Basil

Identification and origin

Ocimum basilicum belongs to the family Lamiaceae, the mint family. It is native to a broad tropical area stretching from India and the Himalayas to Sri Lanka, Southeast Asia, southern China, and Taiwan, and on to New Guinea and northern Australia.

It is an herbaceous plant grown as an annual because it cannot take cold. Its square stems are hollow and branch readily. The opposite, oval leaves have smooth edges, bulge slightly between the veins, usually measure 1–3 in, and are strongly fragrant. Depending on the variety, they range from bright green to purple. The plant generally reaches 12 in – 3 ft tall. In summer it produces spikes of small, white, two-lipped flowers arranged in whorls, sometimes pink or purple in ornamental forms.

There are a great many cultivars. ‘Genovese’, with large leaves, is the classic pesto basil. ‘Spicy Globe’ and ‘Greek Dwarf’ form compact mounds that are ideal in pots; ‘Spicy Globe’ is slower to go to seed than most basils, a plus indoors. ‘Dark Opal’, ‘Rubin’, and ‘Purple Ruffles’ have purple foliage; ‘Siam Queen’ is a Thai basil with a licorice flavor; ‘Sweet Dani’ smells of lemon. ‘Prospera’ and the ‘Rutgers’ series resist downy mildew, and ‘Newton’ resists Fusarium wilt.

Light

Basil wants sun: at least six to eight hours of bright light a day. It still grows reasonably well in part sun with about six hours of direct light. Indoors, set it in a south-facing window. In winter natural light is not enough; a grow light placed about 6 in above the plants and run for ten to sixteen hours a day makes up for the short days.

Watering

Basil likes soil that drains well yet stays evenly moist. Feel the mix: as soon as the surface dries, water until the whole root ball is moist, then empty the saucer, because the roots should never sit in water. Never let it wilt, but soil that stays soggy invites root rot.

Pots dry out much faster than garden beds, so check them closely and always use a container with drainage holes. Water at the base without wetting the foliage, which makes it more prone to disease. In the garden, a 3 in mulch of pine needles or shredded leaves keeps moisture even and the leaves clean.

Potting mix

It needs rich, loose, well-drained soil. In the garden, work in compost or well-rotted manure before planting; the ideal pH is 6 to 6.5. Poor soil gives slow-growing basil with little flavor, but too much fertilizer is no better: overly fast growth dilutes the flavor of the leaves. For potted basil, a liquid fertilizer at half strength every four to six weeks indoors, or every three to four weeks outdoors, is enough.

Temperature

Basil is very tender. It thrives when daytime temperatures reach 70–79 °F. Move it outdoors only after the frosts are over, when nights stay above 50 °F and the soil has warmed to at least 61 °F. Cold soil and air stunt its growth, and the leaves blacken at 50 °F. Below 39 °F the foliage is damaged, and the first fall frost kills it. If a late frost threatens, cover the plants overnight with an upturned pot. It can only be grown in the ground year-round in USDA zones 10a and 10b.

Repotting

When started in a cell or small pot, basil is moved to its own pot once it has two pairs of true leaves. In the garden, space plants 6–12 in apart depending on the variety.

Before the first frost, you can cut back garden plants, dig them up, and pot them to keep indoors in a sunny window. Keep in mind that basil eventually flowers and dies, even indoors, so plan on sowing again every spring.

Pruning

Pinch out stem tips regularly for a bushy plant with more tender leaves. Remove flower buds as soon as they appear: basil that flowers and sets seed turns woody, yields less, and develops a bitter taste. Leave just a few spikes if you want to collect seed or enjoy the blooms of purple varieties.

Harvesting doubles as pruning. Pick young leaves as needed, or cut whole stems just above a pair of leaves: new shoots appear within about a week. Leave enough foliage so the plant is not set back, and avoid harvesting in wet weather, because fresh cuts are an entry point for gray mold for a day or two. Keep cut stems in a glass of water at room temperature; in the refrigerator the leaves turn black.

Propagation

Seed is the usual method. Sow indoors a few weeks before the end of the frost season, barely covered, in a mix kept at 70 °F or warmer: seeds sprout in five to ten days. Seedlings are easy to spot by their two D-shaped seed leaves. In the garden, sow directly once all danger of frost has passed and thin to 6–12 in. On a sunny windowsill, sowing again every two weeks keeps a steady supply, with harvests starting after four to five weeks.

Basil also grows from cuttings: take stem tips from a plant kept indoors over winter. This is the only method for sterile forms such as ‘African Blue’ basil. To save your own seed true to type, keep cultivars well apart, and never save seed from a crop that had Fusarium wilt or bacterial leaf spot.

Common problems

• Basil downy mildew: yellowing bounded by the leaf veins, then brown spots and purplish-gray fuzz on the leaf undersides before the leaves drop. Space plants out, water at the base, and choose resistant varieties.
• Fusarium wilt: stunted growth, browning tips, then sudden wilting; the fungus survives in the soil for years.
• Gray mold: dieback starting from harvest cuts.
• Root rots: yellowing, unthrifty plants, and branch dieback from the tips, often in poorly drained soil.
• Pests: aphids, spider mites, whiteflies, slugs, and, outdoors, Japanese beetles.

The same symptom can have several causes. Look at the whole plant before acting.

Toxicity

No toxicity is reported for dogs or cats, and both the leaves and flowers are eaten. Basil is a common culinary herb with no toxicity noted by veterinary references.