Most plants require differing day and night temperatures for optimum growth. Definite knowledge of day and night temperatures necessary for maximum yield of high quality crops is vital for intelligent agricultural planning. This applies to choice of crops for a single farm or for an agricultural community. It is also essential in determining potential soundness and ultimate economy of land-and water-development projects. Tremendous losses, both of money and time, often result from trial and error processes which eventually prove suitability of certain crops in a specific area. The study reported here describes a new method of phenological determination of temperature requirements for plants.