I was checking on the development of the 2017 pecan crop last week and made some interesting observations about pecan flowering and nut set. Whenever I look at pistillate flower clusters that are fully receptive and ready to be pollinated, I become extremely optimistic about the current season's nut crop. A cluster of five nutlets with bright red stigmas just looks so impressive (photo above, right).
However, rarely do all the pistillate flowers in a cluster turn into harvestable pecans. When a pecan tree creates an pistillate flower cluster, the flowers at the end of the pedicel are often smaller or ill-formed (photo at left). Its like the tree just runs out of gas at the end of the flowering stalk.
Weak or poorly formed flowers may capture pollen but these flowers don't have the strength to produce a fertilized nut. This is why you might find that terminal flowers turn brown and eventually fall off the tree (photo at right). What the casual observer might blame on poor pollination, is actually weakly formed pistillate flowers being shed by the tree.
Pecan tree will only hold on to nuts that have become fertilized. The developing seed produces a plant hormone that sends a signal to the rest of the plant that basically says-- "hold me and feed me". The photo at left illustrates how a pecan tree shed unwanted unfertilized pistillate flowers. At first the weak flower turns brown (labeled A). The tree responds by forming an abscission layer between the flower and the pedicel. The location marked B once held a pistillate flower that has since aborted. If the pedicel does not contain a single fertilized nut, the entire pedicel will be removed from the tree. You can see an abscission layer forming at the base of the pedicel (marked C).
Without an actively growing nut cluster, you will note that the buds just below the pedicel are starting to swell. A second flush of vegetative growth will soon appear on this branch terminal.