A lot depends on the size of your window boxes. One thing that will give you more choices of plants is to have window boxes that are deeper than typical. Shallower boxes dry out more quickly and have greater temperature swings. I use a moisture absorbing polymer in my pots as well to help maintain a more even moisture level.

Do you like having plants that grow up in front of your windows, or do you only want low-growing and trailing plants?

A couple plants I have grown in containers that are quite tolerant of heat and don't mind if it is a bit dry are Portulaca (moss rose) and the foliage plant Dichondra argentea 'Silver Falls', both of which will spill over the edge. If the boxes are large enough, vines like ornamental sweet potato (foliage plant) and Black-eyed Susan vines (Thunbergia) can be trained upwards on trellises or left to trail down over the side. Nicotiana/ornamental tobacco likes warmth and will bloom continuously for a long time, but you will probalby want to choose a relatively short variety like Little Nicky. I love its evening perfume, rather vanilla-like. Some of the shorter varieties of Cosmos, which likes heat, would probably do well and give you a bit of height also. Verbena is available as both short upright plants and trailing types. I have grown all these plants in full sun conditions that get up into the 90's, though our nights are probably cooler than yours, so I can't be sure that the flowering ones will continue to flower well in your conditions.

You could do a web search for heat-tolerant annuals and read about their size and moisture requirements to decide if you think they fit your needs.

I'll link to a blog that has some stunning cottage style window boxes to give you some inspiration.

Here is a link that might be useful: Deborah Silver's window boxes

Ahhhhhh! That link explains quite a bit!! It looks like the problem wasn't the heat, but the size of my planter (it's only 7" wide and 7" deep, 55" long). Thank you so much for the link and ideas, nhbabs! I'm far from and green thumb, and it can all get overwhelming! I'll rebuild the planter and keep plugging away (I'll get it at some point, right?!). Thanks again!

Btw, those window boxes in the link are breathtaking!!

I killed a lot of annuals in pots (I don't have window boxes) before I realized that larger pots and the water-absorbing polymer gel added to my potting soil would help my plants survive.

An alternative is to just plant succulents in the smaller boxes. There are a lot of Sedums, Echeveria, Sempervirens/hensand chicks, and other succulents that you can use in a shallower box, though you will still need to pay attention to the soil type (succulent potting soil has better drainage) and moisture levels to keep them from getting too dry for too long or too wet. They will probably have flowers for a shorter time period, but you can find a range of foliage and textures that will make them attention grabbing, though perhaps a bit less cottagey. If you can find one of Debra Lee Balwin's succulent books at the public library, it may give you some ideas for smaller window boxes.

Here is a link that might be useful: succulent containers

That is by far the best, most straight forward article on window boxes I have ever read!

I bookmarked that site. Some great ideas there. Thanks for sharing.

I think the plant called 'million bells" or calibrachoa is a very floriferous and heat tolerant plant and is self-cleaning meaning you don't have to cut off the dead flowers (deadhead). Do a search and see if I've got that plant right.
Dwarf sunflowers might work too. In my experience, sweet potato vine needs watering more. You can also plant herbs, which several are known for their tolerance of drought conditions. Thyme is low growing and I've also grown oregano which just took care of itself and bloomed alot.

I love the window boxes in that link, especially the white/purple exuberant ones. Love having the vines growing up on the sides, too. I have always wanted window boxes, but have been sure I would end up with ugly, dead boxes. Maybe it is time to take a chance! She is so right about making them wider than the windows and wide and deep.

So glad you asked swaybaby and glad nhbabs posted her link. I bookmarked it also, schoolhouse.

I'll add that other aspects of 'Dirt Simple' are well worth perusing IMO. Deborah Silver tends to create rather more structured gardens than cottage gardens, but some have cottagey aspects, and her winter holiday decorations have made a huge difference to the way I decorate outdoors for the holidays. She also maintains the FB page for Detroit Garden Works, her company, so there are lots of great photos there also. She's my favorite garden reading after GW. (and I have no stake in her company, just an interest in really good garden writing that mixes in excellent photos and wonderful design.)

I have to say I agree with you nhbabs. The first time I saw her work was in the link you posted at the top of the thread. I spent the better part of two days going through her blog and have bookmarked it. I think good design is good design period. It is figuring out why something works, the artists thoughts behind it, problems, solutions, documenting the journey so to speak that gets and holds my interest. I have found that true especially with gardens...perhaps because they are living and under constant change. I suspect that many gardeners who have a preference for a certain style or type of gardening are not hampered in their appreciation for other styles. Again, great link!

Nhbabs, I'm going back through the link tonight, you mentioning the outdoor holiday planting piqued my interest even more!! I'm rebuilding my window boxes next weekend :)