I have a soft spot for the garden without either lawn or soil. There's something so determinedly cheerful about a tower block balcony festooned with geraniums, or a side alley glimpse of a tiny shady fire escape-cum-patio with tall bamboos in pots.
Their owners may bemoan their lot, but to me these gardens achieved entirely against nature are some of the most enchanting – not least when I think of the backbreaking work involved in transporting all that compost up five flights of stairs.
My friend Hetty has one of the best of these tiny gardens on her terrace in Waterloo. Whilst the inside of Hetty's flat is deliciously calm, verging on pure white minimalism, the balcony outside rages with Pop Art colour.
Frondy fennel and baby-blue lobelia sit side by side in this west-facing plot, which measures just 1m x 6m, also serving as a very welcoming entranceway to the front door. Seed germination is done on the kitchen windowsill but the display is added to with little plants from neighbourhood markets. The result is a brightly coloured mix, favouring tiny orange-flowered mimulus, postbox-red verbena and Barbie pink fuchsias. On a summer evening, a table is unfolded and Hetty and her boyfriend can eat supper outside watching the sun going down behind the London Eye.
So what's the secret of this kind of display? Certainly enthusiastic compost-lugging is a factor. New compost usually contains a couple of months' worth of plant food, which will see your young plants well on their way. At big DIY shops and garden centres you can buy moisture-retaining compost (20 litres, £2.98 at B&Q) or even specialist hanging basket compost (Westland, 60 litres, £7.99).
And watering is all-important, done carefully to avoid over-watering which will quickly leach all nutrients from the soil. However carefully you water, eventually your compost will lose its fertility: at this point, acquire a large bottle of a flower feed such as Miracle-Gro. The balcony is no place to garden organically: most bedding plants were brought back from South Africa and Latin America during the nineteenth century and need tropical levels of water, light and feeding.
But the real key to Hetty's balcony extravaganza, I think, is the intensity of the planting. Container gardening is one place to ignore all the rules about giving plants enough room to grow and expand into. When you are planting, squash as much in as you can, planting far closer together than you would in a flowerbed. Windowboxes should look "done" from the moment they are planted, or you'll wait all summer for them to fill out.
In terms of composition, take a look at the hanging baskets at your local pub – think about the angle you'll be seeing the plants from. Hanging and trailing varieties will dramatically increase the amount of flowers you get, making the most of the space underneath the basket – for example, ivy leaf pelargoniums like the Decora varieties will create a gorgeous wave of flowers. If you choose a wire-sided basket rather than a solid one, you can also cut carefully through the liner with a knife and plant the outsides of the basket, doubling the growing surface.
I have fuchsias and geraniums downstairs for neighbour-pleasing colour, but my bedroom windows have smart granite windowboxes planted with sedums in gravel. To me these spiralling mathematical rosettes of green and plum have a Zen-like quality of unassuming calm and grace.