FORT WAYNE, Ind. (WANE) – Tuesday night brings our first frost/freeze alerts of the spring season for northeast Indiana and northwest Ohio. The counties in blue are under a Frost Advisory for temperatures dipping as low as 33° to 36°. The counties in purple are under a Freeze Warning for temperatures dropping down to between 29° and 32°.

Even though temperatures were around freezing Monday night, the decision was made Tuesday to begin issuing frost/freeze alerts because some crops have progressed farther along in their growth. While the growing season has not yet been officially declared, last week’s stretch of very warm April days has brought us very close to it.

Meteorologist Kyle Brown with the National Weather Service Northern Indiana office says the issuing of frost/freeze alerts arrives pretty close to when we typically experience our last frost/freeze. The average last date for a 32° freeze is approximately April 23rd in Fort Wayne. The earliest last 32° freeze was April 1st, 1922, and the latest was May 27th, 1961. The average last date for a 28° freeze is approximately April 11th. The earliest last 28° freeze was March 10th, 1929, and the latest was May 13th, 1946.

Brown says the office decided to issue the alerts based on how certain crops are progressing. We accumulated several hours over a certain temperature threshold during our warm stretch last week and the crops, trees, and flowers are all responding by showing growth.

However, the growing season has not been declared just yet. This is because agricultural experts in crop health and vulnerability at Michigan State University and Purdue University agree that only one type of crop is susceptible to damage at this juncture of the spring. This would be fruit crops and they are only concerned about temperatures in the upper 20s causing damage. This includes pears, apricots, apples, and plums, to name a few.

From the meteorological side of things, growing degree days are used to predict crop vulnerability. Growing degree days involve setting a base threshold temperature. In the case of fruit crops, they respond well to growth at temperatures higher than 42°. So the weather service will take the average temperature from a given day and subtract that temperature by 42 degrees. If the number is positive, that is how many growing degree days have occurred on that day.

In the case of corn and soybeans (which most farmers have not yet planted), the base value is 50°. Therefore, these crops respond better in warmer temperatures and have not accumulated as many growing degree days.

Ultimately, the weather service looks at issuing frost/freeze products when around 150 growing-degree days have been accumulated since the last 28° freeze. We accumulated a sizable amount of growing degree days during our warm spell last week and now Fort Wayne is right around that 150 threshold. The combination of meeting this meteorological threshold, the reported vulnerability of fruit crops, and the agreement between nearby weather service offices resulted in the NWS Northern Indiana office issuing frost/freeze alerts Tuesday.

As we warm back up and cool back down through the rest of this week, crops and plants will continue to grow. Brown anticipates more alerts will be needed by the weekend.

Remember, Mother’s Day or after remains the best time to plant, according to the Purdue Extension Office. Do not be tempted to plant by warm April stretches of weather. If you have already planted, make sure you cover your plants up or bring your potted plants indoors.

We’ll watch the temperatures closely throughout the rest of the spring. To view if any future frosts and freezes are expected, keep an eye on our WANE 15 Forecast Page.