Sweet Basil (Ocimum basilicum) — identification, habitat & look-alikes

Sweet basil is the classic Italian herb behind pesto, and Genovese is its best-known type. It is native to tropical Asia, including India and Southeast Asia. In the US it is grown almost entirely as a summer annual, though it has gone wild in Hawaii.

Appearance

Growth form: Upright annual herb, about 1–5 ft (30–150 cm) tall depending on the variety, with square stems.
Leaves: Glossy, oval leaves about 0.8–2 in (2–5 cm) long, in opposite pairs. Usually green; purple in some cultivars. Crushed leaves smell sweet, with anise or clove notes.
Flowers: Small white to purplish flowers in tiered spikes at the stem tips, with four or more flowers at each tier.
Fruit: Each flower produces four tiny seeds. They turn gel-coated when soaked, and the swollen seeds are used in drinks and desserts.

Look-alikes

Perilla (beefsteak plant)Perilla frutescens

Basil carries four or more flowers at each tier of the flower spike. Perilla has only two. Up close, basil's upper lobe is rounded and winged, while perilla's is broadly triangular.

Why they get mixed upBoth are aromatic, square-stemmed mint-family plants with similar flower spikes.

Holy basilOcimum tenuiflorum

It is hard to tell apart in photos. Holy basil has hairy stems and stays smaller, about 1–2 ft (30–60 cm) tall.

Why they get mixed upIt is a close relative, and a basil hybrid (spice basil) is sometimes sold under its name.

Where it lives

Typical habitats: Vegetable and herb gardens
Containers
Roadsides and waste ground (escapes)
Range: Native to tropical Asia. Naturalized on every main Hawaiian island, mostly in dry, disturbed lowland spots. On the mainland it occasionally escapes near gardens, as in Connecticut, Massachusetts, and Rhode Island, but these plants are usually short-lived.
Preferred conditions: Needs full sun and well-drained soil rich in organic matter. Frost kills it, so in most of the US it is started indoors and planted out after the last frost.

Growing it

Care: Downy mildew has become a serious problem in the eastern US since it reached Florida in 2007. It first yellows the leaves, then leaves gray-purple fuzz underneath. Sweet basil is the most susceptible type. Genovese-type cultivars with partial resistance are now available.

Name and classification

About the name: The name comes from the Greek word for king. Holy basil is a separate species, Ocimum tenuiflorum.
Varieties and forms: Popular cultivars include Genovese (large, dark green leaves), Sweet Thai (spicy flavor, purple stems), and 'Purple Ruffles' (purple-leaved).

Conservation

IUCN Red List: NENot EvaluatedNot yet assessed

Culture and stories

Culture and stories: A pot of basil appears in a tale in Boccaccio's Decameron. Keats retold it in his poem "Isabella, or the Pot of Basil."
Several Orthodox churches use basil to prepare holy water.