Caltha leptosepala aka Howell's Marsh Marigold

Caltha leptosepala is a small plant that lives for years. It belongs to Ranunculaceae, the buttercup family. Gardeners may call it white-flowered marsh marigold. Its native range starts in central Alaska. It extends through the west of North America. Records include western Canada and several western US states. It grows in subalpine or subarctic climates. These are cold zones near high peaks or the Arctic. It is not native to Britain or Ireland. Its mountain range helps explain how well it withstands cold.

The plant forms clumps and sheds its leaves each year. The RHS gives its height and width as about 1 foot [0.3 metres]. The wider garden size range reaches 1.6 feet [0.5 metres] both ways. Even at that size, it is a small plant. Its dark green leaves are glossy and shaped like hearts. They make up much of its low, leafy form. The RHS says it takes two to five years to reach full size.

White flowers open in spring, each with a yellow centre. Each bloom can reach about 0.13 feet [0.04 metres] across. The pale blooms stand out against the dark leaves. The evidence for this species gives no more exact bloom time. It does not say what the fruit or seeds look like. The spring blooms and low form suit wet garden sites. The RHS lists it as a plant for wildlife gardens. It does not name any animals that visit it.

Choose an open spot with full sun and rich, boggy soil. Grow it at the water's edge, not on dry ground. The RHS lists loam and soil that drains poorly as suitable. The soil can be acid, neutral, or alkaline. The stated growing needs allow for both exposed and sheltered sites. Its RHS rating of H7 means it withstands cold below −4°F [−20°C]. This rating applies to UK growing conditions. It does not state a USDA zone.

Routine care needs no pruning. Grow new plants from seed, or split clumps you have. The RHS lists it among plants that need little care. Pests tend not to cause problems, but powdery mildew can occur. The species evidence warns that the plant contains toxic compounds. It warns against eating it without thorough cooking. This does not prove that home cooking makes it safe. The supplied species evidence does not show that pets are safe. Nor does it name any parts that are safe to eat.