The root of 3

The Mexican Agave americana is an architectural plant with few equals, its bold, spiky shape drawing the eye in most settings. It is an excellent choice for a pot on a sun-drenched patio and creating striking combinations with other plants is child's play.

A desert succulent, it is well adapted to periods of drought, so there will be no need to get the neighbours round to water every time you go away. The thick, fleshy, broad, sword-shaped leaves (the source of rope fibre and tequila) are bluish-green and covered in a feint bloom, as though they have been lime washed. They are arranged in a rosette and can reach 6ft in length in the wild, snaking skywards like fossilised cobras ready to strike. Here they usually reach half that height. Whatever their size, they are tipped and edged by vicious, skin-tearing spines. You have been warned.

Owning an Agave has other drawbacks. In most areas of the British Isles they will not survive the winter outside. They can deal with a certain amount of cold, but the combination of low winter temperatures and high rainfall is a lethal mixture. To ensure their survival, put them in a greenhouse, porch or garden shed and always plant them in gritty, free- draining compost.

Agaves are usually given a pot to themselves, and they look splendid in their isolation, but they also look good if you grow other plants that are finer and softer in texture in among their broad leaves.

Parrots' beak, Lotus maculatus, an evergreen, shrubby trailer from the Canary Islands, makes a good contrast. Its leaves are a similar colour to the Agave but utterly different in shape - clumps of short needles on trailing branches that will encircle the Agave and spill over the pot. In summer, they produce strange, exotic, fiery, golden-amber flowers an inch long and the shape of a lobster's claw. In our climate, treat them as annuals and renewed each season.

The final and most familiar plant to British gardeners is the evergreen, perennial wallflower, Erysimum "Bowles Mauve".

This will need a pot of its own and will make a mound a couple of feet high. It usually needs replacing every couple of years as, quite literally, it flowers, itself to death. Its small, cross-shaped, mauve blooms clustered on tall spikes and produced relentlessly are the reason for its inclusion here. Their colour adds intensity to the Agave's blue leaves and in turn seems to be deepened by the association.

John the Gardener