This year I'm growing a new introduction, `Maverick Mixed'. "The ultimate top-quality garden geranium" says the seed supplier Thompson & Morgan. We shall see.
To flower most abundantly geraniums need sun, but a window-box in full sun is more difficult to manage than one in part shade. Compost dries out faster, and watering may need to be done twice a day.
A window-box looks best if it fits as exactly as possible the size of the window. Small plastic containers balanced on large sills have an uneasy, we're-not-stopping feel to them. Made-to-measure wooden containers are ideal. You can line them with heavy-duty black polythene, with a few holes punched in the bottom. The bigger the container and the larger the volume of compost, the easier it will be to keep.
Painting wooden boxes can trap you in a tyranny of annual repainting. Wood stains are less demanding and, watered down, give you some pleasantly murky greens and blues. Let the plants boss the box rather than the other way around.
Profusion is the effect that you want, but in a window-box you won't get it without a dedicated regime of feeding and watering. Slow-release fertilisers are excellent for this kind of gardening. So are the expanded polystyrene granules you can buy, which sop up water faster than sponges and then release it slowly as the plants need it. Both these can be mixed into the compost at planting-time.
Plastic boxes are cheap, but are not always sturdily enough made to take a full load of compost. They start buckling in the middle and sag outwards in a dispirited way. Whatever you use, make sure it is securely anchored. Some town houses still have the ornamental cast-iron surrounds of the late 18th and early 19th centuries to fence in window-boxes. If you've got them, flaunt them.
Often boxes look better from the outside than they do from within. Flowers naturally turn towards the light, so from inside, you are backstage, as it were, looking at the supports rather than the painted backdrop. You can get over this by dropping the level of the window box slightly, so that you look down on your flowers, but whether this is possible depends on the type of windows and sills that you have.
For a suitably lush effect (and window-boxes must be lush - nothing looks meaner than a mean window-box, with its plants as distant as oases in the desert) you need plenty of leaf. Include at least one good foliage plant in each mixture.
Helichrysum petiolare is a natural, because it is not too bossy, threads itself about well, climbs and cascades. The standard version has grey, felted leaves, but there is a good lime-coloured variety, `Limelight', and a cream variegated one as well. The grey-leaved one loves heat and will not flag in a sun-baked position. The sulphurous one is better in part shade. Use them with argyranthemums, white petunias and shrubby santolina, with perhaps some variegated ginger mint binding the whole cast together.
An average size window-box, 30-32 inches long, will easily accommodate a dozen plants, perhaps four each of the marguerites and petunias, one helichrysum, one santolina and two ginger mints. This will give a cool effect - lost, though, if you live in a white painted house. White-flowered window-boxes look best set against grubby stone, especially the dark grits and granites of the north.
Grey helichrysum also looks good weaving its way round ivy-leaved geraniums, such as the pale pink `Madame Crousse', with a mixture of lobelias stuffed into the gaps. `Cascade Mixture' trails elegantly and has flowers in many colours. Add some deep purple heliotrope if you want an extra benefit from your window box. It smells gorgeous.
For a much warmer effect use the greeny-yellow helichrysum `Limelight' with a golden variegated ivy and some brilliant yellow and blue pansies. These need regular dead-heading if you want them to give a long display, but they are reliable flowerers and have the sort of squidged-up faces that always make you smile. Ivy is very slow-growing, so you will need to splash out on some decent-sized plants. They are at least perennial and, with care, could be with you for several seasons. If you get tired of them in a window-box, you can pot them up and use them as pot plants inside.
Variegated ivy also combines well with white-flowered, variegated pelargonium. Add the pale lemon lance leaves of hosta, a golden-leaved vinca, campanula, purple pansies and feathery, bleached-looking grasses. If the window-box will stand it, interplant with a lemon-coloured lily such as `Limelight'.
Nasturtiums are also plants that raise the spirits. There is nothing restrained about a nasturtium. It is a helter-skelter opportunist and will probably swamp any plants that you put with it. Use it on its own, sowing seeds direct into the window-box. When you get rid of your spring display, replace the old compost before replanting for summer.
You will get less of a hiccup between displays if you start off your nasturtium seeds in pots inside and set them out as growing plants. `Alaska' (Thompson and Morgan, pounds 1.49) is a good mixture, with leaves splashed and mottled with cream. I'm also fond of `Empress of India' (Thompson and Morgan, pounds 1.39), with dark velvety leaves and equally velvety flowers, deep, deep red.
Any annual that lolls gracefully is a natural contender for a window- box. The swan river daisy, Brachyscome iberidifolia, could easily be used this way. It has lax stems topped with small daisy flowers and the foliage is lush and ferny enough to furnish a window-box with ease. The plants do best if you pinch them back early in the season to encourage thick, bulky growth. In the wild, they favour slightly acid soils, so you may find that you need to dose them occasionally with iron sulphate (Sequestrene). If the leaves turn yellow, they need treatment.
The wild species have been interbred to provide a series of different colours and types: `Blue Mist', `Yellow Mist', `Pink Mist'. Though they rarely grow more than 6in high, each will spread to cover 2ft or more. Swan river daisies do not immediately flag if they are dry at the roots, and they will flower for months.
As they are quite delicate they do not need anything too bossy with them. If you want height, interplant them in a window-box with daisy-flowered argyranthemums. This summer, I'm trying a new white one called `Summer Stars'. The leaves are grey and very finely dissected, as in the better- known variety `Chelsea Girl'. We are spoiled for choice here.