Fertilising guide

How to fertilise Thai Basil (Ocimum basilicum var. thyrsiflora 'Siam Queen')— schedule & NPK

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.

Growth habit: 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 fertiliser thai basil actually wants — and why

Thai Basil is a soft, fast leafy herb that you harvest hard — a modest balanced feed keeps tender growth coming without tipping it into bland or bolting.

A balanced general feed (even N-P-K) at modest strength — enough nitrogen to keep replacing the leaves you pick, but not so much that flavour thins or it bolts to seed.

For the language behind the three numbers on the bottle — what nitrogen, phosphorus and potassium each do — see the NPK ratio explained entry. The short version for thai basil: match the feed to the job the plant is doing right now, not to a generic “plant food” on the shelf.

How often to feed thai basil, and which months

Feeding only earns its keep while the plant is in active growth and can use the nutrients — pour feed into a dormant or low-light plant and it simply builds up as root-burning salt. For thai basil:

Light-to-moderate feeder. In good soil it needs little; in pots feed every 2-3 weeks with a balanced liquid feed at half strength. Avoid heavy nitrogen, which produces lush leaves at the expense of the aromatic oils. In practice: a balanced liquid feed every few weeks through the main growing and harvesting season (spring through early autumn), more often the harder you are picking it.

The dormant-season rule matters more than the exact interval: skip feeding entirely when thai basil is resting. For the wider context on indoor feeding rhythms across the seasons, the houseplant fertiliser schedule walks through the year month by month.

What strength to mix for thai basil

Half strength is a sensible default for thai basil — enough to fuel regrowth after cutting, gentle enough that the leaves stay aromatic rather than watery.

Feeding always goes onto already-damp soil, never dry roots — water thai basil first if the soil is dry, then apply the diluted feed. The companion question is when to water at all, covered in the thai basil watering schedule.

Signs you are over-feeding thai basil

Over-feeding is far more common — and more damaging — than under-feeding for most plants. The classic tells for thai basil:

Fast, soft, pale growth with diluted, less aromatic flavour.
Early bolting (running to flower) and a bitter edge.
Salt crust and scorched tips on container plants.

Signs you are under-feeding thai basil

Pale, slow regrowth after cutting and small leaves.
A tired, stalled plant that cannot keep up with harvesting.
Yellowing older leaves in a long-spent pot.

If the symptoms point at watering, light or roots rather than nutrition, the full thai basil care brief covers soil, humidity and the common problems for this species.

Flushing and leaching the salts

Pot-grown thai basil builds up feed salts quickly — water until it drains each time and flush the pot with plain water every few weeks, especially on a sunny windowsill.

Organic vs synthetic feeds for thai basil

Organic options

A diluted seaweed feed or worm-casting tea keeps soft growth coming without overdoing it. UK: dilute seaweed or Westland; US: Espoma Garden-tone or Neptune's Harvest. Gentle, hard to overdo, flavour-friendly.

Synthetic / liquid feeds

A balanced liquid feed at half strength through harvesting — UK: Phostrogen, Baby Bio or Westland; US: Miracle-Gro all-purpose at half strength. Fast regrowth; just do not overdo the nitrogen.

Brand names are examples, not endorsements, and UK and US ranges differ — check the label’s own NPK and dilution rate, since formulations change.

Fertilising thai basil — frequently asked questions

What fertiliser does thai basil need?

A balanced general feed (even N-P-K) at modest strength — enough nitrogen to keep replacing the leaves you pick, but not so much that flavour thins or it bolts to seed. Thai Basil is a soft, fast leafy herb that you harvest hard — a modest balanced feed keeps tender growth coming without tipping it into bland or bolting.

How often should I feed thai basil?

Light-to-moderate feeder. In good soil it needs little; in pots feed every 2-3 weeks with a balanced liquid feed at half strength. Avoid heavy nitrogen, which produces lush leaves at the expense of the aromatic oils. Light-to-moderate feeder. In good soil it needs little; in pots feed every 2-3 weeks with a balanced liquid feed at half strength. Avoid heavy nitrogen, which produces lush leaves at the expense of the aromatic oils. In practice: a balanced liquid feed every few weeks through the main growing and harvesting season (spring through early autumn), more often the harder you are picking it.

What strength of feed for thai basil?