Inadequate fertilizer can severely limit new growth, predisposes the plant to insect and disease problems, and increases the susceptibility of tender perennials to winter injury. A light application of fertilizer to perennials in early spring should promote new root and shoot growth and ensure vigor in the new growing season. Sequential harvests of annual herbs will be facilitated by light applications of fertilizer after each heavy harvest. The high concentration of essential oils in healthy, actively growing herbs repels most insects. However, aphids and spider mites can be a problem.
Aphids seem to be more prevalent in crowded conditions with rapidly growing, succulent plants.
Spider mites thrive in dry conditions and can be controlled by spraying the plants with plain water at regular intervals, especially during periods of drought. Since there are very few labeled pesticides for use on herbs, the best defense against pests is preventative cultural management, such as good sanitation, removal of weak or infested growth, and regular pruning. Periodic, judicious pruning promotes vigorous, sturdy plants that are less susceptible to disease and winter injury. If they are allowed to grow unchecked, some herbs will take on a gangly, unkempt appearance. If you are lavish in your use of herbs, regular harvesting for use in cooking, potpourri, and flower arrangements should keep your herbs sufficiently pruned.