Plant growth and development are affected by several ecological factors and these same factors will influence the production of essential oils as well as other active agents in herb plants. The effect of external conditions on chemical composition of plants has been recognized for more than 80 years. The environment affects the essential oil concentration directly through metabolic processes and indirectly through plant growth. Metabolic changes affect the amounts of substances synthesized, and plant growth alters dry matter production and the proportion of different plant organs. Certain conditions may favor either the production of roots, above ground vegetative organs, or flowers and seeds (Bernáth 1986, Bernáth and Tétényi 1978, Clark and Menary 1979, Flück 1955, Savchuk 1976).