Light, fresh pesto and savory tomato sauces start with fresh basil leaves from the basil plant (Ocimum basilicum), an annual herb that grows best in full sunlight. While basil is relatively easy to grow, you might be alarmed to notice leaves drying up on your basil plant. Dried leaves are usually a sign of improper care, but in some cases, are a normal part of basil growth.
Water is the first thing to evaluate when you notice basil plants begin drying. Basil requires constant moisture for best growth and may need to be watered daily during periods of drought with intense heat. With too little water, the leaves begin wilting and dry out. Water the plant often enough to keep the soil evenly moist so that a ball of soil squeezed in your hand holds shape but is slightly crumbly. Likewise, too much watering can be problematic, leading to root rot that causes the leaves to shrivel and dry out as the plant dies. As a general rule, only water basil when the top 1 inch of soil -- in ground or in planters -- feels dry. Planters and pots must have a drainage hole so excess water can escape. If there's a saucer or drip tray, empty it out if it has water in it.
Too Much Sun
Basil plants need full sun -- six to 10 hours of direct sun daily. Intense sunlight on a hot day can cause stress on the basil plant, visible first as wilted leaves that eventually begin to dry out. If the plant has enough water, it plant should return to normal as the sun goes down and temperatures drop. If the problem persists, add a sun shade to help filter the rays during the hottest part of the day. For container plants, move them to a spot that receives filtered sunlight between 2 p.m. and 4 p.m., when the afternoon sun and heat is most intense.
Pests and Diseases
Basil is fairly resistant to most pests and diseases, but a few problems can occur. Dried leaves are not a symptom of these pest problems, but can occur as plant health declines and the plant begins to die. In addition to fungal root rot caused by overwatering, basil can suffer from another fungal disease, basil shoot blight. Look for black streaks on the leaves and sudden leaf loss as signs of this disease, in addition to drying. Enough spacing for air circulation around plants helps prevent shoot blight. Water the basil in the morning so the leaves don't stay wet too long. Healthy leaves may begin to wilt and dry up if too many leaves are eaten by grasshoppers, Japanese beetles or slugs. You can remove these pests by hand.
Too Much Fertilizer
Fertilizer helps boost plant growth, but too much fertilizer can burn plants and affect flavor. Fertilizer application rates vary among different products, but as a general rule, apply 3 pounds of 5-10-10 fertilizer to a 100 square foot garden of 16 plants, or roughly 3 ounces for each plant before planting. Repeat application once every two months. Spread the fertilizer evenly over the soil, then water it in well. If you notice dried leaves from too much fertilizer, you may be able to save the plants by flushing the soil with water. The extra water can also damage plants if there isn't adequate drainage. In some cases, the plant can grow back from its roots even if you didn't catch the problem in time to save the existing leaves.
It's natural for a few of the lower leaves of a basil plant to dry up and turn brown. Simply pull these dried leaves off and the basil will continue to grow. Basil plants are annuals that die at the end of the growing season, so dried leaves might just be a sign of the end of its life cycle. Basil dies prematurely if you let the plant form flowers and go to seed. A plant's goal is to reproduce and with seed production, it has completed its life goal and no longer has cause to grow. Pinch the tips of branches regularly to promote new leaf growth, bushy form and to prevent flower formation. As the weather cools and the first frost hits, there is no way to save the dying plant.