron metal flower box cages and self watering liners. They specialize in custom built window boxes that ship out fully assembled and built to order all over the United States.