Every year around mid-February people ask the same questions – “Have we had our last frost?” “Is it safe to plant my vegetables?” – and the answers always seem to favor two key indicators, the date of Easter and Pecan leaves.
Growing up I heard the same things – watch for the pecan leaves to bud and the date of Easter. The dates of both events were sure-fire signs that winter weather was officially over. There is a mixture of truth in both, as pecan trees are one of the last trees to bud out in spring, and the date of Easter is after the first date of spring, with the most common dates being March 31st, April 5th, and April 17th over the last 100 years. However, there are two other things we need to consider – history and heat.
When it comes to gardening, looking at historical weather data is critical to determining current practices. I did a quick analysis of weather data from the past 100 years, available on the National Oceanic and Atmospheric Administration website, for the Greater Baton Rouge Area. I was looking specifically for three important dates when it comes to spring gardening, last date for a minimum temperature of 32 F, last date for a minimum temperature of 36 F, and first date for a maximum temperature of 90F.
Why those temperatures? While 32 F is important for freezing weather, 33 F to 36 F is typically the temperature range at which a frost can form. Some vegetables are more susceptible to frost damage such as tomatoes and peppers; therefore, the last frost date is more important for these than the last freeze date. Lastly, I looked at the first date for a maximum temperature of 90 F. The ideal daytime temperature range for fruit set in tomatoes is between 70 – 85 F and up to 90 F for heat tolerate varieties. Above these temperatures flowers fail to pollinate and the blooms drop.
Looking at the data for the Greater Baton Rouge area, since 1926, our mean date for the last freeze is February 24th, with the earliest and latest last freezes occurring on January 8, 2017, and April 13, 1940, respectively. Our mean date for a minimum temperature of 36 F is March 13th, with the earliest and latest last 36 F low dates being January 27, 2017, and April 15, 2008, respectively. The mean date for first 90 F high date is May 13th, with the earliest and latest high of 90 F occurring on March 17, 1963, and June 10, 1976, respectively.
How does historical weather data line up with pecan leaves and Easter dates? When pecan leaves emerge can vary greatly depending on several factors such as variety, location, plant health, number of chill hours, and number of heat units. Not enough chill hours during the winter and warm springs can cause erratic and delayed budbreak, potentially occurring long after the last frost date.
Easter always falls after both of our mean dates for last freeze and last low of 36 F. The possible date range for Easter is between March 22nd and April 25th. Over the last 100 years, the earliest date was March 23, 2008, and the latest date was April 25, 1943.
Since 1926, the last minimum low of 36 F occurred before Easter 93 out of the last 100 years. It did this by 79 days in 2017. When we look at our mean date of March 13th being our last frost date, this still proceeds the most frequent, early Easter date of March 31st by 18 days. Historically, we do not always have frost a few days right before Easter. Over the past 100 years the last low of 36 F occurred an average of 26.7 days before Easter.
If we always wait until after Easter to plant our spring garden, we could miss out on anywhere from 9 to as much as 43 days of favorable growing conditions. This is important because in south Louisiana it gets hot early in the year. Excessive heat is generally not good for many of our spring vegetable crops. The milder temperatures we generally experience in March and April are the best for producing vegetables that yield a good harvest. Waiting too long to plant can mean hot weather limiting crop productivity.
Oh, our latest 36 F (probable frost) low date since 1926 was April 15, 2008. Easter was March 23rd that year, the 2nd earliest possible date for Easter to occur.
