I have several pink flowering primroses around the garden. Folklore has it that the traditional yellow flowering primula vulgaris can be stimulated into producing pink flowers by planting in ground which is rich in cow manure. There does not appear to be any substantive scientific support for this. The likely explanation for the appearance of pink flowering plants is much more prosaic. What we are seeing are in all probability hybrid plants, a genetic amalgam of wild primroses and garden planted polyanthuses.