A few years ago I purchased an over-the-window plant hanger that had a curved metal bar that just slipped behind the window frame. It was not screwed into the wall or frame, the tension would hold it in place. It had a swivel hook on the other end. I would like to find more of these hangers if anyone knows where to purchase them.
By RocknChr from Ord, NE
Have you tried Lowe's? Good luck.
If you can't find your plant hanger, you can probably also use an over-the-door wreath hanger. These are fairly easy to find. I bought mine after Christmas at a dollar store. You can bend the wreath hanger in several places if you need it to jut out from the window more. You can use most of the over-the-door hangers in the same way you'd use your plant hanger. Buy the flat metal ones, not the ones that are round or the clear plastic hangers. They sell from 99 cents to $12. I got mine 2 for a dollar at an after Christmas sale.
Here's one for $1.84: http://www.wasserstrom.com/restaurant-supplies-equipment/Product_112905
Here's a photo of a wreath hanger: