I could also use some ideas. I have (6) window boxes -- 2 face due north on the house, and the remaining (4) face east.

I tried petunias last summer but wasn't particularly pleased with the results -- too hot and too humid for them, I think.

My boxes are wooden, and I use plastic liners in them, about 30" l x 8"w x 10" d. They are standard-size. I use a good, organic potting soil, and change it with each season, but I have been over-run with whiteflies, both in the pansies this fall and winter, and the petunias last summer.

I've actually never done windowboxes per se, so I can't help you with how to secure a windowbox, but I do have a lot of things in pots on the front and back porches. (As well as on the steps, hanging from the fire escape, hanging from the railings....)

I always have a basket of heliotrope/alyssum/catnip, so I have something nice to smell (and out of reach of the cats!) I like to use creeping jenny or lysimachia (usually the gold kind) to soften the edges of the pots.

They need specialized winter space, but I've started growing potted fig trees in pots on the porch.

If you can assure regular water, perhaps you could grow a mini kitchen garden -- with cut-and-grow lettuces, chives, small varieties of annual herbs you can clip and use. (Useful is always nice, too!)

If the water and light are good, your options are nearly endless! You can grow things to trail down below the window; string fishing line up to the top of the window and you can grow things up, to give you a nice green "curtain" -- all sorts of possibilities!

Here is a link that might be useful: Sample of plants suitable for windowboxes

I have several window boxes. Most of my face north or east. My favorite combo is this: impatians, victoria blue annual salvia and vinca vine. On the east side I add a couple of wave petunias. I tried the wave petunias on the north side but they did not work...probably not enough sun. The combo I use seems to work really well. I have window boxes on my 2nd story and the vinca vine usually grows long enough to reach the top of my 1st story windows. I put fertilizer granuals in as well as soil moist (water asorbing polymers). This helps keep everything moist.

This year I am trying lime colored licorice vine. The jury is still out on it.

Does anyone know where I can find a specific wood window boxes with this size other than making my own? thanks.