A quick explanation…
Annuals – hardy and half-hardy
These have a one-year or one growing season cycle – from seed to flowering to dying. If they’re hardy they’re likely to survive the winter and withstand frost. If they’re half-hardy then frosts might be a problem, except that window-boxes close to heated buildings tend to benefit from that warmth. Depending on the exposition of a window-box, it should be possible to take more risks leaving plants in situ, outdoors, over winter.
Hardy annuals include sweetpeas, dill, ammi, cornflowers, ammi, calendula… and half-hardy annuals include cosmos, nicotiana, zinnias and verbenas.
These usually have a two-year cycle. After flowering in the first year or cycle, the plants put out seeds, and repeat a second cycle before their inevitable demise.
Biennials include certain types of poppies, wallflowers, euphorbia and dianthus.
Unlike annuals and biennials, perennials repeat their life cycles several times over, year after year. Perennials include several types of euphorbia, hellebores, peonies, phlox, daphne, and hydrangea – and of course the ever-reliable geraniums as well as many types of pelargoniums.
Window-boxes which consist mostly of perennials will help maximise greenery and minimise time, effort and cost.