Growli
Let's go

Plant care

Double King Cup (Double Marsh Marigold) care

Caltha palustris 'Multiplex'

Also called Double King Cup, Double Marsh Marigold, Multiplex Kingcup.

RHS H7USDA 3-7Mildly toxic to petsIndoor 30–45 cm (12–18 in) tall in flower

Watering rhythm

Bright indirect light (just back from a sunny window)

Permanently moist to standing water 0–5 cm deep

Jump to schedule→Get smart reminders in the app

Light

Bright indirect light (just back from a sunny window)

Soil

Rich, heavy boggy soil or aquatic compost

Humidity

High (pond margin and bog garden habitat)

Temp

-30 to 25°C

Pet safety

Mildly toxic to pets

Mature size

30–45 cm (12–18 in) tall in flower

Care at a glance

Light

Bright but filtered. Double King Cup burns within days in unfiltered south-facing summer sun, and stops growing within months in deep shade. Thrives in full sun to very light partial shade at the pond's edge. An open, sunny position maximises flower production and creates the most vibrant gold display. Tolerates brief periods of dappled shade but in consistently shady spots flowering is reduced and the plant becomes lax. If you only have a south window, set the plant back 1.5 m or hang a sheer curtain — both knock the intensity down into the right range.

Watering

Watering double king cup: permanently moist to standing water 0–5 cm deep. The number that matters isn't the day of the week — it's how dry the top 2-3 cm of the pot feels. A finger in the soil tells you more than a watering app. After every watering, tip the saucer. Grow at a pond margin in permanently moist to boggy soil, or in very shallow standing water no deeper than 5 cm over the crown. Never allow the root zone to dry out during the growing season. In aquatic baskets, position on a submerged shelf at the correct depth. Tolerates winter wetness well.

Soil and pot

Double King Cup grows best in rich, heavy boggy soil or aquatic compost. Requires fertile, moisture-retentive, clay-loam or dedicated aquatic basket compost. Thin or sandy soils must be heavily amended with well-rotted organic matter. In pond baskets, use proprietary aquatic compost topped with pea gravel to prevent soil loss into the water. pH 5.5–7.0. A pot with a working drainage hole is non-negotiable for this species — even free-draining mix will turn soggy in a closed planter. If you love the look of a decorative pot without a hole, use it as a cachepot around an inner nursery pot you can lift out to water.

Humidity and temperature

Double King Cup sits happiest at around High (pond margin and bog garden habitat) humidity and -30 to 25°C (-22 to 77°F). Naturally suited to the high-humidity microclimate of pond margins and bog gardens. No supplemental misting needed. Mulching around planting in a bog bed helps maintain a cool, moist environment for the roots and raises local humidity slightly. If you keep the room above year-round and avoid placing the plant near a cold draught, a hot radiator, or an air-conditioning vent, you have already handled the two biggest indoor stressors.

Fertilising

Feed double king cup sparingly. Apply a single slow-release aquatic fertiliser tablet pushed into the basket compost in early spring as new growth emerges. Repeat in early summer if growth appears weak. Avoid high-nitrogen liquid feeds near pond water. A light mulch of well-rotted compost around bog-garden plantings in autumn also helps. Skip fertiliser entirely on a stressed, recently-repotted, or actively wilting plant — fertiliser salts make damage worse, not better. Wait for a round of healthy new growth before resuming a feeding rhythm.

Common problems

Below are the issues we see most often on double king cup in the Growli community. Each is annotated with the most common cause so you know where to start.

  • Powdery mildew on summer foliage — White powdery patches may appear on leaves in warm, dry or poorly ventilated conditions. Cut back the foliage hard immediately after spring flowering; the plant will regrow with clean leaves and the stress of summer heat is avoided as the plant is semi-dormant. Good airflow reduces recurrence.
  • Crown rot in excessive water depth — Planting too deeply in standing water — more than 5 cm over the crown — risks crown and rhizome rot, particularly in winter. Always position at the pond margin or on a raised basket shelf so the crown is at or just below the waterline. Lift and reposition if planted too deeply.
  • Clump becoming congested and flower-shy — After several years, dense clumps become congested and flowering decreases. Lift and divide every 3–4 years in late summer or early spring, discarding the old woody central portions and replanting vigorous outer sections into refreshed aquatic compost.

Propagation

Divide clumps every 3–4 years in late summer after flowering or in early spring as buds emerge. Separate into sections of 3–5 shoots with roots attached and replant immediately into moist, fertile aquatic compost or bog soil. Does not reproduce true to type from seed; division is the only reliable method for this double-flowered cultivar. Propagation is the cheapest, most satisfying way to expand a collection — and it doubles as insurance against losing a mature plant to an accident. Take a backup cutting once the parent is established and healthy.

Toxicity to pets

Double King Cup is mildly toxic to pets. Caltha palustris 'Multiplex', like all Caltha palustris cultivars, contains protoanemonin when bruised — a lachrymatory irritant causing mouth irritation, drooling, vomiting, and diarrhoea in pets and humans if ingested in quantity. The Dogs Trust includes the species on its harmful plants list. Wear gloves when handling. Not individually listed by the ASPCA but the genus irritant is well established. If you keep cats, dogs, or curious children in the house, weigh placement carefully — a high shelf or a hanging planter is enough for casual safety. For severe ingestion incidents, call your local vet and the ASPCA Animal Poison Control Center (in the US, 888-426-4435).

Pet-safety status is sourced from the ASPCA Toxic and Non-Toxic Plant List, which catalogues the most-asked-about plants for cats, dogs, and horses.

Double King Cup care — frequently asked questions

What is the common name for Caltha palustris 'Multiplex'?

Caltha palustris 'Multiplex' is most commonly called Double King Cup, but it is also known as Double King Cup, Double Marsh Marigold, Multiplex Kingcup. The names refer to the same species, so care instructions for Double King Cup apply identically to anything sold as Double Marsh Marigold.

How much light does double king cup need?

Double King Cup grows best in bright indirect light (just back from a sunny window). Thrives in full sun to very light partial shade at the pond's edge. An open, sunny position maximises flower production and creates the most vibrant gold display. Tolerates brief periods of dappled shade but in consistently shady spots flowering is reduced and the plant becomes lax.

How often should I water double king cup?

Water double king cup permanently moist to standing water 0–5 cm deep. Grow at a pond margin in permanently moist to boggy soil, or in very shallow standing water no deeper than 5 cm over the crown. Never allow the root zone to dry out during the growing season. In aquatic baskets, position on a submerged shelf at the correct depth. Tolerates winter wetness well. The finger-test (or lifting the pot to feel its weight) beats a fixed weekly calendar because pot size, light, and season all change how fast the soil dries.

Is double king cup toxic to cats and dogs?

Double King Cup is mildly toxic to pets. Caltha palustris 'Multiplex', like all Caltha palustris cultivars, contains protoanemonin when bruised — a lachrymatory irritant causing mouth irritation, drooling, vomiting, and diarrhoea in pets and humans if ingested in quantity. The Dogs Trust includes the species on its harmful plants list. Wear gloves when handling. Not individually listed by the ASPCA but the genus irritant is well established.

What USDA hardiness zone does double king cup grow in?

Double King Cup is rated for USDA zone 3-7 and RHS hardiness H7. Outside that range, grow it as a container plant that overwinters indoors before the first hard frost.

Double King Cup deep-dive guides

Every aspect of double king cup care, each with its own calibrated guide:

Featured in these plant shortlists

Double King Cup qualifies for 3 curated Growli shortlists — each one filtered objectively from our structured plant-care library, so the selection is consistent and checkable:

Related guides

Double King Cup is also known as Double King Cup, Double Marsh Marigold, and Multiplex Kingcup.