American Horticultural Society (AHS) heat zones
This was developed to recognize that plant tolerance to summer heat is as crucial as its survival in cold temperatures. It categorises zones by the number of days when the temperature will be at least 86°F/30°C. This not only helps to indicate where a plant might be at risk from prolonged high temperatures but also, in cooler climates, it can indicate whether a plant will have sufficient warmth to flourish.
