Generally, the period of the year during which the temperature of cultivated vegetation (i.e., the temperature of the vegetal microclimate) remains sufficiently high to allow plant growth. This is an important concept in agricultural climatology, but it suffers greatly from vagueness and complexity. The growing season is highly variable due to plant varieties as related to temperature sensitivity. Currently, the most common measure of this period, “the average length of growing season,” is defined as the number of days between the average dates of the last killing frost (see frost) in spring and the first killing frost of autumn. The lack of a positive, practical definition for (and means of determining) a “killing” frost seriously limits the scientific usefulness of this measure. To provide some economic significance, the effective growing season is defined as the length of growing season that prevails in 80% of the years. Another measure, the frost-free season, is defined as the interval between the last and first occurrences of 32°F temperatures in spring and fall. This may be observed exactly, but its relationship to the local microclimate is variable and nonspecific, and it does not consider differences in types of vegetation. Still a fourth measure, the vegetative period or vegetation season, attempts to allow for the greater microclimatic temperature range and for the general growth retardation by cold temperatures, and is defined as the summer period confined between occurrences of 42°F (or 41°F or 43°F) temperatures. At best, any of the above is an index of growing season length, rather than a direct measure of it. Basically, the growing season (and “killing frost”) should be defined biologically rather than meteorologically and should consider the detailed microclimate, plant resistance to frost, growth rate versus temperature, and probably other factors.