How to Grow Thai Basil
'Large Leaf' (Horapa) from Seed
Bai Horapa — the glossy-leaved, purple-stemmed Thai basil with its sweet anise-liquorice character that holds through cooking, essential for green and red Thai curries, Vietnamese phở, and any dish where the anise dimension of Thai cooking is needed
When you buy 'Thai basil' in a supermarket or Asian grocery shop, this is what you are buying — Bai Horapa, the sweet anise Thai basil (กะเพรา in Thai), distinguished from Italian Genovese by its glossier, more pointed leaves, its distinctive purple-tinged stems, its pink-purple flowers, and above all by its fundamentally different flavour profile. Where Genovese basil tastes of warm sweet herbaceousness with clove undertones, Horapa tastes of sweet anise and liquorice — the methyl chavicol compounds that define the Thai basil flavour are quite different from the linalool-dominant chemistry of Italian basil, producing a taste that is both recognisably "basil" and entirely distinct from anything used in European cooking.
The 'Large Leaf' selection offers the same Horapa flavour character in a larger-leafed form than standard varieties — individual leaves up to twice the size of regular Thai basil, making harvest more productive and garnishing more practical. The variety is a selection of Ocimum basilicum var. thyrsiflora — botanically it is a sweet basil variety, sharing its species with Classic Italian, but the variety-level flavour difference is profound. Horapa's flavour is also notably more heat-stable than Italian basil — it holds its anise character through cooking in a way that Genovese cannot, which is why it is used in curries and stir-fries rather than just as a finishing 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, sheltered sun; min 15°C
Height
30–50cm
Difficulty Rating
2 out of 5 — Similar to Classic Italian
Understanding the Plant
Thai Large Leaf is a selection of Ocimum basilicum var. thyrsiflora — the Horapa type of Thai basil, selected for larger-than-standard leaf size. The glossy, pointed leaves with their purple-veined undersides and purple stems are visually distinct from both Italian Genovese (with its rounder, lighter-green leaves) and Holy Basil (with its smaller, slightly hairy, more serrated leaves). The purple coloration in the stems is the result of anthocyanin pigmentation triggered by the plant's genetics — a reliable visual identifier for the Horapa type.
The distinctive Horapa flavour comes primarily from methyl chavicol (also called estragole) — a compound found in star anise and tarragon as well as in Thai basil — which is responsible for the sweet anise-liquorice character. The concentration of this compound in Horapa is significantly higher than in Italian Genovese, producing a flavour that reads as a completely different herb to an untrained palate despite both being basil.
Horapa vs Kaprao — Two Completely Different Herbs
The two Thai basils available in this range — Large Leaf Horapa and Holy Basil Kaprao — taste completely different and are not interchangeable in cooking. Horapa (this variety): sweet anise-liquorice, methyl-chavicol dominant, used in curries, phở and garnishing. Kaprao (Holy Basil): intensely peppery clove-camphor, eugenol-dominant, used in Pad Kra Pao stir-fries. Thai cooks consider these two herbs as distinct as, say, rosemary and tarragon. Using one in place of the other in an authentic recipe produces a fundamentally different result. If you cook Thai food regularly, grow both.
The Heat-Stable Advantage
Unlike Italian Genovese, which rapidly loses its aromatic character when heated, Horapa holds its anise-liquorice flavour through moderate cooking. This is why it works in Thai green and red curries — it can be added earlier in the cooking process than Italian basil without complete loss of flavour. The most intense Horapa flavour is released at the end of cooking (a handful of fresh leaves added in the last minute), but the herb also works added fifteen to twenty minutes before the end, unlike Genovese which would need to be added at the very last moment.
Sowing & Germination
Surface Sow — Same Light Requirement as Classic Italian
Thai Large Leaf, like all basils, is photoblastic — it needs light to germinate. Surface sow onto moist seed compost with only the finest dusting of vermiculite covering the seed. The seed should still be partially visible. Place in warmth (20–25°C) and bright light. Germination takes 7–14 days.
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Sow indoors from March to May at 20–25°C. The temperature requirements are the same as Classic Italian Genovese — this is the main practical similarity between the two species, despite their completely different flavour. On a warm, sunny windowsill from March, or in a propagator from late February.
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Pinch out when 15cm tall. Slightly later than the three-leaf-sets pinch point for Classic Italian, reflecting the slightly more vigorous initial growth habit of Thai basil. Pinch the central growing tip to encourage branching.
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Plant out in June in a warm, sheltered position. Thai Large Leaf performs reasonably well outdoors in a warm UK summer — better than Holy Basil but still preferring a sheltered, south-facing position or a greenhouse. Space 25cm apart. Water at the base in the morning only.
Care Through the Season
Warm and Sunny
Thai Large Leaf needs the same warmth as Italian basil — minimum 15°C at all times, and ideally above 20°C for best growth and flavour intensity. In a warm UK summer, outdoor growing in a sheltered south-facing position works well. In a cool summer, a greenhouse or conservatory produces significantly more productive plants.
Water at Base, Morning
Water deeply at the base only in the morning. Thai basil is moderately more tolerant of humidity than Italian Genovese, but wet overnight foliage still encourages mildew. Allow the top cm of compost to dry slightly between waterings. Outdoors, avoid overhead irrigation.
Harvest Regularly
Harvest from the stem tips — always cut above a pair of leaves to encourage branching. Thai basil flowers more readily than Genovese — the purple flower spikes appear earlier and more frequently. Remove buds promptly to extend productive leaf growth. If flowers are left, leaf production slows and eventually stops.
Edible Flowers
Unlike Italian basil where flowering is purely a problem to delay, Thai basil flowers are edible and flavorful — they taste of intensely concentrated Horapa and make a beautiful garnish scattered over curries, phở and salads. The pink-purple flowers can be left to develop if the primary purpose is garnishing rather than leaf production.
Added During Cooking
Horapa's heat-stable character means it can be added to curries ten to fifteen minutes before the end of cooking — longer than Italian basil, shorter than dried spices. For the fullest anise-liquorice flavour, add a second handful of fresh leaves in the last minute before serving in addition to the earlier addition.
The Complete Thai Basil Pair
Growing Thai Large Leaf Horapa alongside Holy Basil Kaprao provides the full range of Thai basil applications from a small growing space. Together they cover the sweet-anise dimension (curries, phở, garnishing) and the peppery-clove dimension (stir-fries, Pad Kra Pao). The two plants share broadly similar growing conditions and can be grown side by side in adjacent pots.
Harvesting & Cooking
Horapa in the Kitchen — Uses and Techniques
Thai green curry: Add whole Horapa leaves five to ten minutes before the end of cooking; repeat with fresh leaves at serving. The cooked leaves infuse the curry base with their anise character; the fresh leaves at serving deliver a bright, fragrant top note.
Vietnamese phở: Served as a fresh garnish alongside bean sprouts and lime wedges — diners add Horapa leaves to their bowl at the table, where they soften slightly in the hot broth and release their anise fragrance. The 'Large Leaf' selection's larger leaves are particularly suited to this application.
Fresh garnishing: Scattered over finished Thai noodle dishes, fried rice, and curries at serving. The large leaves of this variety make particularly attractive garnishes — whole leaves rather than torn or shredded pieces.
Not for pesto: The anise-liquorice character of Horapa produces a very different result from Italian Genovese in Western applications. It can be used experimentally in fusion contexts — Horapa pesto with fish, for example — but is not a like-for-like substitute in traditional Italian recipes.
Storage: Store fresh stems in a small jar of water at room temperature, as with Italian basil. Do not refrigerate. Use within three to five days for best flavour. The large leaves of this variety can be dried on a rack in a warm, ventilated space — dried Horapa retains more of its anise character than dried Genovese basil.
Common Problems & How to Fix Them
| Problem | Likely Cause | What to Do |
|---|---|---|
| Weak stems, floppy growth | Insufficient light or warmth | Thai basil needs strong light and warmth to develop properly. Move to a brighter, warmer position. In the UK, a south-facing greenhouse or warm conservatory produces significantly stronger plants than an outdoor border position in most summers. |
| Plants bolt quickly | Heat stress, drought, or genetics | Thai basil flowers more readily than Italian Genovese. Remove flower buds as they appear. Consistent moisture and warmth delays bolting. Accept that Thai basil has a shorter productive leaf period than Genovese — grow successionally, sowing a new pot every six to eight weeks. |
| Flavour not as anise-forward as expected | Cool growing conditions | The methyl chavicol compounds responsible for the anise character are produced more abundantly in warm conditions. Plants grown in maximum warmth — greenhouse or conservatory — produce noticeably more intense anise flavour than cool windowsill plants. Harvest in the morning for peak flavour. |
| Black leaves after chilling | Temperature drop below 10°C | All basils are damaged below 10°C. Bring indoors as soon as temperatures drop, including at night in late summer. A single cold night can damage or kill outdoor basil plants. |
Plant Specifications
The Thai basil in the supermarket — grown fresh in your own kitchen
Fresh-grown Large Leaf Horapa bears the same relationship to supermarket Thai basil that fresh garden Genovese bears to the supermarket pot: incomparably superior in flavour, aroma and productivity. Grown in a warm position and harvested regularly, this variety provides the anise-forward Thai basil that every curry, every phở, and every noodle dish needs — the large leaves particularly suited to generous garnishing. Grow alongside Holy Basil Kaprao for the complete Thai basil collection.
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