There are a number of factors to consider when choosing fruit species and varieties for a particular location. Tolerance of the amount of rainfall and the high and low temperatures that a location experiences, adaptation to soil conditions (e.g., pH, drainage, and soil texture), and resistance to common diseases are some of the factors that affect what fruits are well-suited to an area.

Another consideration is the number of chilling hours that a location reliably receives. Among fruit species and varieties, there is wide variation with respect to the number of chilling hours needed to break dormancy and allow abundant flowering and fruit production.

As anyone who has lived in Louisiana (or most of the South) knows, our winters are not consistently cold. We experience periods of cold weather followed by warm days and, often, more temperatures below freezing. For some fruit crops that bloom early in the growing season, it is the case that once a plant has received enough chilling, it is likely to bloom when we have warm days.

If a variety has a chilling requirement too low for its location, it is likely to bloom too early and have flowers or young fruit damaged by late freezes. This is a concern for fruits like peaches and blueberries that bloom early in the growing season. ­­­Some fruits – like muscadine grape – do not tend to bloom prior to late freezes even if their chilling requirements are satisfied.

On the other hand, if a variety requires too many chilling hours for its location, it will not bloom and fruit abundantly on a regular basis.

Plants have chilling requirements for both coming out of dormancy vegetatively (i.e., producing leaves and shoots) and for flowering. For some plants, the chilling requirements for these are the same or similar, while for others they are different. While not having regular fruit production is undesirable, if a plant does not regularly get enough chilling to produce leaves, so that it can produce food for itself (photosynthesize), it is likely to eventually die.

For a given location, there are Goldilocks varieties that require neither too little nor too much chilling.

Dr. Vincent Brown and colleagues with the LSU Department of Geography and Anthropology and Southern Climate Impacts Planning Program have produced maps of Louisiana based on numbers of chill hours received between October 1 and February 28, over the 30-year period between 1991 and 2020.

They produced maps using two common chilling models. One model counts temperatures at and below 45 degrees F, and the other only counts temperatures between 32 and 45 degrees F.

Using each of these models, they produced three maps. One shows the average (median) number of chill hours received over the 30-year period (Figures 1 and 3). In other words, over a recent thirty-year period, in half of those years, locations received at least the number of chill hours indicated. In half of those years, they received fewer.

Because we do not want to grow varieties that only receive adequate chilling in half of years, Dr. Brown and his associates also produced tenth percentile maps (Figures 2 and 4). In other words, in all of but three of thirty years, locations received at least the indicated number of chill hours.

They also produced ninetieth percentile maps that show that, in three of thirty years, locations received more than the indicated number of hours. (Because these maps do not have a lot of practical relevance for variety selection, these are not shown here.)

When chill requirements are listed for a variety, it is not always apparent on what model the number is based. Furthermore, while chilling models are helpful, other factors, like the numbers of hours above certain temperatures, can affect when a plant comes out of dormancy.

Therefore, it is preferable to know what varieties have track records of performing well in an area. One value of these maps is being able to see what parts of Louisiana are similar with respect to the amount of chilling they typically receive. For example, if a location typically gets a similar number of chill hours as Baton Rouge, varieties that have performed well in the Baton Rouge area are likely to be well-suited – chilling-wise – in that location. Other factors, such as soil conditions, would still need to be considered.

Contact Mary Helen Ferguson.

Figure 1. Hours below or equal to 45 degrees F model, 50th percentile (median) contour map. Half of years had fewer and half had more than the indicated number of hours below or equal to 45 degrees F.

Figure 2. Hours below or equal to 45 degrees F model, 10th percentile contour map. Three of 30 years had fewer than and 27 years had more than the indicated number of hours below or equal to 45 degrees F.

Figure 3. Hours 32 to 45 degrees F model, 50th percentile (median) contour map. Half of years had fewer and half had more than the indicated number of hours above or equal to 32 degrees F and below or equal to 45 degrees F.

Figure 4. Hours 32 to 45 degrees F model, 10th percentiles contour map. Three of 30 years had fewer than and 27 years had more than the indicated number of hours above or equal to 32 degrees F and below or equal to 45 degrees F.