A frost date is an estimated calendar date used primarily by gardeners and farmers to manage the risk of cold weather damage to plants. It represents a prediction for a specific location based on historical meteorological data. These statistical averages help determine the safest window for starting and ending the outdoor growing season. Frost dates are not weather forecasts or guarantees, but provide a framework for planning the cultivation of cold-sensitive plants.
The Two Critical Frost Dates
The growing season is bracketed by two distinct, statistically derived events: the Last Spring Frost Date (LSPD) and the First Fall Frost Date (FFD). These dates define the window when most warm-season crops can thrive without protection. A light frost, the baseline for these calculations, occurs when the air temperature drops to \(32^\circ\)F (\(0^\circ\)C).
The Last Spring Frost Date marks the average date in spring when the final temperature dip to \(32^\circ\)F or below is expected. Planting tender plants before this date significantly increases the risk of damage. Conversely, the First Fall Frost Date signals the average arrival of the season’s first \(32^\circ\)F temperature in autumn, marking the end of the natural growing season for cold-sensitive crops.
Applying Frost Dates to Your Garden Schedule
Knowing these two dates allows gardeners to precisely time their activities for maximum success and yield. The Last Spring Frost Date is used to calculate when to start seeds indoors. For example, warm-season plants like tomatoes and peppers require a 6-to-8-week head start indoors before transplanting. By counting backward from the LSPD, a gardener determines the specific week to sow these seeds.
Once the LSPD has passed, the danger of a killing frost has diminished, allowing for the safe transplanting of tender seedlings. Hardy cool-weather crops like spinach and peas can be sown directly into the ground several weeks before the LSPD, as they tolerate light frost. This strategic timing allows for an earlier harvest of spring vegetables that prefer cooler soil temperatures.
The First Fall Frost Date dictates the final phase of the growing season and prompts planning for harvest and protection. Cold-sensitive crops, such as squash and cucumbers, must be harvested before the FFD arrives to prevent damage. Gardeners extending their harvest use the FFD to time the installation of season extension materials, like row covers or cold frames, several weeks in advance. This allows for continued growth of semi-hardy vegetables like kale and carrots past the average frost date.
How Frost Dates are Determined and Located
Frost dates are determined through statistical analysis of decades of historical weather data collected by meteorological stations. Government agencies, like the National Oceanic and Atmospheric Administration (NOAA), compile minimum temperature records over a 30-year period to calculate these averages. Frost dates are probability statistics based on past weather patterns, not weather predictions.
The most common frost date published uses a low probability threshold, typically a 10% chance. This means there is only a 10% probability that the temperature will drop to \(32^\circ\)F or below on that date. Farmers and gardeners use this low-risk date for planting decisions, though some sources may use a 30% or 50% probability date, which carries a higher risk.
The public can locate specific frost dates using online calculators that reference data from the nearest official weather station. Local conditions, known as microclimates, can cause slight variations from the published dates due to factors like elevation or proximity to large bodies of water. Therefore, a gardener’s personal observation of their specific location complements the generalized frost date data.