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Repotting guide

When & how to repot Sweet Flag (Acorus calamus)

Also called sweet flag, calamus, sweet rush.

More about sweet flag

About Sweet Flag

Acorus calamus · also called sweet flag, calamus · herb

Sweet flag is a vigorous marginal aquatic perennial grown for its aromatic, iris-like blades that release a sweet, spicy scent when crushed. It thrives at pond edges, bog gardens and consistently wet ground in sun to part shade. Long used in folk medicine and perfumery, it spreads by stout rhizomes. The foliage contains β-asarone, so handle the plant knowingly.

Mature size: Typically 60-100 cm tall, spreading indefinitely by rhizome to form broad clumps.

How to tell sweet flag needs repotting

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

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 sweet flag

Pot on seedlings as they grow; not a perennial repot. Sweet Flagis grown for one season, so the question is really “how often to pot on” — keep moving it up before the roots circle. Vigorous, rhizomatous, deciduous to semi-evergreen marginal perennial forming spreading colonies of erect, sword-shaped, fan-arranged blades from a creeping aromatic rhizome..

What size pot to step sweet flag up to

Pot sweet flag 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 sweet flag

Pot sweet flag 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 sweet flag

  1. Pot on before it is root-bound. Check sweet flag regularly; move it up as soon as roots reach the edge of the cell or pot, not after they have circled.
  2. 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.
  3. 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.
  4. Pot into rich mix. Set it into fresh heavy, fertile, permanently wet loam or aquatic compost at the same depth (tomatoes are the exception — they can go deeper to root along the stem).
  5. Water in and grow on. Water well, keep it in good light, and resume feeding once it is established and growing again.

Aftercare

Water sweet flag 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 sweet flag

Sweet Flag wants heavy, fertile, permanently wet loam or aquatic compost. Loves rich, mucky pond margins and bog soil. Plant in aquatic baskets with loam-based aquatic compost; tolerates a wide pH range in waterlogged conditions. Always use fresh mix when you repot — reusing old, broken-down soil reintroduces the compaction and poor drainage you are repotting to fix.

Repotting sweet flag — frequently asked questions

How often should you repot sweet flag?

Pot on seedlings as they grow; not a perennial repot for sweet flag. Sweet Flag is a seasonal crop, so you pot it on as a growing plant rather than repotting a perennial. Step seedlings up gradually into heavy, fertile, permanently wet loam or aquatic 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 sweet flag need?

Pot sweet flag 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. Use our pot size calculator to size it from the plant's current pot and root spread.

When is the best time of year to repot sweet flag?

Pot sweet flag 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.

Can you put sweet flag straight into a much bigger pot?

No. Even a fast-growing sweet flag 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 sweet flag after repotting?

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

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