No Pity for Green Flowers!
Yes, there are a few plants with green flowers: green roses, green zinnias, green glads, green echinaceas, etc. These plants are hybrids developed for their unusual color. But if a plant you choose for flowers of another color suddenly starts producing green flowers, the chances are pretty good it is suffering from a disease: aster yellows.
Aster yellows is a very common in the wild, found especially in goldenrods, asters and other wildflowers, and is caused by a phytoplasma (formerly it was called a mycoplasma), a bizarre entity closely related to bacteria, but behaving like a virus. Notably, once a plant is infested, no treatment can cure it.
Aster yellows is transmitted by leafhoppers: by piercing plant tissues, they inject the phytoplasma which then extends gradually throughout the plant, affecting its growth and gradually weakening it, but the first symptom is usually the greening of the flowers. This is called phyllody, the abnormal development of floral parts into leafy structures. In the case of echinaceas, a kind of green growth starts to form on the cone in the flower’s center . Over time, the cone produces small satellite flowers that are completely green. Weird… and really not too wonderful!
The only treatment for aster yellows is to destroy the plant. If left alive, the disease will gradually gain the neighboring plants, including echinaceas, asters, cosmos, strawberries, daisies, marigolds, zinnias and more than 300 other plants.
This is a case where a quick strike solves a ton of future problems: yank the infected plant out at the first sign of symptoms and you can prevent the disease from spreading to your other garden plants.