Why is my purple coneflower black?
Black starts at top, works down stem
Plant purple coneflower in early fall. The following spring, the coneflower sprouts green leaves in a mounding clump. Sturdy green flower stems rise from the mounding foliage to present blooms, which have drooping purple-pink petals. At the center of the bloom is a brown-orange cone. After several weeks, as the flower matures, the petals drop and the cone converts to seeds. The cone and stem darken and turn black as the plant dries. This process continues for all new growth. The drying conehead of seeds attracts bird, like finch, who dine on the seeds. The coneheads that remain will drops seeds in late fall through winter to create new plants.
You have two care options for coneflowers that turn black. If you want to feed the birds or increase the number of purple coneflower in your garden, then allow the stems and seed cone to remain through winter. In the spring, cut the stems back when the leafy base shows new green growth. If you prefer not to have the birds around (they can drop seeds from plants you prefer not having in your garden) or if you prefer not seeing the darks stems in the flower garden all winter, then cut the flowers at the base once the blooms fade. I'm between the two options. I like leaving the cones for the birds and to allow the plant to self-seed. However, I dislike seeing those black stems standing in the snow. My preferred care method is to leave the stems until late fall at which time I cut them back and toss the cone head onto the ground.
Black starts at base, works up stem
Is the base of your coneflower black while the top still displays a bloom, though it may not look healthy? The base of the purple coneflower plant, known as the crown, may rot. By the time the base of the plant turns black, the roots will have rotted, too. This situation may be caused by too much moisture from over watering (manually or from rainfall) or crowding with other plants that can cause poor air circulation around the plant. Unfortunately, a plant will not survive crown rot. Remove and dispose of the plant.
Black spots on leaves only
Is your purple coneflower showing leaves with black spots? If most of the plant is still green, the spotted leaves may have a bacterial infection such as Pseudomonas spp. or Xanthomonas spp. or other moisture-driven bacteria. The leaf spots may be the result of improper watering such as using a sprinkler. The water drops stay on the leaves and that may lead to bacterial infection. To prevent leaf spot caused by bacteria, water the purple coneflower at ground level. Though you may have success using bacterial sprays to reduce spread of the bacteria, my preference is to snip off and dispose of the stems of the affected plants.
Less likely, the cause may be a fungal infection called Botrytis. In addition to black, dying plants, gray mold will display. If only the top portion of the plant is infected, cut the plant back to the ground. If the whole plant is black, remove it.