Caltha palustris (King cup or Marsh marigold) Native to the UK. Caltha palustris, known as marsh-marigold and kingcup, is a small to medium size perennial herbaceous plant of the buttercup family, native to marshes, fens, ditches and wet woodland. Rich golden yellow flowers, with heart shaped leaves loves being in or near the water and if dead headed will flower from early spring through the summer, produces almost a ball of yellow