Holy Basil (Ocimum tenuiflorum)

Holy basil (Ocimum tenuiflorum) is a fragrant herb that’s indigenous to India. It’s a popular ingredient in Thai cuisine. When used for this purpose, it’s called Thai holy basil. Adherents of the Vaishnava strain of Hinduism use it in religious ceremonies. Holistic practitioners worldwide place immense value on the aromatic essential oil derived from the plant.

I. Appearance and Characteristics

Ocimum tenuiflorum, commonly known as holy basil or tulsi, is an aromatic perennial plant in the family Lamiaceae. It is native to tropical and subtropical regions of Australia, Malesia, Asia, and the western Pacific. It is widely cultivated throughout the Southeast Asian tropics. This plant has escaped from cultivation and has naturalized in many tropical regions of the Americas. It is an agricultural and environmental weed.

Holy basil is an erect, many-branched subshrub, 30–60 cm (12–24 in) tall with hairy stems. Leaves are green or purple; they are simple, petioled, with an ovate blade up to 5 cm (2 in) long, which usually has a slightly toothed margin; they are strongly scented and have a decussate phyllotaxy. The purplish flowers are placed in close whorls on elongated racemes.

The three main morphotypes cultivated in India and Nepal are Ram tulsi (the most common type, with broad bright green leaves that are slightly sweet), the less common purplish green-leaved (Krishna or Shyam tulsi) and the common wild vana tulsi (e.g., Ocimum gratissimum).

Holy Basil (Ocimum tenuiflorum)

II. How to Grow and Care

Sunlight

Holy basil thrives best under full sun exposure, requiring a substantial amount of light for optimal growth and development. It performs most favorably when it receives at least 6 to 8 hours of direct sunlight daily. While holy basil has a tolerance for partial sun, meaning some shadow for a part of the day, inadequate sunlight can impede its vigor and reduce essential oil concentrations in its leaves, affecting both flavor and medicinal potency.

With its ability to tolerate somewhat lower light levels, holy basil exhibits resilience, yet consistently bright conditions are fundamental to cultivate a robust and healthy plant. When planted outdoors, a spot that receives ample morning sunlight with some afternoon shade can be beneficial, particularly in hotter climates to prevent scorching.

Temperature

Holy basil is native to Tropical Asia and likes a warm and humid environment. It is more resistant to heat than cold, and to drought than waterlogging. The optimum growth temperature is between 22 to 28 ℃ in the daytime and 13 to 18 ℃ at night. The leaves will turn yellow if the ambient temperature is lower than 13 ℃. Damage may be caused to the plant if the temperature is lower than 10 ℃, and the leaves will droop if the temperature is higher than 35 ℃.

Watering

Native to the tropical climate of South Asia, holy basil thrives in warm and humid conditions, reflecting its preference for consistent moisture. This species is drought-tolerant but performs best when the soil remains slightly moist without becoming waterlogged. Watering should be done every week to maintain ideal moisture levels. As an aromatic herb commonly grown outdoors, holy basil benefits significantly from mulching which helps retain soil moisture and reduces the need for frequent watering.

Soil

Holy basil is a deep-rooted plant, with the roots growing up to 51 to 102 cm deep. Therefore, well-drained, fertile, loose sandy loam or humus soil is the best culture soil. Potted plants can be planted in a mixed culture of peat and vermiculite at 2:1, or garden soil and fully decomposed organic fertilizer at 2:1.

Holy basil does not have strict requirements on soil conditions. It can be planted in both acid and alkaline soil and can tolerate pH values of 5-8, but neutral soil is the best.

Fertilizing

Fertilize holy basil biweekly with high nitrogen fertilizer during active growth for lush foliage. Use half-strength liquid formulation to encourage healthy growth without root burn. Decrease frequency in winter to monthly, aligning with holy basil’s slower growth. Over-fertilization can lead to excess foliage over flower production; observe holy basil for cues. Proper dosage is critical; refer to product-specific guidelines. Always water holy basil before and after application to facilitate nutrient absorption and prevent fertilizer buildup.

Pruning

Holy basil can be pruned when they reach 20 cm in height. Yellow leaves, once found, should be cut off promptly. If the plant grows too exuberantly and the ornamental effect deteriorates, redundant branches can be cut off. After pruning, add a small amount of slow-release fertilizer. Holy basil usually dies after the seeds are ripened, and its life can be extended by cutting off the flowers to prevent the seeds from ripening.

Propagation

Holy basil is best propagated from cuttings during the spring and summer months. Propagation is moderately easy, and successful development can be observed through new growth and root formation. Keep the cuttings well-humidified and provide indirect sunlight.

Transplanting

The prime time for transplanting holy basil is during the warm embrace of mid-spring to early summer. Choose a sunny location with well-draining soil for ideal growth. Be gentle with the roots while transplanting to maintain their health and vigor.

Overwintering

Holy basil hails from a tropical climate, lacking inbuilt mechanisms to handle frosty winters. Hardy only until 50°F, holy basil slides into dormancy when exposed to continuous, intense chill. Gardeners should mimic its sun-drenched dive into subtropics, using light insulation, indoor heating or wintering inside the home. Otherwise, treat it as an annual, sowing anew when springtime warmth beckons.

Repotting

Repot holy basil in early spring every 2-3 years to support its bushy growth and enhance root health. Choose a container one size larger to accommodate expansion. Post-repotting, ensure holy basil gets ample sunlight and regular watering for recovery. This mindful attention will amplify its aromatic presence, making the extra care worthwhile.

III. Uses and Benefits

  • Ornamental uses

An ornamental species with an intriguing scent, holy basil is planted in gardens for its good looks and sweet perfume. It is often planted near Hindu shrines, including outdoor versions of household shrines. It is also planted in flowerbeds and outdoor containers, and used as a garden border.

  • Culinary uses

Tulasi (Sanskrit: Surasa) has been used in Ayurvedic and Siddha practices for its supposed medicinal properties.

Thai cuisine

The leaves of holy basil, known as kaphrao in the Thai language (Thai: กะเพรา), are commonly used in Thai cuisine for certain stir-fries and curries such as phat kaphrao (Thai: ผัดกะเพรา) — a stir-fry of Thai holy basil with meats, seafood or, as in khao phat kraphao, with rice. Two different types of holy basil are used in Thailand, a “red” variant which tends to be more pungent, and a “white” version for seafood dishes. Kaphrao should not be confused with horapha (Thai: โหระพา), which is normally known as Thai basil, or with Thai lemon basil (maenglak; Thai: แมงลัก).

  • Insect repellent

For centuries, the dried leaves have been mixed with stored grains to repel insects.

  • Nematicidal

The essential oil may have nematicidal properties against Tylenchulus semipenetrans, Meloidogyne javanica, Anguina tritici, and Heterodera cajani.

  • Disinfection

Water disinfection using O. tenuiflorum extracts was tested by Bhattacharjee et al. 2013 and Sadul et al. 2009. Both found an alcoholic extract to be more effective than aqueous or leaf juice. Sundaramurthi et al 2012 finds the result to be safe to drink and antimicrobial. A constituent analysis by Sadul found alkaloids, steroids, and tannins in the aqueous, and alkaloids and steroids only in the alcoholic extract.

IV. Harvesting and Storage

Holy basil can be used as an ornamental plant or picked as a fragrant herb to cook with at any time. Its leaves can be harvested before flowers bloom. Cut leaves with a small knife and try to avoid damaging the roots. Its leaves can be used in cold dishes or salads, or to remove the fishy taste of seafood, and dried leaves can be used as a spice. Its stem, leaves, and flowers can be harvested before blooming. They can be used to extract essential oils. It is best to harvest them at 9:00-17:00 after 2-3 consecutive sunny days. Its seeds are generally harvested in late summer.

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