Ocimum basilicum var. thyrsiflora, cultivar Siam Queen.
Not to be confused with tulsi or holy basil (Ocimum tenuiflorum), a sacred Indian relative with a peppery-clove profile, nor with Italian Genovese basil (the classic pesto basil). Thai basil has a distinctive aromatic profile — anise, lightly licorice, slightly spicy — that defines the cooking of Southeast Asia.
Thai basil — horapha (ใบโหระพา) in Thai — is one of the aromatic pillars of Southeast Asian cooking, indispensable to modern Thai, Vietnamese, Lao and Cambodian cuisines. Native to the same biogeographic region as the other basils (the tropical lowlands of South Asia where Ocimum basilicum evolved), it diverges through its adaptation to the hot, humid climate of Southeast Asia, which has shaped its physiology — purple stems and veins, narrow lanceolate leaves — as well as its very particular aromatic profile, dominated by anise and licorice notes from methylchavicol (estragole), markedly more present than in Italian Genovese basil.
The Siam Queen variety is a modern American selection (an All-America Selections winner in 1997), from a breeding program aimed at adapting traditional Thai basil's characteristics — flavour, habit, fertility, vigour — to North American climate and growing conditions. An upright, vigorous, well-branched plant 35 to 45 cm tall, with deep purple stems and veins, bright green lanceolate leaves 4 to 6 cm long, and dense magenta-purple flower spikes that form in late summer. Intense, clearly recognizable flavour: anise and lightly licorice, with a warm clove bottom note 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. Essential as a final garnish on Vietnamese phở (whole leaves added just before eating, never cooked), in Thai coconut-milk-and-galangal chicken (tom kha gai), in all Thai green and red curries (gaeng khiao wan, gaeng phet) where it is added at the very end of cooking, in basil-and-chili stir-fried noodles (pad kee mao, "drunken noodles"), in Cantonese seafood or beef stir-fries, and in Vietnamese summer rolls (gỏi cuốn) where whole leaves bring freshness and aromatic complexity. Worth knowing: the famous pad krapow (basil-fried chicken or pork) traditionally uses holy basil (tulsi or krapow), not Thai sweet basil — though the substitution has become common in Western Thai restaurants where fresh tulsi is hard to find.
Grower's tip: Like all basils, Siam Queen is an absolute heat lover that hates cold — germination stalls below 18 °C, and a young plant that catches cold early in the season stays stunted all summer. Start indoors 6 to 8 weeks before last frost, at 22-25 °C, ideally on a heat mat. Transplant only once nights have stabilized above 12 °C — early to mid-June in Québec, never before. Pinch the tips regularly to stimulate branching and delay flowering: Thai basil bolts faster than Genovese, and the leaves become more pungent (less aromatic) after flowering. For the kitchen, harvest by whole stems rather than leaf by leaf, and use quickly — Thai basil's aromatics are volatile and fade fast after drying (use fresh whenever possible).
- Open-pollinated. Tender annual. Insect-pollinated; crosses readily with other Ocimum basilicum nearby (Genovese, Purple, Cinnamon, Lemon) — isolate for seed saving.
- Height: 35 to 45 cm.
- Maturity: 60 to 70 days for first full-sized leaves.
- Exposure: full sun, warmth.
- Rich, well-drained, warm soil. Space plants 25-30 cm apart.
- Indoor start 6 to 8 weeks before last frost, at 22-25 °C. Transplant early-to-mid June once nights are above 12 °C.
- All-America Selections variety 1997, selected for performance in North American temperate climates.
