The ability to predict whether a newly collected species or cultivars can be successfully grown in a location is fundamental to the continued productivity of America's agriculture and to the survival of our landscapes. The plants in our urban spaces, farms, fields, and forests consist of native vegetation and exotic plants introduced into our land from all over the world. Day length is usually the most critical factor in regulating vegetative growth, flower initiation and development, and the induction of dormancy. Plants survive only when the day length promotes their growth and prepares them for the seasonal changes. Cloudy, rainy days coupled with the shade provided by nearby plants and structures can significantly reduce the amount of radiation available.
Plants survive only where the amount is within a specified range.
Plants survive only where temperatures allow them to metabolize. Plants differ in ability to survive frost, their responses varying from immediate death to sustained performance. The previous environmental and cultural conditions of plants can often shift, but not permanently alter, their tolerance to freezing. Plants survive only when they are adapted to subfreezing weather. The thermal cutoff temperature varies widely from species to species.