Insect damage, lack of sufficient light or water, overgrowth, tired soil, or bound roots can all affect the health of your basil plant. Identify and alleviate the problems, and your plant will revive with a surprising growth spurt.
Check your plant for insect damage. Partially chewed leaves are a big clue. Spray your plant with soapy water, mixed with a teaspoon of jalapeño pepper juice, to keep pests off.
Hold your plant by the base and turn the pot upside down, allowing the plant to slide partially out of the pot. Examine the roots. A densely entwined root system is a symptom that your plant has become "root bound." Fill the bottom of a second, larger pot with a thin layer of gravel, followed by torn newspaper, crushed eggshells, used coffee grounds and used tea leaves. Fill the remainder of the pot with clean, pest-free potting soil.
Make a hole in the middle of the soil in the new pot, big enough to accommodate your basil plant. Gently and carefully squeeze the roots of the plant to break them apart, tearing only a little. HGTV recommends trimming away any roots that appear to be lifeless, as well as cutting an equal amount of top growth from the plant. Place the plant in the hole, being careful that the roots are spread out a little but well covered in the hole.
Pull soil into the hole and around the base of your basil plant. Do not press the soil down. Water the plant just enough to soak the soil but not enough to puddle around the base.
Cut away excess growth using kitchen shears, or pinch leaves from the plant. Discard any leaves that are brown or discoloured, or that appear to have insect eggs attached. Keep the remaining leaves to dry for use in your kitchen. Be sure to trim your basil plant whenever leaves reach 1 inch or more in length.
Do not discard a plant that appears to be dead if there is even a hint of green remaining. Cut away all brown, dead foliage, down to 1/2 inch from the soil, if necessary. Water the plant well with compost tea made from coffee grounds, tea leaves and crushed eggshells. Place it in full sun. Watch for a few days and you will see your plant begin growing again.
Although it may appear to be dead, it is possible to revive a plant by repotting, watering well and adding a little organic fertiliser.
Do not discard a dead or dying plant before attempting to identify the cause of the problem.