Last year I bought two plastic rectangular planters (I think they're 24"). I planted some spring bloomers in them, but once the heat hit they looked terrible and I ended up replacing them with some geranium and impatiens plants.

This year I would like to do something that will bloom all season, and also would like to try some combinations rather than a single type of plant. I've decided on a blue theme for my balcony this year. I bought borage seeds and forget-me-nots for the planters, and would like to find some yellow or white flowers to complement them. They would need to tolerate part shade, and I'd also like to have some things that won't bloom all at the same time, especially plants that will withstand the summer heat. Any suggestions?