The Importance of Sunlight in Pecan Production
The last two growing seasons have been extremely dry in the Ardmore area. The extreme drought stress caused some of the developing pecans and leaves to abort. Surprisingly, the nuts that survived the stress in 1997 filled quite well, while in 1998, nut fill in thin soil areas was marginal. Even with limited moisture into mid-September, fall rains allowed most nuts to fill in 1998. It seems impossible!
We have been taught as a fundamental of pecan production that nut size is determined in the spring, and nut fill is determined late summer into early fall. The extremely dry spring suggested that the nuts would be small, and that was proven true. Most pecans were half-size. The interesting part is that those half-sized nuts were full of meat. There were some areas that didn't get rainfall until mid-September, limiting the early nuts to fill less than their potential.
Where pecans were irrigated in 1998 with adequate water early (May and June), nut size was normal. Further, where the pecans were irrigated adequately early and throughout the production cycle, the pecans filled normally. Even though irrigation is not the answer for native and most introduced pecan production in Oklahoma, it provides a better understanding of the production process.
Most everyone was surprised at the quality of pecans the last two years. This provides an opportunity for us to understand the importance of sunlight to the assimilation of carbohydrates in the plant. Certainly it is important to have adequate water to translocate nutrients and cool the plant. However, without adequate sunlight intensity, the pecan tree cannot capture the energy necessary to fill its pecans.
Notice the years, such as 1990, with excessive amounts of moisture in the late summer and into the fall, where cloud cover reduced sunlight intensity. That year, the nuts were large due to the good spring moisture, but filled poorly. Further, when the carbohydrate manufacturing was restricted with a shortage of sunlight intensity, the pecan tree reserves were in a deficit, limiting return bloom. In 1991, there was a very poor nut set.
Producers consider rainfall more limiting to pecan production in Oklahoma than sunlight. Maybe we should rethink some of our lifelong assumptions. In many native groves, the vastness of the root volume and the accessibility of the water table often overcome the severity of drought stress. Water relationships will certainly be a greater factor in thin, droughty soils with no water table for the trees to draw from.
In so many of our production techniques, I have found the phrase "it depends" to be so true. If we understand the effects of rainfall and sunlight or the lack of them, we can better respond to the conditions that develop.