Basil (Ocimum species) is an annual herb that matures about 18 inches tall. Depending on species and cultivar, the leaves range from green to dark purple and broad to pointed. The flowers are small, white or purplish, and produced in clustered upright spikes at the top of the stems.
Types of basil
Several species of basil can be cultivated in Maryland, but sweet basil (Ocimum basilicum) is the most commonly-grown type for culinary use. Where possible, select cultivars that are resistant to downy mildew. "Bush basil" is a compact variant of sweet basil that has smaller leaves and a rounded growth habit.
Holy basil (Ocimum sanctum) is used in Indian and Thai cuisine. Herbal medicines are made from the leaves, stems, and seeds.
Several purple-leaf basil varieties exist, and their foliage can be used decoratively in the garden as well as for consumption or garnishes.
Planting and caring for basil
Plant basil in a full sun location, where it receives 6 or more hours of direct sun daily in summer. The soil should be moist but well-drained.
Basil grows easily from seed sown after all danger of frost has passed; it is not tolerant of chilly temperatures. Pinch stems as they lengthen to promote bushy, compact growth. Avoid encouraging overly-lush growth (such as from heavy fertilization) as it may reduce flavor. If planted in rows, thin seedlings (remove extra plants) to a spacing of 12 inches between plants and 18 inches between rows.
Preventing basil downy mildew on sweet basil
Warm, wet, humid weather encourages the spread of fungal downy mildew disease. Don’t crowd the plants because good air circulation around the foliage helps reduce the risk of infection.
Plant resistant basil cultivars such as 'Obsession', 'Devotion', 'Prospera', and 'Amazel' to help prevent the disease. Seeds can be ordered from some online vegetable seed and plant suppliers, and some local garden centers may stock one or more of the resistant cultivars as transplants.
Monitor basil plants closely for symptoms and be prepared to remove and throw out infected plants. It is safe to eat leaves from infected plants - the disease does not harm people. If basil plants decline or die due to downy mildew, sow fresh seeds in containers or in another part of the garden.
Harvesting and using basil
Basil leaves can be picked about 6 weeks after planting. It is best to cut leaves for drying just before flowers open.
Spicy-scented basil leaves are one of the most popular of the Mediterranean herbs used in cooking. Cooks favor basil for tomato dishes in either fresh or dried form.