About Caltha palustris L.
Caltha palustris L. is a hairless, fleshy perennial herbaceous plant that grows 10–80 cm (4–31+1⁄2 in) tall. It dies back to the ground each autumn and overwinters using buds located near the surface of marshy soil. This species has many strongly branching roots that are 2–3 mm (3⁄32–1⁄8 in) thick. Its flowering stems are hollow, and either grow upright or are somewhat trailing along the ground. Alternate true leaves form a rosette; each leaf has a petiole around four times as long as its kidney-shaped leaf blade. The leaf blade measures 3–25 cm (1+1⁄4–9+3⁄4 in) long and 3–20 cm (1+1⁄4–7+3⁄4 in) wide, with a heart-shaped base, a blunt tip, and a margin that ranges from scalloped to toothed, and is sometimes almost smooth particularly near the tip. Young leaves are protected by a membranous sheath, which can reach up to 3 cm (1+1⁄4 in) long in fully grown plants.
Caltha palustris is native to marshes, fens, ditches, and wet woodland across temperate regions of the Northern Hemisphere, and can be found throughout much of the northeastern United States. It grows in locations with oxygen-rich water near the soil surface. It prefers nutrient-rich soils, but avoids fertilized sites with high concentrations of phosphate and ammonium, and does not grow in brackish water. It is often found near iron-rich seepage, because iron ions react with phosphate to make it unavailable to plants, forming insoluble "rusty" flocs that appear on wet soil and the surface of marsh plant stems. It grows between reeds along the edges of lakes and rivers, and can be found in black alder coppices and other regularly flooded, consistently moist forests. When in bloom, it often visually dominates the areas where it grows. It was once common on wet meadows, but agricultural rationalization has now restricted it mostly to ditches. It is a component of purple moor grass and rush pastures, a Biodiversity Action Plan habitat in the UK, where it occurs on poorly drained neutral and acidic soils in lowlands and upland fringe areas.
In western Europe, the marsh-marigold moth Micropterix calthella bites open the anthers of Caltha palustris and other plants to eat their pollen. Its summer and autumn caterpillars also feed on Caltha palustris, though they may sometimes be found feeding on mosses. Another insect visitor to this species in western Europe is the leaf beetle Prasocuris phellandrii, which is black with four orange stripes, grows around ½ cm long, and feeds on the plant's sepals; the larvae of this beetle live inside hollow stems of plants in the parsley family. In Illinois, USA, two species of leaf beetle (Plateumaris nitida and Hydrothassa vittata) can be found living on Caltha palustris. The maggots of some Phytomyza species in the family Agromyzidae are leaf miners that develop inside Caltha leaves.
The flowers of Caltha palustris produce both nectar and large amounts of pollen that attract many insect visitors. Hoverflies in the family Syrphidae are thought to be the most common pollinators. In Canada, beetles from the families Cantharidae, Nitidulidae, Coccinellidae, Chrysomelidae, and Cerambycidae, plus thrips (Thripidae), bugs (Miridae), butterflies (Pieridae), sawflies (Tenthredinidae), bees (Apidae, Halictidae, Andrenidae), ants (Formicidae), and flies from multiple families including Syrphidae have been observed visiting the leaves or flowers, and many of these insects carry Caltha pollen. In addition to other pollination methods, this species is adapted to rain-pollination. Caltha palustris is infertile when self-pollinated, and relatively high fertility in crosses between sibling plants suggests this self-incompatibility is genetically regulated by multiple genes. This regulatory mechanism is also found in the genus Ranunculus, and is only known to occur in these two genera.
Caltha palustris contains several active compounds, with protoanemonin being the most significant from a toxicological perspective. Consuming large quantities of the plant may cause convulsions, throat burning, vomiting, bloody diarrhea, dizziness, and fainting. Contact between the plant's juice and skin or mucous membranes can cause blistering or inflammation, and ingestion leads to gastric illness. Younger plant parts contain lower amounts of toxins, and heat breaks down these toxic substances. Small amounts of Caltha in hay do not cause issues when fed to livestock, but larger quantities result in gastric illness. Additionally, plants growing in raw water may carry toxic organisms, which can be neutralized by cooking.
Early spring greens and flower buds of Caltha palustris are edible after cooking, but are poisonous when raw. To prepare, young leaves or buds are soaked several times in fresh boiling water until just tender, cut into bite-sized pieces, lightly salted, and served with melted butter and vinegar. Very young flower buds can be prepared like capers and used as a spice. The common marsh marigold is planted as an ornamental across temperate regions worldwide, and is sometimes recommended for low-maintenance wildlife gardens. The double-flowered cultivar 'Flore Pleno' has received the Royal Horticultural Society's Award of Garden Merit.
