Frost Free Days

Frost Free Days describes the number of continuous days between the last freeze in spring and the first freeze in fall. It is often associated with the growing season for many farmed and wild plants.

For the Forest Health Index, data are drawn from the Nation Climatic Data Center and the National Weather Service.

What We're Seeing

Near average: the number of frost free days was near average last year. The time for seasonal growth and flowering was similar to what it has been in other years, and forests are well adapted to these conditions.

What We're Monitoring

Frost free days are a useful indicator of climate change. Just a small amount of warming on a spring or fall day can make the difference between temperatures dropping below freezing or not. Freezing days are important to the forest because they determine whether precipitation falls as rain or snow and drive timing of natural events such as budding, flowering, and seed production. Frost free days even influence large-scale forest disturbance: a low number of very cold days can allow pine beetle populations to grow rapidly, while an early arriving frost-free period means that ecosystems may be warm and dry early in the year—leading to a long fire season.

Future Outlook

Although the FHI focuses on temperatures since the 1980s, records in many parts of Colorado date back decades further. These records reveal a trend of increasing frost free days over time. A trend is a general pattern found across data: this means that there is some variation—some years may have fewer frost free days that others—but, generally speaking, over time the number of frost free days seen in a year are likely to be higher than was common in the past. As climate change continues to warm Colorado, forest ecosystems will be likely to continue to experience longer warm, frost free periods, and fewer freezing days. Although we cannot control the air temperature on any given day, we do have the ability to influence how quickly climate change is happening. Choices such as reducing energy use, producing less garbage, driving less, and using renewable energy sources can help slow the speed at which the Earth is warming.