Dahlia is a popular genus of around 30 species of tuber-rooted perennials in the daisy family (Asteraceae), native to central Mexico. Dahlias were well-established in Aztec gardens when Hernando Cortez invaded Mexico in 1519, according to Oregon State University extension. Dahlia species' variation of height, flower form and color make them a popular garden feature.
Dahlias range in height from 1 to 6 feet tall, with flowers ranging in size from 2 to 12 inches. They have bright to deep green, lobed leaves and bold flowerheads that vary greatly among cultivated species. Dahlia forms include tall border plants and low-growing bedding types. Other classifications order them by type of flowerhead. Flower size, shape and arrangement of petals vary depending on the species and cultivar. Flower forms include: single-flowering, orchid-flowering, double-flowering, cactus dahlias, and ball or pompon dahlias. Dahlia flower colors range from white and cream to bright yellow, deep red and purple, or tipped and striped with different colors.
Dahlias are popular border plants and look lovely planted in massed beds. Fresh-cut dahlias are commonly used in flower arrangements. The wide variety of flower colors, shapes and forms make dahlias a versatile plant suitable in either formal or informal garden settings.
Plant dahlias after danger of frost has passed in a sunny location. Dahlias prefer nutrient-rich, well-drained organic soils. Dahlias grown in cold climates or where soil is waterlogged in winter should be dug up and stored in a dry place until spring and the soil is warm to prevent "hardening" of the plant. Dahlias are cold hardy in USDA zones 8 through 11.
Place dahlia tuberous roots in furrows or trenches about 5 or 6 inches deep and cover with 2 or 3 inches of soil, adding more as the plant grows, according to Oregon State University extension literature. Plant large dahlia varieties 36 inches apart, and smaller varieties 16 to 24 inches apart.
Water plants thoroughly and keep the soil moist, but not wet. Dahlias are fast growing plants requiring a good deal of irrigation. Large varieties benefit from staking to support their blooms.
Dahlia Ted's Choice is a decorative culitivar with 4 to 6 inch purple-pink blooms.
Lilac Tarathi is a cactus dahlia with delicate, lilac-pink petals borne on tall stems.
My Love is a semi-cactus dahlia popular in fresh cut flower arrangements and bridal bouquets.
Aurwen's Violet is a pompon dahlia purple-red petals forming a spherical head on thin stems up to 3 feet tall.
The most serious diseases of dahlias are caused by viruses, according to Oregon State University Extension literature. Choosing disease-resistant stock is a recommended control of diseases affecting dahlia, such as mosaic disease and spotted wilt. Insects affecting dahlia vary by region. Local county extension offices are a good source of information regarding insect and disease identification and management.