Description
Flowers: Gigantic, fully double blooms available in almost every color except true blue. Petal forms vary from the neat, flat petals of the Decorative type (e.g., 'Café au Lait') to the spiky, quilled petals of Cactus types.
Size: These plants are vigorous, typically reaching heights of 3 to 5 feet (sometimes up to 6 feet). Because they grow so large, they are best suited for the back of flower borders or dedicated cutting gardens.
Bloom Period: They provide late-season drama, blooming from mid-summer until the first hard frost in autumn. 
Care and Growing Instructions
Expert gardeners often say if you can grow beefsteak tomatoes, you can grow dinnerplate dahlias, as their needs for sun, support, and feeding are very similar. 
Longfield Gardens
Sunlight: They are "sun worshippers" requiring 6 to 8 hours of direct sunlight daily.
Soil: Plant in rich, well-draining soil with a neutral pH. Amend heavy clay with compost or sand to prevent the tubers from rotting.
Planting: Wait until the soil has warmed to 60°F and all danger of frost has passed. Plant tubers horizontally 4 to 6 inches deep with the "eye" facing up, spacing them 24 to 36 inches apart.
Watering: Do not water immediately after planting; wait until green shoots emerge to avoid rotting the tuber. Once established, they are thirsty and need 1 to 2 inches of water per week.
Support: Because the blooms are incredibly top-heavy, staking is essential. Drive a 5- to 6-foot stake into the ground at planting time to avoid damaging the roots later. 
Expert Tips for Massive Blooms
Pinching: When the plant is 12 inches tall, pinch off the top 3–4 inches of the center stem. This forces the plant to branch out, creating a sturdier, bushier plant with more flowers.
Disbudding: For the absolute largest "dinnerplate" size, remove the two smaller side buds next to the central bud in each flower cluster.
Feeding: Use a low-nitrogen fertilizer (like 5-10-10) monthly. High nitrogen will give you lush green leaves but very few flowers. 
Winter Care
Dahlias are only hardy in Zones 8–11. In colder climates, wait until the first frost kills the foliage, then lift the tubers, dry them for a few days, and store them in a cool (40–50°F), dark place in peat moss or wood shavings until spring.