Perhaps 50% of the seedling groundsels are not growing well, or at all, and their young leaves have turned purplish and yellow like bruises.
I pulled one up today as I thought these plantlets could be suffering some sort of fungal attack and discovered that aphids were living under the leaves. As well as weakening the plants by sucking out the sap, they may well have introduced some virus.
Several plants have not beeen affected and are growing away quickly. Maybe if this cycle continues a new, aphid-resistant strain of 'windowbox groundsel' will evolve.