How to Grow Basil Thai
'Siam Queen' from Seed
The award-winning Thai basil that is as beautiful as it is useful — an All-America Selections winner with dense ornamental purple flower spikes over emerald green leaves, a bold anise-clove flavour, and the delayed bolting that makes it the most productive Thai basil available from seed
'Siam Queen' is the most decorated Thai basil available from seed — a 1997 All-America Selections (AAS) winner, the North American equivalent of a Fleuroselect Quality Mark, awarded in recognition of its outstanding performance, flavour, productivity and ornamental value compared with other Thai basil varieties in rigorous comparative trials. The award recognises four qualities that set 'Siam Queen' apart: first, the unusually delayed bolting that extends the productive leaf period significantly beyond standard Thai basil varieties; second, the extra-large leaves — up to 4 inches (10cm) long — that make harvesting more productive; third, the strong, compact branching habit that produces bushy, high-yielding plants; and fourth, the ornamental purple flower spikes so intensely coloured and so densely packed that the plant is genuinely beautiful as well as useful.
The flavour is the characteristic Horapa Thai basil character — sweet anise and liquorice with a slightly spicy depth — but more concentrated and more complex than standard Horapa varieties, with an additional clove note that adds dimension. The purple stems and flower spikes are not merely ornamental; the stems carry the same aromatic oils as the leaves in higher concentration, and the flowers are intensely flavoured, making 'Siam Queen' one of the most versatile basils to cook with. In Thai cuisine it serves the same Horapa role as other sweet Thai basils; in the garden or on a patio, it pulls double duty as an outstandingly decorative herb.
Quick Facts at a Glance
Plant Type
Tender Annual (H1c)
Sowing Time
Mar–May indoors at 20–25°C
Harvest
8–10 weeks from sowing
Position
Warm, sunny; container champion
Height
30–45cm — compact and bushy
Difficulty Rating
2 out of 5 — Easy; best bolting resistance of range
Understanding the Plant
'Siam Queen' is an improved cultivar of Ocimum basilicum var. thyrsiflora — the Horapa Thai basil type — bred specifically for better garden and kitchen performance than standard Horapa varieties. The All-America Selections award, given after rigorous North American trial garden comparisons with existing varieties, confirmed that 'Siam Queen' outperformed its competitors on the key metrics of leaf size, plant vigour, bolting resistance, and ornamental value simultaneously — a genuinely unusual combination of improvements in a single variety.
The glossy emerald-green leaves are broader and larger than standard Thai basil, the stems are a rich, deep purple rather than the lighter purple-tinged stems of standard Horapa, and the flower spikes — when allowed to develop — are dense, intensely purple, and held erectly above the foliage in a way that makes the plant unmistakably ornamental. The compact, well-branched habit means that in a 20–25cm pot, 'Siam Queen' produces an extremely productive plant that also serves as a decorative feature beside the kitchen door or on a sunny patio table.
⭐ All-America Selections 1997 — What It Means
All-America Selections is the North American horticultural award system equivalent to Europe's Fleuroselect Quality Mark. AAS winners are selected through comparative trials at independent trial gardens across North America, assessing performance against existing varieties across a range of climates and growing conditions. Winning an AAS award requires demonstrating clear, measurable improvements over the best existing varieties in the same category. 'Siam Queen's' 1997 AAS win for herbs recognised specifically its delayed bolting, enlarged leaf size and superior ornamental value — qualities that remain genuinely outstanding nearly thirty years later.
Ornamental or Culinary — Both at Once
Most herbs require a choice between looking good and being productive — when you harvest extensively, the ornamental quality suffers. 'Siam Queen' is one of the few exceptions: the compact, bushy habit produces new growth so readily that regular leaf harvesting maintains the plant's full, rounded appearance, while the spectacular purple flower spikes — if allowed to develop by reducing harvesting — make the plant a genuine garden ornamental. For culinary use, pinch flower buds before opening; for ornamental use, allow the purple spikes to develop fully and scatter the edible flowers as garnish.
Sowing & Germination
Surface Sow — Light Required
Like all basils, 'Siam Queen' is photoblastic — it needs light to germinate. Surface sow onto moist compost with the absolute minimum dusting of vermiculite. Seeds should still be visible. Warmth (20–25°C) and light together produce reliable germination in 7–14 days.
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Sow indoors from March to May at 20–25°C. On a warm windowsill, on a heated propagator, or under a small LED grow light. Three to five seeds per 7cm pot; thin to the strongest one or two when seedlings have their first true leaves.
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Pinch out when plants have developed their first three sets of leaves. The naturally compact, well-branched habit of 'Siam Queen' means pinching is less critical than for other basils — the plant branches freely. However, an early pinch still improves bushiness further, and the resulting plant will be noticeably more productive than an unpinched one.
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Pot into a 20–25cm container for patio or windowsill display. 'Siam Queen' is bred for container growing — it maintains a naturally compact habit without becoming sprawling or leggy. Water at the base only, in the morning. Feed with half-strength balanced liquid fertiliser every two weeks from six weeks after sowing.
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Plant out from June in a warm, sheltered position. Space 30cm apart for border planting. In an outdoor raised herb bed in a warm summer, 'Siam Queen' produces an impressive and productive display. In containers, it can be moved indoors in September to extend the season on a warm windowsill through October.
Care Through the Season
Full Sun for Best Colour and Flavour
The purple stem and flower spike colouration is richest in full sun. In partial shade, stems become greener and the ornamental impact reduces. For both culinary flavour intensity and ornamental quality, position in the maximum available sun — south-facing patio, conservatory, or greenhouse.
Let It Flower — Flowers Are Edible
Unlike most basils where flowering is purely a problem to be delayed, 'Siam Queen' flowers are densely aromatic, intensely purple, and outstandingly beautiful as a garnish scattered over Thai dishes. The compact inflorescences are the ornamental highlight of the plant. Consider allowing some stems to flower fully while pinching others to maintain leaf production — a dual-purpose approach that maximises both uses.
The Best Bolting Resistance
'Siam Queen's' delayed bolting — the key AAS-winning quality — extends the productive leaf period significantly beyond standard Thai basil. Pinch flower buds from any stems you want to maintain as leaf producers; allow flower spikes on ornamental or garnishing stems. Even in a warm UK summer, 'Siam Queen' typically provides six to eight weeks of productive leaf harvest before bolting becomes inevitable.
Container Champion
In a 20–25cm pot in full sun, 'Siam Queen' produces one of the finest herb pot displays available — glossy emerald leaves, rich purple stems, and eventually the spectacular purple flower spikes. Place beside the kitchen door for the most practical combination of ornamental value and culinary access. Bring indoors in September to extend the season.
Cooking With Siam Queen
The flavour is classic Horapa — sweet anise-liquorice — but with additional depth from the clove notes in the purple stems. Use in the same way as Thai Large Leaf: in curries added ten to fifteen minutes before serving, as a finishing herb in phở, and as a fresh garnish. The large leaves are particularly well-suited to garnishing. The intensely flavoured edible flowers are the bonus use that makes this variety exceptional.
Water at Base, Morning
The same rule as all basils — water at the base only, in the morning. 'Siam Queen' in a container needs checking daily in warm weather and watering when the top cm is dry. Ensure containers have drainage holes; the compact, bushy habit of this variety can conceal root problems until they are advanced. Feel the pot weight as a guide to moisture level.
Harvesting & Using
Making the Most of Siam Queen
Leaf harvest: Cut from stem tips above a leaf pair, always leaving at least two sets of leaves below the cut. The large leaves are up to 10cm long — far more substantial than standard Thai basil — making both cooking and garnishing more practical. Harvest in the morning for peak flavour.
Flower garnish: Scatter individual flowers from the purple spikes over finished Thai dishes at serving — curries, noodles, rice, stir-fries. The purple colour against the golden sauce or green curry is visually striking. The flowers are intensely flavoured — a small number has a significant flavour impact.
Stem flavouring: The purple stems of 'Siam Queen' are more strongly flavoured than the leaves — they can be used as flavouring sticks in broths and soups, removed before serving like a bouquet garni. Simmer three or four stems in Thai broth for fifteen minutes for a concentrated Horapa flavour without adding leaf material.
Culinary applications: Thai green and red curry; Vietnamese phở; stir-fries with fish and vegetables; salads where the large leaves can be used whole; as a fresh finishing herb added in the last minute before serving. The same Horapa applications as Thai Large Leaf, but with more ornamental garnishing potential from the purple flowers.
Common Problems & How to Fix Them
| Problem | Likely Cause | What to Do |
|---|---|---|
| Stems less purple than expected | Insufficient light | The purple anthocyanin pigmentation in the stems is light-dependent — in partial shade, stems remain predominantly green. Move to a sunnier position for the richest purple stem colour. Maximum sun produces the most dramatic ornamental contrast between emerald leaves and purple stems. |
| Plant bolts despite good care | Heat stress or plant maturity | Even 'Siam Queen's' delayed bolting is eventually overcome by heat or age. Pinch flower buds promptly as they appear. When bolting does occur, the AAS-quality flowers are sufficiently beautiful to enjoy before pulling the plant and replacing it with fresh seedlings from a succession sowing. |
| Leaves smaller than expected | Underfeeding or rootbound container | Feed with half-strength balanced fertiliser every two weeks. Repot into a larger container if roots are emerging from the drainage holes. 'Siam Queen' leaves reach their maximum size — up to 10cm — only in well-fed, well-rooted plants with adequate growing space. |
| Cold damage in autumn | Temperatures dropping below 10°C | Bring containers indoors before night temperatures drop below 12°C. In the UK this typically means early to mid-September. A warm south-facing kitchen windowsill can extend the season through October with reasonable productivity. |
Plant Specifications
The award-winning Thai basil that earns its place in the garden and the kitchen equally
'Siam Queen' is the Thai basil for anyone who wants one plant to deliver two things simultaneously: a steady supply of large, intensely-flavoured leaves for curries and phở, and an ornamental spectacle of emerald and deep purple that genuinely enriches the patio or kitchen garden display. An All-America Selections winner for three decades of good reason — the delayed bolting alone makes it superior to standard Thai basil for UK growing, where the season is short and every productive week counts.
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