 Fertilization. They are heavy water and heavy fertilizers, meaning they are one of those plants, if you don't fertilize your agapanthus, they kind of turn a lime yellow. The leaves you're talking about. That's a good way to tell. It's by the color of the leaves. It should be a deep green foliage. As far as specific, people talk about phosphorus or anything. I don't think so. I think that just an all-purpose fertilizer would be what an agapanthus needs.