It's really hard to find something for a windowbox that can take the wind. I have generally resorted to wax begonias in my windowboxes most years, although this year I'm trying some hummingbird plants (salvias - eg., S. coccinea 'Lady in Red' and 'Hummingbird Red'). The begonias have a dense habit and thick leaves that seems to help them remain stiff when it gets windy.

I had geraniums (pelargoniums) for a number of years, although not in a windowbox. They are pretty stocky and should do well. The main drawback is that when it gets too hot, they stop blooming. They are actually cool-weather plants - ie., much of Europe except the furthest southern parts, don't have the type of sustained heat and humidity that many U.S. locales (both north and south) have in summer. Although the past couple summers, many Europeans have been treated to a few unusual (for them) heatwaves which have sadly caused many deaths as most have no AC.

Another possibility that I have seen work in hot windy is the vinca flower ("Madagascar vinca"). The plant is remiscent of impatiens but the stems and leaves are very different and these can take full hot windy blazing sun and come in a myriad of colors. Another choice might be the marigold. They have always been standard full-sun plants, come in different colors including the yellows and oranges, and come in different sizes. About 7 years ago when one of my sisters was still in an apartment with a south-facing balcony, she had marigolds and dwarf sunflowers in her windowboxes.

As something totally different - you could consider some sedum, whether the various creeping ones (eg., stonecrop), or the more upright ones (if pruned to keep compact like "Autumn Joy").. They are later bloomers but with the fleshy leaves, they might be less likely to dessicate in the wind.

How about some of the Mexican sages, which come in all sorts of colors and should be pretty heat tolerant? Mix them up with some of the Mediterranean herbs like rosemary, lavender and santolina, which all have nice flowers and grow in hot, dry areas, and you've got a great palate to work with.

Also, in addition to the sedums Jennie mentioned, check out the various ice plants (delospermas, drosantherums, aptenias and saxifragas); all succulent, with pretty flowers and/or leaf colors.

Finally, don't forget small grass species, which you can find in many different shades, to use as filler in between the flowers.

Enjoy!

Jim

Thanks guys. All good suggestions. Anybody have any thoughts on moss roses as an option (portulaca)?

I've used those, but unless you like to water twice a day, they dry up pretty fast. They do come in hot colors, though, and they hang down.

This year I'm using Million Bells, Icicles and Diamond Frost euphorbia. Hope it all works.