Ocimum basilicum var. thyrsiflora – Thai Basil: An In-Depth Look
Ocimum basilicum var. thyrsiflora, commonly known as Thai Basil, is a highly aromatic cousin of Sweet Basil, prized in Southeast Asian cuisine for its spicy-sweet flavor and remarkable heat tolerance. Belonging to the mint family (Lamiaceae), Thai Basil features narrow, serrated green leaves on striking reddish-purple stems. Unlike Sweet Basil, it stands up well to long cooking, making it a culinary powerhouse for broths, stir-fries, and curries.
The mint family also includes notable herbs such as rosemary, mint, sage, oregano, and thyme.
‘Siam Queen’ – An Award-Winning Thai Basil
Ocimum basilicum ‘Siam Queen’ is an improved cultivar of Thai Basil, prized for both its culinary excellence and ornamental value. This compact, bushy annual herb boasts glossy olive-green leaves with a rich licorice-basil fragrance. A distinct improvement over traditional Thai basil varieties, ‘Siam Queen’ offers impressive yields with leaves nearly twice the size, making it easier to harvest and use.
Awarded the All-America Selections title in 1997, ‘Siam Queen’ stands out for its vigorous growth and delayed flowering—allowing for prolonged harvests without bitterness. Its tall purple-red stems support clusters of showy lavender-purple tubular flowers in late summer, adding beauty to herb gardens and edible landscapes.
Native Range
Thai Basil is native to Southeast Asia. While its precise origin is debated, it is believed to have originated in either India or Asia before spreading to other parts of the world. It is now widely cultivated globally, especially in warm climates and home gardens.
Plant Type and Habit
Thai Basil is a tender perennial in frost-free zones but is grown as an annual elsewhere. It has a bushy, upright form with glossy, narrow leaves and firm purple stems. The square stems, a hallmark of mint family members, support branching growth and beautiful flower spikes. ‘Siam Queen’ is more compact but still vigorous, making it ideal for containers and tight garden spaces.
Size
Thai Basil typically grows 12–18 inches tall (30–45 cm), while ‘Siam Queen’ may reach up to 20–24 inches (50–60 cm) tall and wide, making it slightly larger and more productive than standard Thai basil varieties.
Lifespan
In USDA zones 10–11, it can be grown as a short-lived perennial. In cooler climates, it is treated as an annual and replaced each season or grown indoors to extend its lifespan. Keep it Fresh: 20 Best Herbs to Grow Indoors All Year
Flowers
In summer, ‘Siam Queen’ produces elegant spikes of lavender to dark-violet flowers at the top of their stems. This basil variety is especially valued for blooming late in the season, helping preserve its flavor longer. These blooms are attractive to bees and butterflies, and are edible.
Fruits
After flowering, it produces small nutlets containing seeds. These can be collected for future planting.
Foliage
The narrow, lance-shaped leaves of Thai Basil are firm and bright green with serrated edges, often tinged with purple. ‘Siam Queen’ features larger, glossy olive-green leaves with a powerful aroma and bold, spicy-sweet flavor. The foliage is perfect for use in hot dishes, maintaining flavor even after cooking.
Flavor Profile
- Anise-like: Rich licorice and clove notes define its bold flavor.
- Spicy-sweet: Balances sweetness with a touch of peppery heat.
- Heat-Stable: Retains its aroma and flavor when cooked, unlike Sweet Basil.
Hardiness
Thai Basil is perennial in USDA zones 10–11 and annual elsewhere. Protect it from frost and cold winds for optimal growth.
Uses
- Culinary: Ideal for pho, curries, stir-fries, noodle soups, and Thai dishes—often added just before serving to preserve aroma.
- Medicinal: Rich in essential oils like eugenol and limonene, offering antimicrobial and anti-inflammatory benefits.
- Ornamental: ‘Siam Queen’ adds aesthetic value with its green foliage and lavender flower spikes.
- Aromatic: Used in teas, fresh rolls, and herbal infusions for its fragrance.
- Edible Garden: A flavorful and attractive plant for herb gardens and borders.
Wildlife Value
Its blooms attract bees and butterflies, supporting biodiversity and pollinator populations.
Drought Tolerance
Prefers moist, well-drained soil. Once established, tolerates brief dry spells. Water regularly during heatwaves.
Deer and Rabbit Resistance
Strong aromatic oils help repel deer and rabbits.
Toxicity
Non-toxic to cats and dogs, according to the ASPCA.
Invasiveness
Not considered invasive but may self-seed under favorable conditions.
Benefits of Thai Basil
Health Benefits
- Rich in Vitamins: High in vitamins A, K, and C—supporting vision, bone health, and immune function.
- Essential Minerals: Contains magnesium, calcium, iron, and potassium.
- Antioxidant-Rich: Packed with polyphenols and essential oils such as eugenol and limonene, promoting general wellness.
- Anti-Inflammatory: Traditional uses include soothing digestive discomfort and inflammation.
Culinary Uses
- Essential in pho, curries, stir-fries, and noodle dishes.
- Sturdy leaves retain flavor in cooked dishes, unlike Sweet Basil.
- Used in fresh rolls, herbal teas, fruit salads, and cocktails.
- All parts are edible, including seeds (used like chia in desserts).
Cultural Significance
- Known as “Bai Horapa” in Thai and “Rau Hung Que” in Vietnamese.
- Valued in both cuisine and as a household herb across Southeast Asia.
Drawbacks of Thai Basil
Safety Concerns
- Estragole Content: As with other basils, contains estragole, a compound linked to cancer risk in high doses.
- Pregnancy and Breastfeeding: Culinary use is considered safe, but medicinal amounts should be avoided without medical guidance.
Cultivation Challenges
- Not frost-tolerant—must be protected or overwintered indoors.
- Susceptible to aphids, Japanese beetles, and diseases like downy mildew.
- Requires consistent pruning to maintain bushiness and flavor.
How to Grow and Care for Thai Basil
Planting
- Location: Full sun (6–8 hours). Partial shade is tolerated in very hot climates to prevent leaf scorch.
- Soil: Moist, fertile, well-drained soil with pH 6.0–7.5.
- Spacing: 10–12 inches (25–30 cm) apart to ensure air circulation.
- Depth: Ensure the soil is at least 6 inches (15 cm deep) for optimal root growth, especially in containers.
- Timing: Sow indoors 6–8 weeks before the last frost. Transplant after frost danger passes.
Watering
- Keep evenly moist but not soggy. Avoid overhead watering to reduce disease risk.
- Water lightly and frequently to maintain consistent moisture levels, adjusting to weather conditions.
- Container plants may need more frequent watering in warm weather.
Fertilizing
- Use a balanced liquid fertilizer monthly during active growth.
- Avoid high nitrogen levels, which can reduce flavor intensity.
Pruning
- Pinch growing tips regularly to encourage branching and delay flowering.
- Remove flower spikes unless growing for seed or pollinators.
Overwintering
- In zones below 10, bring indoors before the first frost.
- Grow in bright windows or under grow lights in winter.
How to Harvest Thai Basil
When to Harvest
- Harvest once the plant is 6–8 inches (15-20 cm) tall.
- Morning harvest preserves essential oils for optimal flavor.
How to Harvest
- Pick individual leaves as needed or cut stems just above leaf nodes to encourage regrowth.
- Trim regularly to prevent bolting (flowering).
Preserving
- Wrap stems in a damp paper towel and refrigerate for up to 2 weeks.
- Freeze chopped leaves in oil or water for long-term storage.
- Drying is possible but reduces flavor.
How to Propagate Thai Basil
From Cuttings
- Ensure the scissors or pruners are sharp and clean to prevent infection.
- Cut 4–6 inch (10-15 cm) stem segments below a leaf node.
- Strip lower leaves and place in water until roots form (5–10 days).
- Provide bright, indirect light to promote healthy root development.
- Use room-temperature water and replace it every few days to prevent bacterial growth.
- Transplant to soil when roots are 1–2 inches (2-5 cm) long.
From Seed
- Press seeds into moist seed-starting mix. Do not cover—light aids germination.
- Germinate at 70–75°F (21–24°C) in 5–10 days.
- Transplant outdoors after the last frost and when the soil has warmed.
Companion Planting with Basil
Good Companions
- Tomatoes: Basil enhances tomato flavor and repels tomato hornworms and aphids. Thai Basil, in particular, adds strong aroma and color, making it a beautiful border plant.
- Peppers: Basil provides pest control and traps heat and moisture, benefiting peppers. Thai Basil’s spicy aroma is especially effective at deterring insect pests.
- Oregano, Parsley, Marigolds: All enjoy similar sun and soil needs. Marigolds also help repel nematodes. Thai Basil can create a striking contrast among these companions due to its purple stems and violet flowers.
- Chives: Chives enhance the flavor of your basil and attract pollinators that also benefit Thai Basil’s flowering.
- Asparagus: Basil deters asparagus beetles and attracts beneficial insects like ladybugs, which control aphids.
- Borage: Borage attracts pollinators and improves the flavor of nearby plants. Thai Basil flowers also attract bees and butterflies.
Bad Companions
- Thyme: Prefers drier soil and less water than Thai Basil, which likes more consistent moisture.
- Fennel: Fennel is one of the few plants that has mostly bad companions and may inhibit Thai Basil’s growth.
Common Pests and Diseases of Basil
Pests
- Symptoms: Sticky residue (honeydew), curled or deformed leaves, and yellowing foliage.
- Treatment: Spray with a strong stream of water or apply insecticidal soap or neem oil. Reflective mulch can also help deter them.
- Symptoms: Silvery marks or pale patches on leaves, often causing leaf discoloration.
- Treatment: Remove and dispose of infested plants. Use soap-based insecticides or organic contact sprays.
- Symptoms: Large irregular holes in leaves, leaving a lacy appearance.
- Treatment: Hand-pick beetles early in the morning, apply neem oil or insecticidal soap. Floating row covers can help prevent infestations.
Snails and Slugs
- Symptoms: Irregular holes in leaves, mostly visible in the morning.
- Treatment: Scatter diatomaceous earth or crushed eggshells around plants. Hand-pick at night for best results.
- Symptoms: Ragged holes in leaves, especially overnight.
- Treatment: Raise pots to eliminate hiding spots and apply insecticidal soap or carbaryl dust.
- Symptoms: Seedlings cut off at soil level, especially overnight.
- Treatment: Use collars around the base of young plants to protect stems from damage.
- Symptoms: Small shot-hole damage on leaves, leading to poor growth.
- Treatment: Cover plants with floating row covers and apply neem oil or diatomaceous earth.
Diseases
- Symptoms: Yellow leaves, stunted growth, and brown streaks on stems.
- Treatment: Remove and destroy infected plants. Avoid replanting basil in the same soil for several seasons.
- Symptoms: Powdery mildew appears as white, powdery spots on basil leaves, stems, and buds. Infected leaves may curl, yellow, and drop prematurely.
- Treatment: Remove and destroy affected leaves. Improve air circulation and avoid overhead watering.
- Symptoms: Soft, brown or gray fuzzy mold on leaves and stems.
- Treatment: Remove affected parts, increase air circulation, and avoid overhead watering.
- Symptoms: Small black or brown spots on leaves, sometimes with yellow halos or stem streaking.
- Treatment: Water at the base, avoid wetting leaves, and space plants to allow air flow.
- Symptoms: Sudden wilting of plants despite green foliage.
- Treatment: There is no effective treatment. Remove and destroy infected plants and rotate crops to reduce disease buildup.
Prevention and Control
- Check plants often for early signs of pests and diseases.
- Provide good airflow by spacing basil properly and pruning dense growth.
- Use drip irrigation or water at the base to keep leaves dry.
- Practice crop rotation to limit recurring soil-borne issues.
- Promptly remove and discard infected plants to prevent further spread.
Popular Basil Plants
- Sweet Basil: The most widely grown variety, with large, smooth, glossy green leaves and a sweet flavor. Ideal for salads and Italian dishes, especially pesto. 18–24 in. tall (45–60 cm).
- Genovese Basil: A top choice for culinary use, especially in pesto. Features large, slightly crinkled leaves with a fragrance of jasmine, lemon, and licorice. 24–36 in. tall (60–90 cm).
- Thai Basil (Ocimum basilicum var. thyrsiflora): Known for its savory, spicy, and anise-like flavor. It has pointed green leaves with purple veins and showy purple flowers. Holds up well in stir-fries. 12–18 in. tall (30–45 cm).
- Greek Basil (Ocimum basilicum var. minimum): A compact variety with small leaves and intense flavor. Ideal for containers, pesto, tomato sauces, and pizzas. 8–12 in. tall (20–30 cm).
- Lemon Basil (Ocimum basilicum var. citriodorum): Features narrow, matte green leaves with a fresh lemon-anise taste. Common in Middle Eastern and Southeast Asian dishes. Great for seafood, lemonade, and compound butters. 24 in. tall (60 cm).
- Opal Basil: Also called Purple Basil, it boasts glossy, deep-purple leaves with sweet, earthy flavor notes of anise, ginger, and mint. Produces edible lilac-pink flowers. 18 in. tall (45 cm).
- Lettuce Leaf Basil: Known for its large, wrinkled, bright green leaves with a mild, slightly sweet taste. Often used fresh as a salad base. 18–24 in. tall (45–60 cm).
- Cinnamon Basil: A tall variety with smooth, pointed leaves and a spicy, cinnamon-like flavor. Popular in Asian cooking, especially in marinades, rice, and noodles. 24–36 in. tall (60–90 cm).
While every effort has been made to describe these plants accurately, please keep in mind that height, bloom time, and color may differ in various climates. The description of these plants has been written based on numerous outside resources.
