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Repotting guide

When & how to repot Thai Basil (Ocimum basilicum var. thyrsiflora 'Siam Queen')

Also called Siam Queen Basil.

More about thai basil

About Thai Basil

Ocimum basilicum var. thyrsiflora 'Siam Queen' · also called Siam Queen Basil · herb

Thai basil 'Siam Queen' is a sturdy, heat-loving basil with narrow green leaves, striking purple stems and flower spikes, and a distinctive sweet anise-liquorice flavour central to South-East Asian cooking. More bolt- and heat-tolerant than sweet basil, this tender annual thrives in full sun and warmth, cropping prolifically through summer when pinched regularly to stay bushy.

Mature size: 45-60cm (18-24in) tall, 30cm (12in) wide

Watch for — Root rot in cold, wet soil: Overwatering or cold conditions rot the roots; use free-draining mix and ease off watering in cool spells, as this tropical herb hates soggy feet.

How to tell thai basil needs repotting

Repotting on a calendar is less reliable than reading the plant. For thai basil, 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 thai basil

Pot on seedlings as they grow; not a perennial repot. Thai Basilis grown for one season, so the question is really “how often to pot on” — keep moving it up before the roots circle. Upright, vigorous branching tender annual with purple-tinged stems and dense purple flower spikes. Pinching the tips keeps it compact and leafy; 'Siam Queen' holds its flavour and resists bolting better than sweet basil, though removing flowers still prolongs leaf production..

What size pot to step thai basil up to

Pot thai basil 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 thai basil

Pot thai basil 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 thai basil

  1. Pot on before it is root-bound. Check thai basil 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 rich, well-drained loam or quality potting mix, ph 6.0-7.0 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 thai basil 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 thai basil

Thai Basil wants rich, well-drained loam or quality potting mix, ph 6.0-7.0. Prefers fertile, moisture-retentive yet free-draining soil with organic matter. In containers use a good peat-free mix with sharp drainage; cold, wet soil causes root rot, so let pots drain freely. Always use fresh mix when you repot — reusing old, broken-down soil reintroduces the compaction and poor drainage you are repotting to fix.

Repotting thai basil — frequently asked questions

How often should you repot thai basil?

Pot on seedlings as they grow; not a perennial repot for thai basil. Thai Basil is a seasonal crop, so you pot it on as a growing plant rather than repotting a perennial. Step seedlings up gradually into rich, well-drained loam or quality potting mix, ph 6.0-7.0 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 thai basil need?

Pot thai basil 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 thai basil?

Pot thai basil 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 thai basil straight into a much bigger pot?

No. Even a fast-growing thai basil 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 thai basil after repotting?

Not immediately. Wait about 1 week after repotting thai basil. 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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