How to Grow Siamese Sweet Basil

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Siamese Sweet Basil

Ocimum basilicum var. thyrsiflorum

herb

Siamese Sweet Basil is a Thai heirloom variety of basil known for its distinctive anise or licorice-like flavor with subtle spice notes. It produces attractive purple flower spikes and lush green foliage, making it both ornamental and culinary. This variety is particularly prized in Southeast Asian cooking and thrives in warm, sunny conditions.

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Example Siamese Sweet Basil timeline · Pittsburgh, PA (Zone 6b)

Plant Siamese Sweet Basil
Around Apr 17

Near Pittsburgh's average last spring frost.
Tend & protect
Late spring–summer

Watering, feeding, and pest-watch reminders.
Harvest Siamese Sweet Basil
Around Jun 16

≈ 60 days after planting

Sample dates for Pittsburgh, PA. Sign up to get exact Siamese Sweet Basil dates for your own zip code.

Growing Conditions

☀️

Sun: Full sun, 6-8 hours daily minimum

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Water: Keep soil consistently moist but not waterlogged. Water deeply when top inch feels dry. Reduce watering in cooler months. Prefers humidity.

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Spacing: 12 inches

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Days to maturity: 60-75 days

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Planting depth: 0.25 inches (surface sow or barely cover seeds)

Soil

Type: Well-draining loamy soil

pH: 6.0-7.0

Amendments:

Compost Peat moss Perlite

Growing Zones

Find your zone →

Thrives in zones 9b-11; grown as annual in cooler zones

9b 10a 10b 11a 11b

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Growth Stages

Seedling

2-3 weeks

Tiny leaves emerge; plants are delicate with 2-4 true leaves visible

Maintain warm soil (70-75°F), provide bright light, keep soil moist, thin seedlings to 6 inches apart if direct seeding

Vegetative Growth

4-6 weeks

Bushy plant develops multiple sets of leaves, reaching 12-18 inches tall

Pinch off top leaves regularly to encourage branching, maintain consistent moisture and warmth, apply balanced fertilizer every 3-4 weeks

Flowering

Ongoing if not deadheaded

Purple or violet flower spikes form at stem tips, very ornamental

Pinch off flowers regularly if focusing on leaf production; allow some to flower for pollinator attraction, continue watering

Peak Harvest

4-8 weeks

Plant is full and lush with abundant aromatic leaves and flower spikes at optimal flavor

Harvest leaf tips frequently to maintain flavor and encourage more growth, use leaves fresh or process for storage

Decline

Variable with season

Plant slows growth as temperatures cool or after extended flowering and harvest

Plant naturally declines in cool weather; consider replanting in spring or maintaining indoors

Companion Planting

Plan your garden →

Plant with:

Tomato Pepper Eggplant Marigold Nasturtium Parsley Oregano

Avoid planting near:

Fennel Rue Sage

Common Pests

Spray with neem oil or insecticidal soap; wash off with strong water spray; introduce ladybugs
Increase humidity, spray with water, use neem oil; ensure good air circulation
Use yellow sticky traps, spray with insecticidal soap, apply neem oil
Handpick early morning, use pheromone traps away from plant, apply neem oil

Uses

🍳

Thai Cuisine & Stir-fries

Culinary

Siamese Sweet Basil's anise-licorice flavor is essential in Thai cooking, used fresh in curries, stir-fries, pad thai, and as a garnish. The purple flowers are also edible and add visual appeal. [source]

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Fresh Drinks & Desserts

Culinary

The distinctive flavor works wonderfully in cocktails, lemonades, sweet teas, and Asian desserts. The aroma enhances beverages and infusions. [source]

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Traditional Digestive Aid

Medicinal

In traditional Southeast Asian medicine, Siamese basil is used to support digestion and as a mild anti-inflammatory. The essential oils possess antimicrobial properties. [source]

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Garden Ornament & Pollinator Attractor

Ornamental

The vibrant purple flower spikes make it attractive in containers and garden beds. Flowers attract bees, butterflies, and other beneficial insects. [source]

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Natural Fragrance & Pest Deterrent

Household

Fresh or dried Siamese basil releases a pleasant aroma in the home and is traditionally believed to repel mosquitoes and flies indoors. [source]

This is not medical advice. LizPlants is not a medical resource. Always consult a qualified healthcare professional before using plants medicinally.

Harvest Tips

Begin pinching off leaves once plant has 6-8 true leaves. Harvest in the morning after dew dries for best flavor. Pinch off flower spikes to extend leaf production, or harvest them fresh for garnish and flavoring. Cut stems just above a leaf node to encourage branching. Leaves are best used fresh but can be dried or frozen. Harvest regularly—the more you pick, the bushier the plant grows.

Fun Facts

🌱 Siamese Sweet Basil is believed to hold spiritual significance in Thai culture and is sometimes planted near home altars for blessing and protection.
🌱 Unlike sweet basil, this variety's anise flavor intensifies when the plant flowers, making the purple spikes both a culinary ingredient and aesthetic feature.
🌱 The plant grows vigorously in tropical and subtropical climates and can become perennial in warm zones, sometimes flowering year-round.

Gardening Terms

New to gardening? Learn about terms used in this guide:

Full Sun Heirloom True Leaves

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