Fertilising guide
How to fertilise Double King Cup (Caltha palustris 'Multiplex')— schedule & NPK
Also called Double King Cup, Double Marsh Marigold, Multiplex Kingcup.
More about double king cup
About Double King Cup
Caltha palustris 'Multiplex' · also called Double King Cup, Double Marsh Marigold · flowering
Double King Cup is a striking double-flowered cultivar of the marsh marigold with densely packed, fully double rich-yellow flowers similar to 'Flore Pleno', listed separately by the RHS. Fully hardy (H7), it grows at pond margins and in shallow water up to 5 cm deep, producing an exceptional display of golden pompom blooms in early spring. A long-established cottage-garden and pond-margin favourite.
Growth habit: Clump-forming, rhizomatous herbaceous perennial; fully deciduous and winter-dormant; one of the earliest aquatic marginals to flower in spring
What fertiliser double king cup actually wants — and why
Double King Cup is an easy, light foliage feeder — a half-strength balanced liquid feed through the growing months keeps it green without forcing weak, sappy growth.
A balanced general houseplant feed (roughly even N-P-K) is exactly right — it is grown for foliage, so steady, moderate nitrogen for healthy leaves is the goal, not a bloom or root formula.
For the language behind the three numbers on the bottle — what nitrogen, phosphorus and potassium each do — see the NPK ratio explained entry. The short version for double king cup: match the feed to the job the plant is doing right now, not to a generic “plant food” on the shelf.
How often to feed double king cup, and which months
Feeding only earns its keep while the plant is in active growth and can use the nutrients — pour feed into a dormant or low-light plant and it simply builds up as root-burning salt. For double king cup:
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. Treat that as sparingly through the growing season between spring through early autumn (roughly March to September); ease off in autumn and stop entirely in the low light of winter.
The dormant-season rule matters more than the exact interval: skip feeding entirely when double king cup is resting. For the wider context on indoor feeding rhythms across the seasons, the houseplant fertiliser schedule walks through the year month by month.
What strength to mix for double king cup
Half strength is the safe default for double king cup — houseplant feeds are formulated strong, and the diluted dose is gentler on the roots while still ample for foliage.
Feeding always goes onto already-damp soil, never dry roots — water double king cup first if the soil is dry, then apply the diluted feed. The companion question is when to water at all, covered in the double king cup watering schedule.
Signs you are over-feeding double king cup
Over-feeding is far more common — and more damaging — than under-feeding for most plants. The classic tells for double king cup:
- Brown, crispy leaf tips and edges with no sign of underwatering.
- A white, crusty salt deposit on the soil surface or pot rim.
- Weak, pale, stretched new growth that flops.
- Lower leaves yellow and drop while the soil is correctly watered.
Signs you are under-feeding double king cup
- Uniformly pale or yellow-green leaves, oldest first.
- Noticeably small new leaves and stalled growth in good light and season.
- A generally tired, lacklustre look despite correct watering and light.
If the symptoms point at watering, light or roots rather than nutrition, the full double king cup care brief covers soil, humidity and the common problems for this species.
Flushing and leaching the salts
Flush the pot of double king cup with plain water until it runs freely from the base every couple of months in the feeding season — it washes out the fertiliser salts that cause brown tips.
Organic vs synthetic feeds for double king cup
Organic options
A diluted seaweed or worm-casting feed, or fish emulsion if you can tolerate the smell indoors. UK: Westland or Baby Bio Organic, dilute seaweed; US: Espoma Indoor! or Neptune's Harvest fish & seaweed. Slow, gentle and hard to overdo.
Synthetic / liquid feeds
A general-purpose houseplant liquid at half strength — UK: Baby Bio, Westland Houseplant Feed or Phostrogen; US: Miracle-Gro Indoor Plant Food or Schultz. Convenient and fast-acting; the only risk is overdoing it.
Brand names are examples, not endorsements, and UK and US ranges differ — check the label’s own NPK and dilution rate, since formulations change.
Fertilising double king cup — frequently asked questions
What fertiliser does double king cup need?
A balanced general houseplant feed (roughly even N-P-K) is exactly right — it is grown for foliage, so steady, moderate nitrogen for healthy leaves is the goal, not a bloom or root formula. Double King Cup is an easy, light foliage feeder — a half-strength balanced liquid feed through the growing months keeps it green without forcing weak, sappy growth.
How often should I feed double king cup?
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. 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. Treat that as sparingly through the growing season between spring through early autumn (roughly March to September); ease off in autumn and stop entirely in the low light of winter.
What strength of feed for double king cup?
Half strength is the safe default for double king cup — houseplant feeds are formulated strong, and the diluted dose is gentler on the roots while still ample for foliage.
What does over-feeding double king cup look like?
Brown, crispy leaf tips and edges with no sign of underwatering. A white, crusty salt deposit on the soil surface or pot rim. Weak, pale, stretched new growth that flops. Lower leaves yellow and drop while the soil is correctly watered. Feeding double king cup year-round on a fixed schedule, including dark winter months, is the most common mistake — it cannot use the nutrients in low light and the surplus simply burns the roots and crusts the soil.
Should I flush the soil of double king cup?
Flush the pot of double king cup with plain water until it runs freely from the base every couple of months in the feeding season — it washes out the fertiliser salts that cause brown tips.
Keep reading
- Double King Cup care — the full brief (light, soil, humidity, problems, pet safety)
- How often to water double king cup — the watering schedule
- The houseplant fertiliser schedule — feeding through the year
- NPK ratio explained — what the three numbers on the bottle mean
- How to fertilise oriental spruce
- How to fertilise red spruce
- How to fertilise black spruce
- All 8452 fertilising guides in the Growli library