Herbculinary herbSome care needed

Thai basil

Ocimum basilicum var. thyrsiflora

Tougher than sweet basil in every way that matters here: it stands more heat, holds its leaves longer once flowering starts, and lasts noticeably better in a pot. The purple stems and tight purple flower spikes make it easy to identify, and the aniseed flavour does not survive long cooking.

Position: Full sun
Water: moderate demand
Drainage: Free-draining, Moisture-retentive
Grows to: 45–60 cm × 25–30 cm
Hardiness: H1C
Harvest: July to September
Difficulty: Some care needed

Described view

Thai basil showing purple stems and tight purple flower spikes beside green sweet basil for comparison

August

Harvest

Thai basil’s year

Resolved for United Kingdom (general). Every window is stored against a climate event rather than a date, so a late frost pocket runs later than the band shows. What else August calls for.

Activity	M	A	M	J	J	A	S
Sow under cover	March.	April.	May.
Plant out				June.
Harvest					July.	August.	September.

Timing that travels

The band above resolves this plant's windows into months for one region. These are the windows themselves, stored against a climate event rather than a date — which is the form that survives being carried to a colder garden, a frost pocket, or next year.

What kind of herb this is

Herbs are a kitchen grouping rather than a botanical one, and the plants in it want opposite things. This is the one distinction that decides how to treat it.

Warm-season tender herb

Growing regime

source

Sourced guidancehigh confidence

Royal Horticultural Society: Thai basil — growing

Checked 2026-08-13

Does nothing below about 10 °C and sulks in a cold wet spring. Warmth and shelter matter more than feeding, and outdoors in Britain is usually the wrong place until midsummer.

Treated the same way: basil, shiso. The herb bed, split by regime

Indoors: Indoors spring to autumn

Where the indoor assessment comes from

IndoorsSourced guidancemoderate confidence: Royal Horticultural Society: Thai basil — growing indoors
Checked 2026-08-13

Almost any herb will sit on a windowsill in July. This is about whether it is still there in March, which is the part a pot on a supermarket shelf does not tell you.

What happens when it flowers

On a herb this is rarely a problem and rarely nothing. It is the point at which the plant stops being what you grew it for — by a lot, a little, or not at all.

No real effect on the leaves

source

Sourced guidancehigh confidence

Royal Horticultural Society: Thai basil — harvesting

Checked 2026-08-13

The purple flower spikes are part of the plant's appeal and the leaves stay usable through them, which is where Thai basil differs from sweet basil. Pinching still produces a bushier plant and more leaves, but it is a choice rather than a rescue.

source

Sourced guidancemoderate confidence

Royal Horticultural Society: Basil — pinching out

Checked 2026-08-13

Conditions it needs

Light: Full sun
Frost: Frost tender
Hardiness rating: H1c (RHS)
Grows best at: 20–32 °C
Soil: Peat-free compost, Loam
Soil pH: 6–7.5
Drainage: Free-draining, Moisture-retentive
Water needs: moderate
Drought: poor tolerance
Waterlogging: poor tolerance
Heat: good tolerance
Wind and exposure: poor tolerance

Where these figures come from

LightSourced guidancemoderate confidence

Royal Horticultural Society: Thai basil — position

Checked 2026-08-13

FrostSourced guidancehigh confidence

Royal Horticultural Society: Thai basil — hardiness

Checked 2026-08-13

Hardiness ratingSourced guidancemoderate confidence

Royal Horticultural Society: RHS hardiness ratings

Checked 2026-08-13

Grows best atSourced guidancemoderate confidence

Royal Horticultural Society: Thai basil — temperature

Checked 2026-08-13

SoilSourced guidancemoderate confidence

Royal Horticultural Society: Thai basil — soil

Checked 2026-08-13

Soil pHSourced guidancemoderate confidence

Plant Finder — Missouri Botanical Garden: Ocimum basilicum — culture

Checked 2026-08-13

DrainageSourced guidancemoderate confidence

Royal Horticultural Society: Thai basil — soil

Checked 2026-08-13

Water needsHorticultural consensusmoderate confidence

No external citation — see the evidence type above.

Checked 2026-08-13

DroughtHorticultural consensusmoderate confidence

No external citation — see the evidence type above.

Checked 2026-08-13

WaterloggingHorticultural consensusmoderate confidence

No external citation — see the evidence type above.

Checked 2026-08-13

HeatHorticultural consensusmoderate confidence

Handles heat better than sweet basil, which is its main advantage here.

No external citation — see the evidence type above.

Checked 2026-08-13

Wind and exposureHorticultural consensusmoderate confidence

No external citation — see the evidence type above.

Checked 2026-08-13

On your soil

Computed by comparing this plant's recorded requirements against each soil's recorded characteristics. A recorded horticultural fact overrides the comparison where GrowingHQ holds one.

Soil	Verdict	Why
Heavy clay	Works, with one condition	Thai basil will crop on heavy clay, but one thing has to be dealt with first. It needs ground that drains, and this soil holds water. For a crop lifted before winter the risk is not rot but the season: waterlogged ground is cold and unworkable in spring, so sowing is late and germination patchy.Improve the drainage before planting — a raised bed, or bulky organic matter worked in over several seasons. Do not add sand to clay in small quantities; it makes the problem worse.
Sandy	Works, with one condition	Thai basil will crop on sandy, but one thing has to be dealt with first. It has little drought tolerance and this soil dries out fast — it will check and bolt in the first dry spell without watering.Water and mulch will carry it through. On this soil the watering is not optional in a dry spell, which is the difference between a crop and a failure.
Chalky and alkaline	Works, with one condition	Thai basil will crop on chalky and alkaline, but one thing has to be dealt with first. It has little drought tolerance and this soil dries out fast — it will check and bolt in the first dry spell without watering.Water and mulch will carry it through. On this soil the watering is not optional in a dry spell, which is the difference between a crop and a failure.
Loam	Prefers this soil	Loam suits Thai basil as it stands: The pH suits it: it wants 6 to 7.5, and this soil is in that range. The free drainage is exactly what it wants. Loam is among the soils it is recorded as suiting. The soil holds moisture, which saves a great deal of watering.
Silty	Prefers this soil	Silty suits Thai basil as it stands: The pH suits it: it wants 6 to 7.5, and this soil is in that range. The free drainage is exactly what it wants. The soil holds moisture, which saves a great deal of watering.
Peaty and acid	Works, with one condition	Thai basil will crop on peaty and acid, but one thing has to be dealt with first. It needs ground that drains, and this soil holds water. For a crop lifted before winter the risk is not rot but the season: waterlogged ground is cold and unworkable in spring, so sowing is late and germination patchy.Improve the drainage before planting — a raised bed, or bulky organic matter worked in over several seasons. Do not add sand to clay in small quantities; it makes the problem worse.
Shallow over rock	Works, with one condition	Thai basil will crop on shallow over rock, but one thing has to be dealt with first. It has little drought tolerance and this soil dries out fast — it will check and bolt in the first dry spell without watering.Water and mulch will carry it through. On this soil the watering is not optional in a dry spell, which is the difference between a crop and a failure.
Made ground and urban soil	Works, with one condition	Thai basil will crop on made ground and urban soil, but one thing has to be dealt with first. It needs ground that drains, and this soil holds water. For a crop lifted before winter the risk is not rot but the season: waterlogged ground is cold and unworkable in spring, so sowing is late and germination patchy.Improve the drainage before planting — a raised bed, or bulky organic matter worked in over several seasons. Do not add sand to clay in small quantities; it makes the problem worse.

Not sure which you have? Four hand tests will settle it.

Space and spacing

Mature height: 45–60 cm
Mature spread: 25–30 cm
Rooting depth: 25 cm
Spacing: 25–30 cm apart

Where these figures come from

Mature heightHorticultural consensusmoderate confidence: No external citation — see the evidence type above.
Checked 2026-08-13
Mature spreadHorticultural consensusmoderate confidence: No external citation — see the evidence type above.
Checked 2026-08-13
Rooting depthHorticultural consensusmoderate confidence: No external citation — see the evidence type above.
Checked 2026-08-13
SpacingSourced guidancemoderate confidence: Royal Horticultural Society: Thai basil — spacing
Checked 2026-08-13

Measured centre to centre. Plants set closer than this compete for light and water long before they look crowded, and the crowding cannot be undone once they are in.

In a container

Minimum volume: 5 litres or more
Minimum depth: 20 cm

Where the container figures come from

Minimum volumeHorticultural consensusmoderate confidence: No external citation — see the evidence type above.
Checked 2026-08-13
Minimum depthHorticultural consensusmoderate confidence: No external citation — see the evidence type above.
Checked 2026-08-13

Does better in a pot under cover than outdoors in most British summers, and lasts noticeably longer in one than sweet basil. Water in the morning at the base and give it the brightest position available.

Where and how you can grow it

Each way of growing is assessed separately, and “not assessed” is a real answer rather than a gap we have filled in with a guess.

Method	Suitability	What it needs
Container	Well suited source Sourced guidancemoderate confidence Royal Horticultural Society: Growing method suitability Checked 2026-08-13	5 litres
Greenhouse	Well suited source Sourced guidancemoderate confidence Royal Horticultural Society: Growing method suitability Checked 2026-08-13
Polytunnel	Well suited source Sourced guidancemoderate confidence Royal Horticultural Society: Growing method suitability Checked 2026-08-13
Windowsill	Well suited source Sourced guidancemoderate confidence Royal Horticultural Society: Growing method suitability Checked 2026-08-13
Hydroponics	Well suited source Sourced guidancemoderate confidence Purdue Extension — Purdue University: Hydroponic herbs Checked 2026-08-13
Open ground	Workable source Sourced guidancemoderate confidence Royal Horticultural Society: Growing method suitability Checked 2026-08-13
Traditional dug bed	Workable source Sourced guidancemoderate confidence Royal Horticultural Society: Growing method suitability Checked 2026-08-13
Raised bed	Workable source Sourced guidancemoderate confidence Royal Horticultural Society: Growing method suitability Checked 2026-08-13
Cold frame	Workable source Sourced guidancemoderate confidence Royal Horticultural Society: Growing method suitability Checked 2026-08-13
Indoor pot	Workable source Sourced guidancemoderate confidence Royal Horticultural Society: Growing method suitability Checked 2026-08-13

Growing it

PruningPinch the growing tips out regularly, as with sweet basil. Thai basil is more forgiving if you let it flower — the leaves stay usable — but a pinched plant still produces far more of them.
ProtectionWater in the morning and at the compost rather than over the leaves, and keep it above 12 °C at night. Thai basil takes heat better than sweet basil but has exactly the same intolerance of a cold wet night.
WateringWater in the morning and at the base of the plant, never over the leaves and never in the evening — basil left with wet foliage overnight is what botrytis takes. Let the surface dry slightly between waterings but do not let the plant wilt. Thai basil stands dry conditions rather better than sweet basil, and a pot that is soaked and then allowed to drain properly does better than one kept constantly damp.
FeedingA plant in fresh multipurpose compost needs nothing for the first six weeks. After that, or where a plant is being picked hard, a balanced liquid feed every two or three weeks keeps the leaves large — basil being repeatedly cut is a leaf factory and runs out of nitrogen. Stop feeding as light fades in autumn.
HarvestingPick from the top, not the bottom. Cut whole shoot tips just above a pair of side buds and the plant branches into two; strip the lower leaves off instead and you get a bare stem with a tuft on the end. Thai basil holds its leaf quality longer after flowering than sweet basil does, but pinching the purple flower spikes out still keeps it producing for weeks longer.
SowingSow indoors from March to May at 18–20 °C, barely covering the seed, and do not be tempted outside early — Thai basil planted out before nights are reliably above 10 °C sits and sulks and rarely recovers. Plant out in June once the weather has settled, or keep it in a pot under cover, which in most of Britain gives the better plant.

Where this guidance comes from

PruningSourced guidancemoderate confidence: Royal Horticultural Society: Basil — pinching out
Checked 2026-08-13
ProtectionSourced guidancemoderate confidence: Royal Horticultural Society: Basil — growing
Checked 2026-08-13
WateringSourced guidancehigh confidence: Royal Horticultural Society: Basil — cultivation
Checked 2026-08-13
FeedingSourced guidancehigh confidence: Royal Horticultural Society: Basil — feeding
Checked 2026-08-13
HarvestingSourced guidancehigh confidence: Royal Horticultural Society: Basil — harvesting
Checked 2026-08-13
SowingSourced guidancehigh confidence: OSU Extension Service — Oregon State University: Herb seed germination temperatures
Checked 2026-08-13

The bud you cut to becomes the new shoot, and it grows in the direction it points — which is why an outward-facing bud opens the plant up and an inward-facing one crosses the middle.

Is it worth putting under cover?

Whether it will grow there is on the methods table above. This is whether the space is well spent, which is a different question and usually the one being asked.

Structure	What protection is worth	What decides it
Greenhouse	Only possible under cover source GrowingHQ editorial positionmoderate confidence What protection is worth for this crop, judged against the same standard as every other record in this file. GrowingHQ editorial synthesis — GrowingHQ Checked 2026-08-13	Night temperature, and a longer requirement than sweet basil — it is a tropical perennial being grown as a summer annual.
Polytunnel	Extends the season source GrowingHQ editorial positionmoderate confidence What protection is worth for this crop, judged against the same standard as every other record in this file. GrowingHQ editorial synthesis — GrowingHQ Checked 2026-08-13	Overnight temperature. Glass holds overnight heat that polythene loses, so a tunnel runs cooler at night than a greenhouse in the same garden. For a crop at the edge of its heat requirement that difference decides whether the fruit ripens.

On a windowsill

Spring to autumn

source

GrowingHQ editorial positionmoderate confidence

How long the crop is actually productive on an ordinary British windowsill without added light.

GrowingHQ editorial synthesis — GrowingHQ

Checked 2026-08-13

Productive from roughly March to October and not after. Growth stops when the light does, and a plant pushed through the dark months usually does not recover. Night temperature. More heat-demanding than sweet basil and correspondingly worse in a British winter, but longer-lived on a warm sill through summer.

Growing it without soil

Nearly anything can be grown in a soilless system by somebody with enough equipment. The useful question is whether it is worth it, in which technique, and what goes wrong.

Well suited

An established soilless crop. Grown this way commercially or routinely by home growers, with predictable results.

source

Sourced guidancemoderate confidence

Purdue Extension — Purdue University: Hydroponic herbs

Checked 2026-08-13

Assessed by extension from sweet basil, which is the same species. The variety difference shows in heat tolerance rather than in anything the system does.

Techniques that suit it

Nutrient film (NFT)A shallow stream of solution runs over bare roots in a sloped channel. Cheap and productive for small fast crops; a pump failure kills a crop within hours because there is no reservoir at the roots.
Deep water cultureRoots hang in aerated solution. Very productive and very unforgiving of solution temperature — warm solution holds less oxygen and root disease follows.
Drip-fed mediumSolution is dripped into an inert medium around each plant. The most forgiving technique and the one that suits large, long-season, heavy crops.
Ebb and flowA bed of inert medium floods and drains on a timer. The medium holds moisture between floods, which makes it far more tolerant of interruption than NFT.

source

Sourced guidancehigh confidence

Purdue Extension — Purdue University: Hydroponic herbs

Checked 2026-08-13

Root space: Modest, as sweet basil. It grows taller and may need support in a shallow channel.
Lighting: Needs supplementary light
Solution temperature: 20–26 °CSourced guidanceDepends on conditions
pH band: 5.5–6.5

Where these figures come from

Root spaceHorticultural consensusmoderate confidence

No external citation — see the evidence type above.

Checked 2026-08-13

LightingHorticultural consensusmoderate confidence

No external citation — see the evidence type above.

Checked 2026-08-13

Solution temperatureSourced guidanceDepends on conditionsmoderate confidence

Applies when: A degree or two warmer than sweet basil suits it, and it tolerates the upper end better — which makes it the easier of the two to run alongside fruiting crops.

Purdue Extension — Purdue University: Warm-season crops in solution culture

Checked 2026-08-13

pH bandSourced guidancehigh confidence

Outside roughly this band, individual nutrients become unavailable even when they are present in the solution — which is why a crop can starve in a full reservoir.

Purdue Extension — Purdue University: Hydroponic solution management — pH

Checked 2026-08-13

What actually goes wrong

Cold, exactly as sweet basil. Thai basil takes heat better and is no more tolerant of a cold solution, which is the mistake its reputation invites.
source
Sourced guidancemoderate confidence
- Royal Horticultural Society: Basil — growing
Checked 2026-08-13
Pump or power failure. In nutrient film and aeroponics there is no reserve at the roots, so a crop can be lost within hours; medium-based systems hold moisture and are far more forgiving.
source
Sourced guidancehigh confidence
- Purdue Extension — Purdue University: Hydroponic system design — failure modes
Checked 2026-08-13

No nutrient recipes here

GrowingHQ records pH bands, which are measurable properties of nutrient availability, and deliberately records no EC or PPM targets and no mixing instructions. Published targets vary by cultivar, growth stage, water source and technique, and the figures that apply to your system are on the product you bought.

Harvesting

When it is readyReady once the plant has four or five pairs of leaves and is branching. Unlike sweet basil it holds its leaf quality after the purple flower spikes appear, so a flowering plant is still worth picking — but pinching those spikes out keeps it producing for several weeks longer.
How to pick itCut whole shoot tips just above a pair of side buds so the plant branches. Use fresh; it neither dries nor freezes well.

Where the harvest guidance comes from

When it is readySourced guidancehigh confidence: Royal Horticultural Society: Basil — harvesting
Checked 2026-08-13
How to pick itHorticultural consensusmoderate confidence: No external citation — see the evidence type above.
Checked 2026-08-13

What goes wrong

Problem	How often	Notes
Aphids	occasional source Horticultural consensusmoderate confidence No external citation — see the evidence type above. Checked 2026-08-13
Root rot	common source Horticultural consensusmoderate confidence No external citation — see the evidence type above. Checked 2026-08-13

Sources for this page

GrowingHQ editorial synthesis · OSU Extension Service · Plant Finder · Purdue Extension · Royal Horticultural Society

Each figure on this page carries its own citation, under the “where these figures come from” note in its section. Record last reviewed 2026-08-15.