Seven plants is plenty for a pot 15in or more (not less) across. Choose at least one kind of plant with bulky, interesting foliage and use that as a background. Variegated helichrysum is easy. Choose the small-leaved "Roundabout", which will not swamp its neighbours, and match it with something airier, such as the beautiful Lobelia richardsonii. Bushy at first, this soon begins to trail. You'll need a centrepiece for the pot. I've suggested Begonia fuchsioides, but it could well be a new daisy-flowered brachyscome.
Tomatoes are another imperative. Even (especially) in a concrete yard. Or a balcony. Or on an open stairwell.
You don't need earth. Get a growbag. Each bag will support three tomato plants. Most garden centres have standard varieties such as "Alicante", "Moneymaker" and "Ailsa Craig". "Shirley" is best in a greenhouse. To avoid the problems of staking, plant bush tomatoes such as "Tornado" or "Red Alert" - both excellent in flavour. Don't overwater.
Buy one plant or shrub to look forward to later in the season. I like the hardy Fuchsia magellanica "Versicolor". Only frost stops its profuse flowering, and it foliage is pink, green, grey and bronze. Before it flowers it makes a good backdrop for "Blue Parrot" tulips.
Buy string and twisty plastic ties to keep up with sweet peas, clematis, climbing roses, tomatoes and fan-trained fruit trees that will need tying in over the next couple of months.
1 grow bag (Fisons) pounds 1.76
3 tomato plants pounds 1.50
3 variegated helichrysum pounds 3.60
3 Lobelia richardsonii pounds 3.60
1 Begonia fuchsioides pounds 1.20
1 ball jute (Andersons) pounds 2.39
1 roll Polytwist
(Andersons) pounds 3.35
1 Fuchsia magellanica
`Versicolor' pounds 2.95
TOTAL pounds 20.35