Repotting guide

When & how to repot Japanese Arrowhead (Sagittaria japonica)

Also called Japanese Arrowhead, Arrowhead Water Plant, Kuwai.

More about japanese arrowhead

About Japanese Arrowhead

Sagittaria japonica · also called Japanese Arrowhead, Arrowhead Water Plant · edible

Japanese Arrowhead is an aquatic perennial grown for its arrow-shaped leaves and edible corms, prized in Japanese and Chinese cuisine. It thrives in shallow ponds, bog gardens, or containers of standing water in full sun. Starchy corms are harvested in autumn and can be roasted, boiled, or stir-fried. Hardy in temperate climates.

Mature size: 60–90 cm tall above water; spread 30–45 cm

How to tell japanese arrowhead needs repotting

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

Roots circling the bottom of the module or pot, or poking out of the drainage holes.
The seedling dries out within a day and growth has visibly stalled.
Roots are white and matted in a tight spiral when you tip the plant out.
It has outgrown its current container for the stage of the season — pot japanese arrowhead on before it becomes hard root-bound.

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 japanese arrowhead

Pot on seedlings as they grow; not a perennial repot. Japanese Arrowheadis grown for one season, so the question is really “how often to pot on” — keep moving it up before the roots circle. Upright emergent aquatic perennial with basal rosettes of sagittate (arrow-shaped) leaves and slender floating leaves in deep water. Spreads by stolons and corms..

What size pot to step japanese arrowhead up to

Pot japanese arrowhead on gradually — a seedling jumped straight into a huge pot sits in cold, wet, airless soil and stalls. Step up one or two sizes at a time as the roots fill each container, finishing in a large final pot or the ground. The aim is roots that never circle and never check.

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 japanese arrowhead

Pot japanese arrowhead on through the active growing season, whenever roots fill the current container — there is no single date, just "before it becomes root-bound". Avoid potting on during a cold snap.

Step-by-step: repotting japanese arrowhead

Pot on before it is root-bound. Check japanese arrowhead regularly; move it up as soon as roots reach the edge of the cell or pot, not after they have circled.
Step up one or two sizes. Choose the next container up — not a giant one. Cold, wet, unused soil around a small root system stalls seedlings.
Knock it out gently. Support the stem, tip the pot, and ease the rootball out without breaking it. A little teasing of circled roots at the base is fine.
Pot into rich mix. Set it into fresh heavy clay loam or aquatic planting compost at the same depth (tomatoes are the exception — they can go deeper to root along the stem).
Water in and grow on. Water well, keep it in good light, and resume feeding once it is established and growing again.

Aftercare

Water japanese arrowhead in well and keep it in bright light; a freshly potted-on seedling can wilt for a day while roots settle, so do not overcompensate by drowning it. Do not fertilise for about 1 week — fresh mix already carries nutrients and feeding freshly disturbed roots scorches them.

The right soil mix for japanese arrowhead

Japanese Arrowhead wants heavy clay loam or aquatic planting compost. Use a low-nutrient heavy loam or specialist aquatic basket compost with no added perlite or bark. Plant corms into baskets lined with hessian, top-dress with pea gravel to prevent soil dispersal into water. Always use fresh mix when you repot — reusing old, broken-down soil reintroduces the compaction and poor drainage you are repotting to fix.

Repotting japanese arrowhead — frequently asked questions

How often should you repot japanese arrowhead?

Pot on seedlings as they grow; not a perennial repot for japanese arrowhead. Japanese Arrowhead is a seasonal crop, so you pot it on as a growing plant rather than repotting a perennial. Step seedlings up gradually into heavy clay loam or aquatic planting compost so the roots never circle the cell, ending in a large final container. A root-bound transplant stalls and never fully recovers.

What size pot does japanese arrowhead need?

When is the best time of year to repot japanese arrowhead?

Can you put japanese arrowhead straight into a much bigger pot?

No. Even a fast-growing japanese arrowhead should only go up one pot size at a time. A vastly oversized pot holds a reservoir of wet soil the roots cannot reach, which stays cold and soggy and rots the roots — the opposite of what you wanted.

Should you fertilise japanese arrowhead after repotting?

Not immediately. Wait about 1 week after repotting japanese arrowhead. Fresh mix already contains nutrients, and feeding freshly cut or disturbed roots burns them. Resume your normal feeding routine once you see new growth.