Almost all plants will do well in window boxes providing you take into account the conditions it will be planted in. They are sun or shade, windy or sheltered. Also, the size of the container, cramming an Oak tree into a small window box might not work! 

Scented flower plants like sweet peas planted near open windows are a delight, as are herbs growing in the kitchen window box. I personally like the bright red trailing geraniums you see on the widows sill of houses in Germany and Holland. 

You can check out my other pages to select plants for the conditions they are going to grow in. I have also listed a few plants for full sun or shade here.

Sunny spots. 

Nasturtium has attractive leaves, bright edible flowers, 

Good in all weather. 

Petunia 'Marshmallow'  is good in all weather with  large flowers or choose from the  many other upright and trailing varieties,

Helichrysum petiolare - with its silver grey foliage looks great amongst bright flowers,   

Ivy,

Ivy,  try one of the  variegated types, 

Windy 

Pelargonium will  survive on windswept high-rise balconies. 

Shady

Best plants for a window box in a shady spot Busy Lizzies, Fuchsia 'Marinka' is excellent in window boxes, 

Ferns, smaller variety's are best for window boxes, Cyclamen - great in winter,

All year round colour. japonica - for all year round colour.

My favourite the red trailing geraniums you see on the continent works well in partial shade and full sun.

Most plants that grow in your garden will grow in containers, hanging baskets and flowerpots providing you select for tolerance to shade and full sun, etc,  but the big difference is, size matters

Make sure the plant fits the container.

Spring 
Rhododendron, Azalea, lilac, Primrose, Fritillaries, Ferns, Kalanchoe, Pansies.

Summer
 Abutilon, Bacopa, Callibrachoa-Million Bells, Coleus, Convolvulus, Dahlia, Fuchsia, Geranium, Herbs, Impatiens, Larkspur, Lobelia, Marigolds, Nasturtiums, Oxalis, Petunias, Scabiosa Wave, Petunias, Ornamental grasses, Miniature Roses. 

Autumn
Begonias Fuchsias Impatiens Asters Cyclamen Gentian Heather Heuchera Marguerite Daisies Mums Osteospermum Cotoneasters Pyracanthas, 

Winter plants,

Boxwood Privet Juniper Wintergreen 

To add height to a container

Canna Colocasia (Elephant Ear) Brugmansia (Angels Trumpet) Phormium Salvia Pennisetum setaceum ‘Rubrum’ (Fountain grass)

To fill a container

Artemisia Creeping Jenny Helichrysum Houttuynia Ivy Moss Plectranthus Sedum Spider Plants Sweet Potato Vines Transcantia Wandering Jew 

Climbers for containers,
Clematis Cartmanii Hedera Helix 

Shrubs 

Euonymus Fortunei Pittosporum Tenuifolium Skimmia Japonica Hosta Pennisetum Setaceum

 Plants for both colour and interest to plant in hanging baskets throughout the year

Plants for a Spring hanging basket 

 shade / herbaceous borders / clay / window boxes / pathways / Winter colour  / dry / sandy soil / chalky soil / slopes / bog gardens / ground cover / Low maintenance / home