Basil can turn yellow after it has fully matured or as a result of improper growing conditions. Typically, basil begins to naturally yellow after the plant has produced seeds.
Cool temperatures may cause the basil plant to begin yellowing. The plant should be moved away from windows at night if the temperature is at or below 32 degrees Fahrenheit. Basil that's grown outdoors should be planted in a sunny location and protected from frost. Too much or too little water may cause basil plants to yellow. Water when the top layer of soil feels dry, and plant the basil in a container that has drainage holes.
If the basil has yellow leaves near the bottom of the plant, the plant may require additional nutrients. Add nitrogen-rich, liquid fertilizer to the plant's water until the leaves have returned to their normal color. Then fertilize with potassium-rich fertilizer at least once per week.
Basil that has recently been transplanted into the garden may yellow as a result of the environmental change. Before transplanting herbs to the garden, harden off the seedlings by exposing them to the outdoors in increasing increments of time over the course of a week. To harden off seedlings, take them outdoors for one hour the first day, and then increase the amount of time spent outdoors by one hour each day. After the seventh day, the seedlings are hardy enough to survive outdoors.