Dahlia

Dahlia is agenus of bushy, tuberous, herbaceousperennial plants native to Mexico. A member of the Asteraceae (or Compositae), dicotyledonous plants, related species include the sunflower, daisy, chrysanthemum, and zinnia. There are 42 species of dahlia, with hybrids commonly grown as garden plants. Flower forms are variable, with one head per stem; these can be as small as 5 cm (2 in) diameter or up to 30 cm (1 ft) ("dinner plate"). This great variety results from dahlias being octoploids—that is, they have eight sets of homologous chromosomes, whereas most plants have only two. In addition, dahlias also contain many transposons—genetic pieces that move from place to place upon an allele—which contributes to their manifesting such great diversity.

The stems are leafy, ranging in height from as low as 30 cm (12 in) to more than 1.8–2.4 m (6–8 ft). The majority of species do not produce scented flowers or cultivars. Like most plants that do not attract pollinating insects through scent, they are brightly colored, displaying most hues, with the exception of blue.

The dahlia was declared the national flower of Mexico in 1963. The tubers were grown as a food crop by the Aztecs, but this use largely died out after the Spanish Conquest. Attempts to introduce the tubers as a food crop in Europe were unsuccessful.

Dahlias are annual blooming plants, with mostly tuberous roots. While some have herbaceous stems, others have stems which lignify in the absence of secondary tissue and resprout following winter dormancy, allowing further seasons of growth. As a member of the Asteraceae the flower head is actually a composite (hence the older name Compositae) with both central disc florets and surrounding ray florets. Each floret is a flower in its own right, but is often incorrectly described as a petal, particularly by horticulturists. The modern name Asteraceae refers to the appearance of a star with surrounding rays.

In the language of flowers, Dahlias represent dignity and instability, as well as meaning my gratitude exceeds your care. 

Predominantly Mexico, but some species are found ranging as far south as northern South America. D. australis occurs at least as far south as southwestern Guatemala, while D. coccinea and D. imperialis also occur in parts of Central America and northern South America. Dahlia is a genus of the uplands and mountains, being found at elevations between 1,500 and 3,700 meters, in what has been described as a "pine-oak woodland"vegetative zone. Most species have limited ranges scattered throughout many mountain ranges in Mexico.

Slugs and snails are serious pests in some parts of the world, particularly in spring when new growth is emerging through the soil.Earwigs can also disfigure the blooms. The other main pests likely to be encountered areaphids (usually on young stems and immature flower buds), red spider mite (causing foliage mottling and discolouration, worse in hot and dry conditions) and capsid bugs (resulting in contortion and holes at growing tips). Diseases affecting dahlias include powdery mildew, grey mould (Botrytis cinerea),verticillium wilt, dahlia smut (Entyloma calendulae f. dahliae), phytophthora and someplant viruses. Dahlias are a source of food for the larvae of some Lepidoptera species including angle shades, common swift, ghost moth and large yellow underwing.