Common sweet basil (Ocimum basilicum) thrives in U.S. Department of Agriculture plant hardiness zones 9 through 10, where its pungent leaves add exotic flavors to salads, meats, pastas and more. Other varieties include hoary basil (Ocimum americanum) and lemon basil (Ocimum xcitriodorum), both which grow in zone 10. However, all basil varieties do well as annuals in all zones. Wherever you grow basil, don't let leaf curling ruin the foliage. After all, it's the leaves that are most prized on a basil plant.
Water, Water, Everywhere
Basil grown directly in the soil needs to get water at least once a week, preferably every four days or so. If you have basil grown in containers, they may need water even more frequently than this since the exposed sides of a pot cause moisture to evaporate quicker. Check the soil surface with your fingertips. If leaf curling is happening simultaneously with dry, crumbly soil, the basil plant is likely experiencing drought stress. Irrigate immediately. Use enough water to moisten the soil to a depth of 8 to 12 inches for plants grown in the ground, or apply water to container-grown basil until moisture appears in the pot's bottom drainage holes.
You Are My Sunshine
For the biggest, lushest basil plant, it must receive full sun. That's a minimum of six hours of direct sunlight per day. If your basil plant is in a shadier spot and its leaves are curling, fading and looking lackluster, lack of sun may be the culprit. If possible, trim back overhanging tree branches and nearby shrubs to expose the basil plant to more light. If your basil is in a container, consider moving it to a sunnier spot. When choosing a planting site, east- and west-facing sides of your backyard tend to be sunniest.
Season With Salt
If your basil plant's leaves aren't just curling, but also look stunted in size and are turning yellow, the basil may be experiencing a magnesium deficiency. Epsom salt is naturally rich in magnesium and can help enhance chlorophyll production to return your plant's foliage to its greenest, healthiest state. Measure the basil plant's height in feet. Sprinkle 1 teaspoon of Epsom salt for every foot of plant height, spreading the salt around the base of the basil. Water the basil immediately to help dissolve the salt and carry the magnesium to the plant's roots.
Attack the Aphids
Compared to other plants, basil has relatively few insect pest problems. When it does, one of the most common insects related to leaf curling is aphids. Toxins injected into the plant by the insects as they feed can cause the foliage to curl inward at the edges. Inspect your plant. Aphids may show up as small, oval bugs that are black, brown, green, yellow or red. You will often find them on the underside of the basil leaves. Spraying your basil with a strong blast of water from a garden hose is often enough to dislodge the pests and keep insect populations too low to cause problems. If you have a heavy infestation, mix a couple tablespoons of liquid dish soap in a quart of water. Spray it on your basil to fully coat all foliage and plant stems. Repeat once a week until aphid activity is no longer noticed.
Of the most common plant diseases experienced by basil, none of them directly cause leaf curling. However, the stress created by a disease infestation may sometimes indirectly cause wilting and similar foliage symptoms. All common basil diseases are best managed by changing how you care for your basil, since very few fungicides are registered for appropriate use on basil plants. If you notice mold and other fungus growths on foliage, trim off the affected leaves with pruning shears. Wipe the shears with rubbing alcohol between cuts to prevent the spread of fungus and mold. Additionally, only water your basil plant at its base, not overhead. Overhead watering keeps the leaves wet and increases the risks of plant disease.