Dahlias are native to central Mexico and have the most extensive range of colors, sizes and forms of almost any flower. Flower diameter can be as small as ½ inch up to 12 inches. The colors are just about all imaginable except blue. These are truly spectacular garden and bedding flowers, but they do require some attention. Most people purchase dahlia plants from nurseries or divide their own tubers. Black dahlias are rare and costly so you will want to take good care of them.
Prepare the bed or planting area as soon as the ground can be worked in the spring. Choose a sunny location for the bed. Black dahlias will be darker with more sun, however lighter colors will appreciate some afternoon shade. Dig the bed to 8 inches in depth and remove all weeds and debris.
Add 5-20-20 fertilizer to the soil and mix it in well. Rake the soil out level and let it sit for one month, watering occasionally. At the end of the month, hand-pick any weeds that have grown up.
When all danger of frost is gone, place a 2-inch layer of compost over the soil and work it in by turning the soil and raking it out.
Dig a hole two times the diameter of the root ball and slightly less than the depth of the roots. Space the plants 9 inches apart as the black flowers are on the smaller side. If planting larger cultivars in other colors, space them 12 inches apart.
Place the plants in the soil so the crowns are just above the soil level. Fill the soil in around them and hand tamp down firmly. Black dahlias do not normally need staking but the taller cultivars will.
Water immediately after planting and keep the soil moist throughout the growing season. Dahlias have shallow roots so daily short watering is fine. Water the soil and not the foliage or flowers.
Apply the 5-20-20 fertilizer until mid-August. When using a granular fertilizer, spread on dry soil and water immediately after. Follow manufacturer's recommendations for amounts to use.
Spread a 1-inch layer of compost or well-aged manure around the plants a month after planting as a mulch. This will keep the flowers weed-free and enrich the soil.