An herb garden, either indoors or outdoors, wouldn't be complete without a basil plant (Ocimum basilicum). Known for its large, shiny leaves that have a strong aroma and flavor, basil is usually an easy-to-grow plant. So when a problem develops, it can be especially disappointing and, in some cases, a bit baffling. Identifying the cause -- improper conditions, pests or diseases -- is an important first step in restoring the plant's health.

Green basil plants in a wooden flowerpot
credit: Lukasz Zandecki/iStock/Getty Images
Basil grows well in containers.

Damage from Cold

The basil plant is frost-sensitive and grown as an annual in all parts of the United States. Don't plant basil seedlings in the garden until soil temperature is at least 60 degrees Fahrenheit and frost danger has passed because cold soil and air can slow growth of young plants, causing them to look stunted.

Plants exposed to temperatures of 50 degrees F or less for any period of time might develop limp stems or blackened leaves. If most of the plant appears healthy, trim off any damaged parts with shears you disinfect by wiping with rubbing alcohol between cuts to prevent disease spread.

Damage from a cold snap is best prevented by covering plants with light cloths or plastic sheets, but remove these when weather warms. If you're growing basil in pots, move them indoors or to a warm, protected outdoor spot until cold weather passes.

Cultural Problems

Basil grows best when it receives even moisture that keeps soil lightly moist in an area that receives full sun. If soil becomes totally dry for any length of time, the plant could wilt and stop growing, a problem that's reversible by providing water if the entire plant hasn't died back. To retain soil moisture during dry spells, add 2 or 3 inches of organic mulch under the plants but keep it back from each plant's center to discourage fungal growth.

Fertile soil keeps new leaves coming all season, but nutrient-poor soil can produce slow-growing plants with small leaves that have poor flavor. To prevent this, add 2 or 3 inches of compost to the area before planting, mixing it in well. Then feed plants every two weeks once you start harvesting leaves using a balanced, 5-5-5 granular fertilizer. Mix 1/2 pound into the soil for every 20 square feet, but do this carefully to avoid damaging roots, and check the product label for further directions.

Basil Diseases

Basil is susceptible to several fungal disorders that can damage or destroy plants. For example, Fusarium wilt causes wilted leaves and brown streaks on stems and possibly death of the plant. Gray mold leads to gray or brown growth on stems, while root rot causes sudden wilting, with collapse of the entire plant if severe. Other fungal problems include downy mildew, causing yellowing of leaves with black fuzz on the undersides, and leaf spot, which produces black spots on leaves.

These fungal disorders are best prevented by spacing plants 1 to 1 1/2 feet apart, with more space for large-leafed types, and using a soaker hose or drip irrigation to keep foliage dry when watering. Plant basil in well-drained soil and give it full sun so the foliage dries quickly after rain; regularly clear away plant debris that can harbor fungus. If you grow potted basil, only use containers with drainage holes and never leave the plant in a water-filled saucer.

Insect Problems

Basil plants also attract a few types of pests that can damage leaves and slow growth. These include aphids, tiny green insects that feed on young growth, and leafhoppers, small light-colored insects that cause stippling on foliage. Whiteflies, tiny white flying insects, might also cluster on leaves where they feed.

Control these insects by spraying plants with insecticidal soap until leaves are dripping wet, repeating every two weeks as needed. Dilute the soap at a rate of 5 tablespoons per gallon of water.

Japanese beetles, which are metallic green, can also chew and destroy basil leaves, as can slugs, which are mostly a problem during wet weather. Control these pests by hand-picking or use commercially available traps labeled for slugs or Japanese beetles.