Repotting guide

When & how to repot Salvinia molesta (Salvinia molesta)

Also called Giant Salvinia, Kariba Weed, Aquarium Watermoss.

More about salvinia molesta

About Salvinia molesta

Salvinia molesta · also called Giant Salvinia, Kariba Weed · houseplant

Salvinia molesta is a larger, more vigorous floating fern famous for the eggbeater-shaped split hairs on its leaves that make it almost impossible to wet. Sometimes used in aquariums, it is one of the world's most damaging aquatic weeds, capable of forming thick floating rafts. Grow only in fully contained systems and never release it.

Mature size: Leaves up to 2-4 cm; unchecked colonies can build rafts tens of centimetres thick covering the entire surface.

How to tell salvinia molesta needs repotting

Repotting on a calendar is less reliable than reading the plant. For salvinia molesta, watch for these signs:

Roots creeping out of the drainage holes or matting tightly across the soil surface.
The rootball dries out within a day or two no matter how much you water.
Water channels straight down the gap between rootball and pot without wetting the centre.
Steady decline — thin growth, persistent crispy edges — that good humidity and watering have not fixed. Only then is the disturbance of a repot worth the risk for salvinia molesta.

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 salvinia molesta

Every 1–2 years, disturbing the roots as little as possible. Salvinia molesta's growth habit — free-floating fern that progresses from flat single leaves to a chained surface mat and finally to a thick, folded three-dimensional raft as crowding increases. — sets the pace. Salvinia molesta is a larger, more vigorous floating fern famous for the eggbeater-shaped split hairs on its leaves that make it almost impossible to wet. Sometimes used in aquariums, it is one of the world's most damaging aquatic weeds, capable of forming thick floating rafts. Grow only in fully contained systems and never release it.

What size pot to step salvinia molesta up to

Go up only one size and handle the rootball as little as possible. Salvinia molesta resents root disturbance, so the goal is to slide the intact rootball into slightly more soil — not to tease, wash or prune the roots. A modest step up means less shock and a faster recovery.

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 salvinia molesta

Early spring, just as new growth restarts, is the ideal window for salvinia molesta. 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 salvinia molesta

Keep disturbance to a minimum. Salvinia molesta resents root disturbance, so the plan is to move the intact rootball — not to wash, tease or prune the roots.
Choose just one size up. Pick a pot only one size larger with drainage, and have moisture-retentive none — free-floating, rootless ready.
Slide the rootball out whole. Water the day before, then ease salvinia molesta out keeping the rootball intact. Gently free only the roots that are circling the very bottom.
Nestle it into fresh soil. Add a base layer of fresh mix, set the rootball in at the same depth, and backfill gently around the sides without packing hard.
Water and protect. Water in, then keep it warm, humid and out of direct sun for a few weeks while it re-roots. Expect a short sulk — that is normal.

Aftercare

Expect salvinia molesta to sulk for a couple of weeks — that is normal after any root disturbance for this group. Keep it warm, humid and out of direct sun, water just enough to keep the mix lightly moist, and do not panic and overwater while it re-roots. Do not fertilise for about 4 weeks — fresh mix already carries nutrients and feeding freshly disturbed roots scorches them.

The right soil mix for salvinia molesta

Salvinia molesta wants none — free-floating, rootless. No true roots; nutrients are absorbed by the submerged feathery leaf. Needs no substrate and floats freely above any pond bottom or aquascape. Always use fresh mix when you repot — reusing old, broken-down soil reintroduces the compaction and poor drainage you are repotting to fix.

Repotting salvinia molesta — frequently asked questions

How often should you repot salvinia molesta?

Every 1–2 years, disturbing the roots as little as possible for salvinia molesta. Repot salvinia molesta every 1–2 years, disturbing the roots as little as possible — it sulks for weeks if the rootball is teased apart. Slide it into one size up in spring with fresh none — free-floating, rootless, keep it warm and humid afterwards, and never bare-root or hard-prune the roots.

What size pot does salvinia molesta need?

When is the best time of year to repot salvinia molesta?

Why does salvinia molesta sulk after repotting?

Salvinia molesta resents root disturbance, so a wilt or stall for a week or two after repotting is normal, not a failure. Minimise it by keeping the rootball intact, stepping up just one size, and keeping the plant warm, humid and out of direct sun while it re-roots.

Should you fertilise salvinia molesta after repotting?

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