Basil grows best when it's sunny and warm -- at least 70 degrees F during the day and 50 degrees F at night. Colder temps cause the delicate leaves to turn brown and the plant to die.
Like most herbs, basil doesn't need extra fertilizer, which can alter the flavor of the leaves and promote flowering over leaf production.
Keep the soil uniformly moist. Too much water suffocates and drowns the roots. To test the soil, stick your finger in the soil up to your first knuckle. If it feels dry, water. If not, hold off.
Pinching the stems yields better branching and leaf growth. To pinch, find the intersection of two leaves along the stem, and use your fingers or small scissors to take off the stem above those leaves. New branches grow from each of those two leaf nodes. Pinch off flower buds as soon as they appear unless you wish to save seeds or grow the flowers for their blooms.
Harvest the top leaves first, then continue cutting to encourage new growth. Harvest all the leaves just before the plant begins to flower.
Growing basil in pots is a good alternative if you don't have other garden space. Plant seeds 1/4 inch deep, spacing them 1 inch apart in a seed-starting mix. Seeds should germinate in one to two weeks. Once the seedlings have two sets of leaves, transplant them into larger pots filled with a soilless potting mix, spacing the seeds 8 inches apart. Garden soil is too dense to use in containers.
If you keep your basil in a pot, consider small varieties such as 'Spicy Globe' and 'Windowbox'. When growing taller varieties of basil, successively transplant basil to a larger pot as it grows.
Keep potted basil outdoors in a location that receives at least eight hours of direct sun per day. Keep the soil moist but not wet, and water the soil, not the leaves. Harvest when plants are four to five weeks old. Make successive plantings of basil every two weeks for a fresh supply.
Growing basil from seed outdoors is easy. When the soil temperatures warm up in late spring after all danger of frost has passed, choose a location with at least eight hours of direct sun per day. Sow seeds 1/4 inch deep and 1 inch apart directly in garden soil. When the seedlings are established, thin (pull) them to space plants 8 inches apart for good air circulation.
As with most herbs, basil is difficult to grow indoors because even sunny windows don't provide as much sunlight as the plants get outdoors. The plants grow but may be spindly or leggy. Grow lights, which emit a larger light spectrum that plants need, offer a better chance of success.
Sow seeds 1/4 inch deep and 1 inch apart directly into a soilless potting mix, and thin them to accommodate growth as they start crowding one another.
To grow basil from cuttings, snip a basil stem about 2 inches long just above a pair of leaves. Insert the stem into moist potting mix so the leaves are just above the soil surface. Cover the container with plastic, leaving enough room for air around the leaves; plastic touching the leaves can cause them to rot. Place the pot in a warm location with indirect light. Full sun can burn the leaves and cause the soil to dry out. Check to be sure the soil stays moist but not wet. After about 10 days or when the cutting is rooted, remove the plastic.
There are dozens of basil types, including some with purple, ruffled, or variegated leaves. Most grow as upright bushes or columns.
Sweet basil is an umbrella term for the most widely used basil types of Ocimum basilicum. This includes 'Genovese', which grows 18 to 24 inches tall with large, sweet-spicy leaves often used to make Italian pesto. Other sweet basils include 'Profumo di Genova', 'Italian Cameo', 'Sweet Green', 'Salad Leaf', and 'Aurelia' Bolognese. 'Dolce Fresca' and 'Persian' basils won All-America Selections vegetable awards in 2015.
'Pesto Perpetuo' grows 4 feet tall with small green leaves edged in white. Its columnar form and beautiful leaves make it an outstanding addition to flowerbeds.
Purple basils, such as 'Purple Opal', look great in flowerbeds and cut bouquets. Others include 'Purple Ruffles', 'Red Rubin', and 'Dark Purple Opal'.
'Cardinal' basil boasts gorgeous burgundy flower heads on 2-foot-tall plants.
'Boxwood' basil sports tiny leaves and grows in a rounded form only 12 inches tall. It looks cute in a pot or flowerbed.
Thai basil's strong licorice flavor is commonly used in Asian dishes and holds its flavor better in cooking than sweet basil. Look for 'Siam Queen', 'Thai Magic', and 'Queenette'.
Lemon basil is what its name implies: a spicy, lemony basil with flowers great for making tea. Look for 'Mrs. Burns' Lemon'. You can also grow lime basil.
Holy basil or tulsi (Ocimum tenuiflorum) earned its common name from its use in Asia for medicinal and religious purposes. The leaves and stems are slightly hairy.
You can use basil leaves fresh or preserve them with freeze- or air-drying.
Freezing better preserves color and flavor. Strip, clean, and dry the individual leaves. Transfer them to baking sheets and place in the freezer, then place the frozen leaves in plastic freezer bags. If you prefer, you can immerse the leaves in water or olive oil before freezing in ice cube trays.
To air-dry, bundle stems of several plants and hang them upside down. Or clip individual leaves and place them in a single layer on a plate in a warm, dry room. When completely dry, store the dried basil leaves in an airtight container in a cool, dark place.
Downy mildew is an increasing problem when growing basil, especially in areas with high humidity where basil leaves stay wet. Keep foliage dry by watering only the soil. Space plants widely for good airflow.
If you see yellowing on the top of leaves followed by fuzzy gray growth on the undersides, immediately harvest any unaffected leaves. Remove and destroy infected plants; do not put them in a home compost pile. Sweet basil types are most often affected.
A fungus called fusarium wilt causes wilted leaves, vertical brown streaks on the stems, stunted plants, and sudden plant death. If you suspect fusarium wilt, do a soil test. Purchase seeds labeled resistant to fusarium wilt, such as 'Nufar' and 'Aroma 2'.