Some
clematis cultivars produce green coloured flowers early in
the season. Symptoms range from tepals tinged green along
the centre line to entire tepals failing to colour.
The exact causes are not known but it may be due to low temperatures as the flowers are forming. A number of early flowering hybrids including 'Duchess of Edinburgh' and 'Nelly Moser' appear particularly susceptible.
The problem may be greatest where clematis are planted in north-facing positions. Reference is sometimes made to the application of sulphate of potash as a remedy but there appears no real evidence for this. Plants usually revert to their normal colour later in the season as new flowers open.
It is important to appreciate that where this disorder occurs, the blooms are perfectly formed and do not exhibit the distortion associated with green flower disease. This is caused by organisms related to viruses called phytoplasma. In such cases the infected plant will not recover and subsequent flowers and foliage continue to exhibit symptoms of distortion. There is no cure and such plants should be destroyed.