Repotting guide

When & how to repot Double King Cup (Caltha palustris 'Multiplex')

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.

Mature size: 30–45 cm (12–18 in) tall in flower; clumps spread to 30–45 cm wide at maturity

How to tell double king cup needs repotting

Repotting on a calendar is less reliable than reading the plant. For double king cup, watch for these signs:

Roots spiralling thickly out of the drainage holes or pushing the whole plant up out of the pot.
The pot is so packed that water runs straight through in seconds and barely wets the soil.
It has split a plastic pot, or the rootball is a solid mass with almost no soil left when you slide it out.
Growth and (for double king cup) flowering have clearly stalled despite good light and feeding — but remember this plant likes being snug, so a little crowding alone is not a reason to repot.

For the underlying biology of a pot-bound root system and why it stalls a plant, see our guide to spotting and fixing a root-bound plant.

How often to repot double king cup

Only every 2–4 years, when genuinely crowded. Double King Cup is one of the plants that genuinely prefers a snug pot — it grows and flowers better with its roots a little restricted, so resist the urge to repot it on schedule. Clump-forming, rhizomatous herbaceous perennial; fully deciduous and winter-dormant; one of the earliest aquatic marginals to flower in spring.

What size pot to step double king cup up to

Go up only one pot size — roughly 2–3 cm (about an inch) wider in diameter, no more. Double King Cup positively prefers a snug pot: it flowers and grows better when the roots are a little restricted. The single biggest repotting mistake here is over-potting — dropping double king cup into a pot two or three sizes up. All that surplus soil holds water the small root system cannot use, stays cold and wet, and rots the roots within weeks. When in doubt, choose the smaller pot.

Not sure of the exact diameter? Our pot size calculator takes the current pot and root spread and tells you the right next size — it deliberately recommends a single step up, never a big jump.

The best time of year to repot double king cup

Early spring, just as new growth restarts, is the ideal window for double king cup. The plant is moving into its strongest growth phase and re-roots into fresh soil quickly. Avoid repotting in winter dormancy or, for flowering plants, while it is in bud or bloom — recovery is slowest then and you risk dropping the flowers.

Step-by-step: repotting double king cup

Confirm it actually needs it. Slide double king cup out and check the roots. Only continue if it is genuinely packed — this plant prefers a snug pot, so if there is still soil and room, put it straight back.
Pick a pot only one size up. Choose a pot just 2–3 cm wider with good drainage. Resist anything bigger; over-potting is the main killer here.
Ease it out gently. Water lightly the day before, then tip double king cup out, supporting the base. Tease the outer roots free only enough to stop them circling.
Repot at the same depth. Add a layer of fresh rich, heavy boggy soil or aquatic compost, set the plant so the soil line sits exactly where it did before, and backfill around the sides, firming lightly.
Settle it in. Water once to settle the soil, then let it sit. Hold off on more water until the top of the soil dries — fresh soil around a small root system stays wet for a while.

Aftercare

Because the new soil holds more water than the old crammed rootball did, ease right back on watering — let the top of the soil dry before you water double king cup again, or you will rot the roots in the very pot you just moved it to. Keep it out of harsh direct sun for a fortnight. Do not fertilise for about 4 weeks — fresh mix already carries nutrients and feeding freshly disturbed roots scorches them.

The right soil mix for double king cup

Double King Cup wants 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. Always use fresh mix when you repot — reusing old, broken-down soil reintroduces the compaction and poor drainage you are repotting to fix.

Repotting double king cup — frequently asked questions

How often should you repot double king cup?

Only every 2–4 years, when genuinely crowded for double king cup. Only repot double king cup every 2–4 years, and only when it is genuinely root-bound — it flowers and grows best slightly crowded. Step up just one pot size in spring using rich, heavy boggy soil or aquatic compost. The key mistake is over-potting: a too-big pot stays wet and rots the roots.

What size pot does double king cup need?

When is the best time of year to repot double king cup?

Does double king cup like to be root-bound?

Yes — double king cup genuinely flowers and grows best when slightly pot-bound, so do not rush to repot it. The mistake to avoid is over-potting into a much larger pot: the excess soil stays wet, the roots cannot use it, and the plant rots. Only repot every few years and only one snug size up.

Should you fertilise double king cup after repotting?

Not immediately. Wait about 4 weeks after repotting double king cup. Fresh mix already contains nutrients, and feeding freshly cut or disturbed roots burns them. Resume your normal feeding routine once you see new growth.