Ocimum basilicum var. thyrsiflora, cultivar Siam Queen.
Not to be confused with tulsi or holy basil (Ocimum tenuiflorum), a related species with a spicy, clove-like flavour, or with Italian Genovese basil (classic Ocimum basilicum, the basil used for pesto). Thai basil has a distinctive aromatic profile—anise-like, lightly licorice-flavoured, and slightly spicy—that defines Southeast Asian cuisine.
Thai basil, horapha (ใบโหระพา) in Thai, is one of the aromatic pillars of Southeast Asian cooking and is essential to modern Thai, Vietnamese, Lao, and Cambodian cuisines. Native to the same biogeographic region as other basils—the tropical plains of South Asia where Ocimum basilicum evolved—it is distinguished by its adaptation to the hot, humid climates of Southeast Asia. That adaptation shaped both its physiology (purple stems and veins and narrow, lance-shaped leaves) and its very distinctive aromatic profile, dominated by the anise and licorice notes of methyl chavicol (or estragole), which is present in much greater amounts than in Italian Genovese basil.
Siam Queen is a modern selection from a breeding program designed to adapt the characteristics of traditional Thai basil—flavour, growth habit, fertility, and vigour—to North American climates and growing conditions. It is an upright, vigorous, well-branched plant 35 to 45 cm tall, with deep purple stems and veins, bright green lance-shaped leaves 4 to 6 cm long, and dense magenta-purple flower spikes that form in late summer. Its intense flavour is immediately recognizable: anise-like and lightly licorice-flavoured, with a warm clove undertone and a fresh, almost minty finish.
In the kitchen, Thai basil has very specific uses that should not be confused with those of Italian Genovese basil. It is essential as a final garnish on Vietnamese phở (whole leaves added just before eating, never cooked); in Thai coconut milk and galangal chicken soup (tom kha gai); in all Thai green and red curries (gaeng khiao wan, gaeng phet), where it is added at the end of cooking; in stir-fried noodles with basil and chili (pad kee mao, known as "drunken noodles"); in Cantonese seafood or beef stir-fries; and in Vietnamese spring rolls (gỏi cuốn), where whole leaves add freshness and aromatic complexity.
Worth knowing: the famous pad krapow (stir-fried chicken or pork with basil) traditionally uses holy basil (tulsi or krapow), not Thai basil, although substitution has become common in Western Thai restaurants where fresh tulsi is difficult to find.
Grower's tip: Like all basils, Siam Queen is an absolute heat lover that hates the cold. Germination stalls below 18 °C, and a young plant exposed to cold early in the season remains stunted all summer. Start indoors 6 to 8 weeks before the last frost at 22–25 °C, ideally on a heat mat. Transplant only once nighttime temperatures remain above 12 °C—early to mid-June in Québec, never before. Pinch the tips regularly to encourage branching and delay flowering: Thai basil flowers more quickly than Genovese basil, and its leaves become more pungent (less aromatic) after flowering.
For cooking, harvest whole stems rather than individual leaves and use them quickly. Thai basil's aromatic compounds are volatile and are quickly lost through drying, so use it fresh whenever possible.
- Open-pollinated. Tender annual. Insect-pollinated; crosses readily with other nearby Ocimum basilicum varieties (Genovese, Purple, Cinnamon, Lemon). Isolate for seed saving.
- Height: 35 to 45 cm.
- Maturity: 60 to 70 days to the first full-sized leaves.
- Exposure: full sun and warmth.
- Rich, well-drained, warm soil. Space plants 25 to 30 cm apart.
- Start indoors 6 to 8 weeks before the last frost at 22–25 °C. Transplant in early to mid-June once nighttime temperatures remain above 12 °C.
