Pecan tree pollination is complicated and an entire book could be written in an attempt to explain this in detail. In short, as I have mentioned various times on this website, pecan trees are imperfect in several ways, one being...pollination. Pecan trees have both male and female pollinators and as you might guess, this should result in pollination. But with most varieties of pecan trees, these pollinators do not tend to sync. Therefore, for proper pollination, it is necessary to have two or more pecan trees in close proximity.
Pollination in the pecan orchard is critical to both the yield and quality of nuts. Pecan trees are cross-pollinated (allogamous) and although self-pollination is possible, the result is largely unsuccessful. Pecan trees are wind-pollinated; therefore, pollinators (i.e., bees) are not required to complete pollination. Cross-pollinated pecans are usually larger and higher quality than self-pollinated pecans. Self-pollination can reduce nut quality and greatly reduce crop yield by as much as 75 percent.
Pecan trees are monoecious, which means both the male and female flowers are on the same tree. Female flowers (pistillate) are located at the end of the current season growth, and the male flowers (catkin) are located at the end of last season's growth. Catkins are easy to spot as they dangle from the tree during the early spring. A single catkin can produce as many as 2.64 million pollen grains. Only one pollen grain is required to produce one pecan. One catkin can produce enough pollen to pollinate flowers to produce 50,000 pounds of average-sized pecans. An average bearing tree is likely to produce several thousand catkins, thus further emphasizing how much pollen could be produced.
As you can see in the chart above, when choosing pecan varieties compatible for pollinating, these varieties should overlap somewhat to ensure pollination will occur.