Yellowing leaves on buddleia

by Helen
(Germany)

Last year the leaves on my buddleia started to turn yellow. This didn´t prevent it from blooming profusely but I still thought it might be lacking nutrients. However, this year, despite the addition of compost and mulch, the same problem has occurred. It´s in a sunny position and is well watered. I´d appreciate your advice as to the possible cause and solution.