Ocimum basilicum ‘Dark Opal’ – Dark Opal Basil: An In-Depth Look
Ocimum basilicum ‘Dark Opal’, commonly known as Dark Opal Basil, is a stunning cultivar of Sweet Basil cherished for its dramatic foliage and subtle licorice aroma. Developed at the University of Connecticut in the 1950s, it quickly gained popularity for both culinary use and ornamental appeal. This All-America Selections winner (1962) is ideal for garden borders, containers, and kitchen windowsills.
Basil belongs to the mint family (Lamiaceae), which includes notable herbs such as rosemary, mint, sage, oregano, and thyme.
Native Range
Originally bred in the United States, Dark Opal Basil is a cultivar of tropical Asian and African Sweet Basil. It has since become popular worldwide.
Plant Type and Habit
This tender herbaceous annual has a bushy, upright habit. Its square stems and compact form make it well-suited to small spaces and container gardens.
Size
Dark Opal Basil grows up to 18–24 inches tall and 24 inches wide (45–60 cm), making it an excellent choice for borders and decorative planting.
Lifespan
Typically grown as an annual, Dark Opal Basil completes its life cycle in one season, although it can be overwintered indoors in warm conditions. Keep it Fresh: 20 Best Herbs to Grow Indoors All Year
Flowers
In summer, it produces elegant spikes of small, tubular cerise flowers arranged in whorls above the foliage. These edible blooms attract pollinators.
Fruits
Dark Opal Basil develops small nutlets containing seeds that can be saved for future planting.
Foliage
Its showy deep purple, nearly black leaves are broad, smooth, and glossy, with a mild licorice aroma. The foliage is high in anthocyanins—powerful antioxidants that also give it its vibrant color.
Hardiness
Grown as an annual in USDA zones 3–9. It can be grown year-round in zones 10–11 or indoors with adequate light.
Award
All-America Selections winner (1962).
Uses
- Culinary: Adds color and mild licorice flavor to tomato sauces, pasta, stews, vinegars, and fresh salads.
- Ornamental: Striking foliage creates dramatic contrast in gardens and containers.
- Medicinal: Rich in anthocyanins and traditionally valued for its antioxidant properties.
- Aromatic: Leaves can be used in potpourri or infused into oils.
- Edible Garden: Combines beauty and taste in one plant.
- Pollinator Garden: Summer flowers attract bees and butterflies.
- Container Plant: Ideal for patios and indoor growing.
- Mass Planting: Stunning visual accent in mass displays.
- Natural Dye: High anthocyanin content makes it a candidate for food coloring or dye production.
Wildlife Value
Dark Opal Basil’s cerise blooms offer nectar to beneficial insects, including pollinators like bees and butterflies.
Drought Tolerance
Although basil prefers moist, well-drained soil, established plants can tolerate brief dry spells. Consistent watering, especially during hot weather, helps preserve leaf quality and flavor. Basil grown in containers will require more frequent watering, but overwatering should be avoided to prevent root rot.
Deer and Rabbit Resistance
Basil is generally unappealing to deer and rabbits due to its strong scent and essential oils. While not entirely immune to browsing, it is less likely to be damaged than more tender herbs and vegetables. Planting basil along the edges of vegetable beds can help deter herbivores.
Toxicity
According to the ASPCA, basil is non-toxic to cats and dogs.
Invasiveness
Not invasive. May self-seed mildly in warm climates if flower spikes are not trimmed.
Benefits of Dark Opal Basil
Health Benefits
- Rich in Antioxidants: Dark Opal Basil contains high levels of anthocyanins, which help neutralize free radicals and reduce oxidative stress.
- Anti-Inflammatory: Compounds such as eugenol may provide natural relief from inflammation and discomfort.
- Heart Health: The herb’s polyphenols and essential oils may support cardiovascular health by improving circulation and reducing blood pressure.
- Digestive Support: Traditionally used to ease bloating, gas, and indigestion due to its soothing properties.
Culinary Uses
- Its vibrant color and subtle licorice flavor enhance sauces, salads, stews, and vinaigrettes.
- Can be infused into vinegars or oils for a decorative and flavorful touch.
- Perfect for making purple pesto or garnishing tomato-based dishes.
Aesthetic and Therapeutic Benefits
- Deep purple foliage makes this basil a striking ornamental plant in containers or garden beds.
- The licorice-like scent is calming and may be used in natural aromatherapy applications.
Drawbacks of Dark Opal Basil
Safety Concerns
- Estragole Content: Like other basils, it contains estragole, which may pose health concerns in high doses.
- Pregnancy: Culinary amounts are safe, but essential oils or large quantities should be avoided during pregnancy.
Cultivation Challenges
- Color Sensitivity: Requires full sun to maintain deep purple pigmentation. Shade may result in greener leaves.
- Cold Sensitivity: Not frost-tolerant and must be replanted annually in most climates.
- Pests: Like other basil types, may attract aphids or spider mites.
How to Grow and Care for Dark Opal Basil
Planting
- Location: Choose a sunny spot with at least 6–8 hours of direct sunlight daily to preserve its rich color.
- Soil: Prefers fertile, well-draining soil with a pH of 6.0–7.5. Add compost to improve poor soils.
- Spacing: Space plants 10 to 12 inches (25–30 cm) apart to ensure airflow and prevent disease.
- Timing: Start indoors 4–6 weeks before the last frost or sow directly after frost danger passes.
Watering
- Consistency: Keep soil evenly moist. Water when the top inch feels dry but avoid waterlogging.
- Morning Watering: Water early to allow foliage to dry, minimizing fungal risk.
- Containers: Water more frequently in pots, especially in hot weather.
Fertilizing
- Feeding: Use a balanced, diluted fertilizer monthly during the growing season. Avoid excess nitrogen to maintain flavor and color.
- Organic Options: Compost or liquid seaweed extract can enhance soil fertility naturally.
Pruning
- Pinching: Begin pinching back tips once the plant reaches 6 inches tall to encourage bushiness.
- Deadheading: Remove flower spikes to prolong leaf production unless growing for ornamental value.
Overwintering
- Indoors: Bring containers inside before frost and place in bright light for continued harvest.
- Annual Habit: Typically grown as an annual in all but frost-free regions.
How to Harvest Dark Opal Basil
When to Harvest
- Timing: Begin harvesting once the plant reaches 6 to 8 inches tall and has established several leaf sets.
- Ongoing Harvest: Regular harvesting promotes fuller growth and delays flowering.
How to Harvest
- Leaf Picking: Snip or pinch individual leaves for culinary use.
- Stem Cutting: Cut stems just above a node to encourage branching and regrowth.
Preserving
- Drying: Retains color better than other basils but may lose some flavor.
- Freezing: Chop and freeze with oil in trays or freeze whole leaves in bags for future use.
- Purple Pesto: Combine with garlic, olive oil, and nuts for a vibrant twist on traditional pesto.
How to Propagate Dark Opal Basil
From Cuttings
- Step 1: Take 4–6 inch cuttings from a healthy stem just below a leaf node.
- Step 2: Strip the lower leaves and place the stem in a glass of water in bright, indirect light.
- Step 3: Change the water every 2–3 days. Roots form in about a week.
- Step 4: Transplant rooted cuttings into moist, well-draining soil.
From Seed
- Indoor Start: Sow seeds indoors 6–8 weeks before the last frost. Use a moist seed-starting mix.
- Light Requirement: Basil seeds need light to germinate—press them into the surface but don’t cover.
- Germination: Seeds sprout in 5–10 days at 70–75°F (21–24°C).
- Transplanting: Move seedlings outdoors after the danger of frost has passed.
Companion Planting with Basil
Good Companions
- Tomatoes: Basil enhances tomato flavor and repels tomato hornworms and aphids.
- Peppers: Basil provides pest control and traps heat and moisture, benefiting peppers.
- Oregano, Parsley, Marigolds: All enjoy similar sun and soil needs. Marigolds also help repel nematodes.
- Chives: Chives enhance the flavor of your basil.
- 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.
Bad Companions
- Thyme: Prefers drier soil and less water than basil.
- Fennel: Fennel is one of the few plants that has mostly bad companions.
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).
- 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.
