Alternate Bearing Trees,
or Where Have All The Acorns Gone?

Bill Devlin
Adams County Master Gardener

Upon returning to Pennsylvania from my longest ever pecan- picking expedition to Kansas in early December, I read several comments on the dearth of acorns this year. The Washington Post had published an article on the topic, and several members of my Carroll Valley Garden Club also joined in the chorus.

This surprised me somewhat as I had seen a bumper crop of acorns on my farm in Kansas, along with the truly bumper crop of pecans. It was the best I have seen in my lifetime. I actually brought home about 200 pounds of cracked pecans with me. My interest was piqued, so I searched further. In my Kansas area, I knew precisely what had happened. In the spring of 2007 we had a record breaking warm March, followed by an Easter Freeze that hit 18̊ F. All the winter wheat was damaged beyond economic harvest, the budding trees had their buds turn black, and so on. There were zero pecans, that's 0.000. Thus the 2008 season developed with a reservoir of left over nutrients in the soil. The season also saw an accumulation of some 60 inches of rainfall versus the normal average of 42 inches. Once you got the ensuing crop of weeds cut, the extent of the pecan crop became obvious. It was a doozy, and thus my two month stay. I could not keep up with the picking, so I advertised for pickers "on shares", with them keeping 2/3 of what they picked and me receiving 1/3. One group of 4 picked 114 pounds that shelled out to 54 pounds for the freezer!

Having zero one year and a bumper crop the next year is the very definition of alternate bearing. Trees and shrubs that bear fruit and nuts often vary widely in the crop produced from one autumn to the next. We're all familiar with fruits here in orchard country. The crop of acorns and nuts from oak and hickory trees are often referred to as "mast". There has possibly been more research done on fruits and nuts than oak tree family members, since acorns have little if any commercial value. Fruits and nuts put the bread and meat on the table for more than a few commercial growers. However, forest scientists have studied cycles in mast crops because they are of great interest to hunters and wildlife enthusiasts. Since they grow in the same areas at the same time, and I've experienced lots of huge acorns alongside the bountiful pecans in Kansas, I firmly believe the same lessons apply.

The factors that affect alternate bearing include genetics, timing of weather events, human interventions, and the growth cycle of certain tree species. Abundance of crops differs in various pecan species as an example of genetic influence. Rainfall amounts in a season are also a factor, with drought being the ultimate boundary condition. Late frost in the spring is probably more of a factor than early frost in the fall. Human intervention, such as spraying to reduce blooms or power thinning, can reduce alternate bearing and produce a more predictable crop from year to year. In the case of acorns, it may take one year for trees in the white oak group to produce acorns, but it takes two years for those in the red oak group to mature fully. This makes every other year more bountiful.

Why does Mother Nature do this? No expert has claimed to know all the answers, although the previous factors are known with certainty to contribute. In the case of pecans, it interrupts pest cycles as one possible advantage. For example, my bountiful crop of pecans was nearly untouched by their nemesis the pecan weevil. With no pecans to lay their eggs in last year, there were no carry-over weevils from last year's crops. Mother Nature cares for the weevil also, as a very small percent wait two years to reemerge (most emerge the next year). Thus the species avoids extinction.

If you find yourself really interested in this phenomenon, you can read more detailed research on the internet. Check out for more about pecan crops. To learn more about acorns, explore "acorn" on any of the internet search engines.

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