The Giant marsh marigold (Caltha polypetala) is the larger cousin of the British native Marsh marigold and grows to around twice its size so therefore is not suitable for small ponds.  These early flowering spring perennials bring a splash of colour into the garden from as early as March but are at their best in April.  They have large yellow buttercup-like flowers with glossy green leaves making a spectacular addition to pond margins and bog gardens and quickly grow to form large clumps which are easily divided to provide new plants in spring or autumn.