Holy basil, or tulsi, is a sacred plant in Hindu tradition and a culinary herb across South and Southeast Asia, with a clove, pepper and anise flavour distinct from any other basil. It goes into Thai stir-fries (it is the basil in pad kra pao) and into herbal teas. The purple-stemmed Kapoor type is the most commonly grown; Krishna, with dark purple leaves, and Vana, the wild type with a lighter flavour, are worth seeking out.
Start seed indoors six weeks before last frost; it is slower to establish than sweet basil but vigorous once growing. Set plants out 12 to 18 inches apart in full sun and rich, moist soil that drains, once nights stay above 50°F. Pinch the tips to keep it bushy and harvest from the top. It is less prone to downy mildew than sweet basil.
The flower spikes are beautiful and hum with bees from midsummer to frost, so let a few plants bloom; they set seed freely and give volunteers the next year in milder climates. It blackens at the first frost, so cut and dry what you want to keep. Deer leave it alone. It is native to the Indian subcontinent.
Downy mildew
Yellowing on top of leaves with grey-purple fuzz underneath. Spreads rapidly in humid conditions.
Remove affected leaves. Improve air circulation. Avoid overhead watering. Copper spray as preventive.
Fusarium wilt
Sudden wilting, brown streaks in stems, yellowing leaves. Affects sweet basil varieties most.
No cure. Remove and destroy plants. Do not replant in the same spot for 3+ years. Choose resistant varieties.
Damping Off
Seedlings collapse at the soil line with thin, water-soaked stems; seeds may fail to emerge or rot before sprouting.
Sow in well-draining mix, avoid overwatering, and ensure good airflow. Remove affected seedlings and let the surface dry between waterings.
Aphids
Aphididae →Clusters of small soft insects on new growth and flower buds.
Knock off with a strong jet of water. Ladybirds and lacewings are natural predators. Insecticidal soap as last resort.