Ask Ludwig! How do I stop my plant leaves turning yellow?

Loren Lewis of Randpark Ridge writes: My ‘Boksburg Fantasia’ roses were growing well but now I see yellow leaves on some of the plants. What is causing this?

Ludwig replies: When leaves lower down on the bush turn yellow it usually means they are not receiving enough sunlight or are being naturally discarded by the bush. The top leaves tend to shade the leaves lower down, especially on thinner branches that should have been pruned off in winter. Another reason may be that the stems have thickened considerably and are naturally severing such leaves.