Clematis magnesium deficiency
Clematis leaves can turn yellow when they get too much moisture. But if your clematis is growing in well-drained soil, yellow foliage is likely a symptom of magnesium deficiency, which usually affects the oldest leaves first. One cause is using too much fertilizer that’s high in potassium. The illustration shows how the areas between the leaf veins and around the edges turn yellow in early summer. Without enough chlorophyll, the plant becomes stunted and weak and may even die.
To treat your clematis, mix 1?4 cup Epsom salts (magnesium sulphate) into a gallon of water and spray the leaves in summer. You may need to treat it several times before you see improvement. Or sprinkle the dry Epsom salts right on the soil (1?4 to 1?2 cup per 10 sq. ft.) around the plant. This can be done once, at any time of the year.