The growing season is the period when temperatures are warm enough to support plant growth, including both cultivated crops and wild vegetation. Traditionally, this time frame is defined by a continuous stretch of frost-free days in a given location. Observations show this period is lengthening globally, meaning many regions are experiencing an extended window for plant life.
How the Growing Season is Measured
The length of the growing season is primarily determined by tracking the dates of the last freezing event in spring and the first freezing event in fall. This is known as the frost-free period, defined by the number of days between the last and first occurrence of temperatures dropping to 32°F. This metric is significant for agriculture because most crops cannot survive a hard frost.
Scientists also use other temperature thresholds, such as the number of consecutive days with average temperatures above 40°F or 50°F, as a more refined measure of a “thermal growing season.” Phenology, the study of the timing of natural events, helps monitor the season by observing biological indicators like leaf-out, bud burst, or the start of autumn leaf senescence. These combined metrics provide a comprehensive picture of the environmental conditions that permit active plant growth.
The Primary Climate Driver
The main mechanism driving the extension of the growing season is a sustained rise in global average temperatures. This warming trend causes the last spring frost to arrive earlier, advancing the start of the growing season. Simultaneously, the onset of colder weather is delayed, pushing the first fall frost date later. In the contiguous United States, this has resulted in an average extension of the growing season by about 12 days over the past century, with spring warming accounting for the majority of the change.
A related measure of this warming is the increase in Growing Degree Days (GDD), a metric used to quantify the accumulation of heat available for plant development. GDD is calculated as the daily mean temperature minus a base temperature below which a specific crop will not grow, such as 50°F for corn. Since 1970, the number of accumulated GDD has increased at 97% of the US weather stations analyzed, providing plants and insects with a greater overall thermal budget for growth and development.
Impacts on Crop Production
A longer growing season offers a theoretical benefit by allowing farmers to cultivate longer-season crop varieties. In some regions, the extended time may also enable double cropping, where a second crop can be planted and harvested after the first, increasing annual productivity. This potential, however, is dependent on other factors like water availability and the timing of the warming.
Increased heat accumulation and earlier warming also introduce significant challenges for crop health. Many staple crops, such as corn and wheat, are susceptible to heat stress, where excessively high temperatures during key reproductive stages can severely reduce grain fill and overall yield. Furthermore, accelerated phenological development means that crops may rush through their growth stages, leading to an earlier harvest that may not allow sufficient time for maximum yield establishment. The risk of plants initiating growth too early only to be damaged by a subsequent late-season cold snap creates new uncertainties for agricultural planning.
Ecological Shifts and Water Resources
The extended growing period creates a longer window for biological activity in natural ecosystems. Warmer conditions mean that insect pests, such as the mountain pine beetle, and invasive weeds have more time to reproduce or migrate to new areas, leading to longer and more damaging pest seasons. This also creates a risk of phenological mismatch, where the timing of a plant’s flowering no longer coincides with the emergence of its specific pollinator species, disrupting ecological relationships.
The most significant consequence for the environment is the impact on regional water resources. Plants transpire, or release water vapor, for a longer duration when the growing season is extended, which increases the total water demand across the landscape. This higher rate of evapotranspiration, especially when combined with hotter temperatures, accelerates the drying of soils and increases the risk and severity of seasonal drought conditions. An extended growing season can create an “ecological drought,” amplifying soil moisture deficits and putting additional strain on water supplies for both natural ecosystems and human use.