In pictures: Growing khat

Khat buds are harvested every 30 days.

Each time, Mr Ntu'mirungu has to employ about 20 casual workers to assist him.

They earn about $30 for the dawn-to-dusk shift.

He harvests 100kg (220lb) each month and may earn about $2,000.

"This tree is unique. It is not affected by any disease and we grow it alongside other crops like tea and vegetables."