A garden is rarely so straightforward as to have smooth flat lawns and borders of rich, easily dug soil. Some of us have to put up with damp sunless corridors between neighbour's houses, awkward slopes or plots shaded by trees or neighbouring buildings. Equally difficult to plant are seaside gardens exposed to gale force winds and salt spray, waterlogged plots with poor drainage, and dry soils exposed to the merciless glare of the sun day after day, without the slightest shade. Try to plant up any of these areas with a standard selection of garden-centre plants and the chances are you'll end up with a display of unsightly brown leaves and dried stems as the plants become defeated by the unsuitable habitat. What you need is an unkillable plant - one that will shrug off all the worst conditions in your garden and actually thrive in them and make good growth. Of course the plants aren't literally unkillable- put a drought-tolerant broom into a bog garden and it will die, of course - but they are invincible in the specific hostile conditions they have evolved to cope with.