FORCED INDOOR BULBS. Once these have flowered don't throw them out. Cut off the heads (unless you want seed) then put them somewhere that the leaves can get the sun. This will feed the bulb for the next year. Once the leaves have died you can plant the bulbs outside and they will flower at the normal (unforced) time next year. The narcissus Tete-a-tete is particularly good, and provides early colour and a delicate fragrance too.
Below I have listed groups of plants. I have tried to include at least four plants in each list as you may not be able to find all of them, although, unless you have a very large windowbox, I would recommend that you have just three in each box.

Theme	Plants	Comments
Thyme	Thymus praecox, wild thyme Thymus pulegioides Thymus leucotrichus Thymus citriodorus	Thymes make a very fragrant, easy to care for windowbox, and an excellent choice for windy sites. The flower colour will be pinky/purple, and you can eat the leaves if your air is not too polluted. Try to get one variegated thyme to add a little colour when there are no flowers.
Herb	Sage, mint, chives, thyme, rosemary	Get the plants from the herb section of the supermarket, so you can eat the leaves. Do not include basil as it need greater fertility than the others. Pot the rosemary up separately if it grows too large.
Mints	Mentha longifolia, horse mint Mentha spicata, spear mint Mentha pulgium, pennyroyal Mentha piperita, peppermint Mentha suaveolens, apple mint	Mints are fairly fast growers, so you could start this box with seed. They are thugs, though, and will very soon be fighting for space. So you will either have to thin and cut back or else you will end up with one species - the strongest. The very best mint tea I ever had was in Marrakesh. A glass full of fresh mint was placed in front of me, and boiling water was poured into it. Then I was given a cube of sugar to hold between my teeth while I sipped the tea. Plant this box and you can have mint tea for months.
Heather	Too many to list	For year-round colour try to plant varieties that flower at different times of year. Heather requires acid soils, so fertilise with an ericaceous fertilser, and plant in ericaceous compost. Cut back after flowering and remove the cuttings. It is best to buy plants as heather is slow growing.
Blue	Ajuga reptans, bugle Endymion non-scriptus, bluebell Myosotis spp., forget-me-not Pentaglottis sempervirens, alkanet	This will give you flowers from March till July. The bluebells should be bought as bulbs, as seed will take a few years to flower. The others can be started from seed.
Yellow	Anthyllis vulneraria, kidney vetch Geum urbanum, wood avens Lathryus pratensis, meadow vetchling Linaria vulgaris, toadflax Lotus corniculatus, birdsfoot trefoil Primula vulgaris, primrose Ranunculus acris, meadow buttercup Ranunculus ficaria, lesser celandine	These will give you flowers from May to October, and if you include the primrose, from February. Try to include a vetch as they can climb or trail so occupy the space that other plants can't. All can be grown from seed.
White	Trifolium repens, white clover Bellis perennis, daisy Digitalis purpurea alba, white foxglove Alyssum maritimum Redsea odorata, mignonette	All can be grown from seed. The clover and daisy will have to be cut back as they will take over. The clover roots add nitrogen to the soil. The mignonette flower doesn't look very special, but the fragrance is wonderful, and the alyssum smells of honey.
Pink	Lychnis flos-cucli, ragged robin Scabiosa columbaria, small scabious Symphytum officinale, comfrey	The comfrey will try to take over. Its leaves make an excellent fertiliser, and are very good on the compost heap, though windowbox gardeners rarely have one.
Fragrant	Lonicera spp., honeysuckle Alyssum maritimum Redsea odorata, mignonette Lathyrus odoratus, sweet pea	The sweet pea will need twine or something to climb up, so is suitable if you have sliding windows or window that open inwards. You will be rewarded by a fragrant curtain every time you open your window.
Spring bulbs and late wildflowers	Galanthus nivalis, snowdrop Narcissus pseudonarcissus, narcissius Crocus purpureus, crocus Cyclamen spp.	The idea of this box is to maximize your space. The bulbs (cyclamen has a corm) will flower and do their stuff early in the year. After flowering cut the heads off as you don't want them making seed, but leave the leaves as they fatten up the bulbs to store energy for next year. The foliage of the wildflowers will hide the bulb leaves to some extent. Then the wildflowers take over and flower till autumn
Spring bulbs and late wildflowers	Aster spp., Michaelmas daisy Linaria vulgaris, toadflax Lonicera spp., honeysuckle Succisa pratensis, devil's bit scabious Mentha pulgium, pennyroyal