Dahlias are tuberous herbaceous perennials with usually dark green, toothed, pinnately divided leaves. The showy flowers - often double - are produced in summer and autumn. They are very popular with gardeners, as cut flowers and as show exhibits. Dahlias come in many colours and from tall to dwarf varieties. All need to be planted in a sheltered, sunny position in humus-rich, well-drained soil. They can be treated as perennials or annuals. Originating from the mountainous regions of Mexico and Central America, the Dahlia genus contains around 30 species and over 20,000 cultivars, most of which are derived from D. pinnata and D. coccinea, and divided into groups based on the form of their flower heads. These striking plants will add interest to almost all garden styles and many Dahlia cultivars have earned a coveted 'RHS Award of Garden Merit'. They are also nectar-rich and excellent for attracting bees and other pollinators to your garden - especially the single-flowered varieties, from which it is easier for them to feed.