Andrew worked as a cook in high school and college, and the old-timers in the kitchens where he toiled swore that the weirdest customers always came out during a full moon. Similarly, you may have heard friends or family in health care or law enforcement report the same pattern—though studies in both the medical and criminal justice literature have cast doubt on such claims. Beyond human behavior, folklore and tradition have long linked lunar phases to a wide range of biological phenomena, including the growth of trees after pruning.
This is the very topic addressed in the latest installment of the Tree Research Journal Club, where we discuss the paper:
Read, H., Rubio, S. Á., Wheater, C. P., & Garcia, Á. S. (2018). Assessing the impact of moon phase on the cutting of lapsed beech pollards. Arboricultural Journal, 40(3), 137–152. https://doi.org/10.1080/03071375.2018.1485388
We weren’t sure if this article would have surfaced in any of our targeted literature searches. Instead, Andrew stumbled upon it when he noticed it was published directly before one of his earliest works in Arboricultural Journal. Intrigued, he read it before even reviewing his own article.
What was done?
According to the authors, local tradition in Spain’s Basque region holds that pollarding beech trees during a waxing (growing) moon phase promotes more vigorous growth than pollarding during a waning (shrinking) moon phase. To test this belief, researchers selected 18 mature, non-uniform beech pollards that had exceeded their typical pruning cycle. These trees were divided into two groups: nine were cut during a waxing moon phase, and nine during a waning moon phase.
Due to the trees’ maturity and variability, achieving uniform pruning was challenging. To identify potential confounding factors, the researchers measured the number of retained branches, branch stub length, stem diameter, and branch stub diameter. Approximately three and five years post-pruning, they reassessed the trees to determine survival rates and evaluated various parameters, including the number of new shoots or shoot clusters, regrowth length, presence and extent of callus formation, and any observed dieback.
What was discovered?
By the end of the experiment, three trees pruned during the waxing moon phase—traditionally considered more favorable—had died, whereas only one tree pruned during the waning moon phase succumbed. Furthermore, trees cut during the waning moon exhibited significantly more shoots and shoot clusters, with these clusters being notably longer than those on trees pruned during the waxing moon.
However, upon examining potential confounding factors, researchers observed that stub diameters were, on average, larger in the waxing moon group, which experienced higher mortality and reduced regrowth. They identified a significant correlation between stub diameter and the number of new shoots: as stub diameter increased, the number of new shoots decreased. Similarly, greater stub diameters were associated with shorter regrowth lengths and increased dieback.
What is the biggest missed opportunity of this paper?
If we had had our way, this article would have been titled When the Moon Hits Your Eye Like a Pollard Grown Too High, but that is a matter of creative differences. More importantly, they should have controlled the confounding (see below) observed as it is impossible to attribute the differences in pruning response to either lunar phase or the differences in stub diameter. It could be one or the other or both (Our bet is not on lunar phase).
What is confounding?
A confounding variable is a variable that is related to both the predictor variable (moon phase) and the dependent variable (survival and growth response). In this case, stub diameter is related to both and confounding. A good experimental design controls for confounding, often by ensuring conditions are completely uniform outside of the predictor variables of concern.
As another example, many years ago during the peak of self-diagnosed gluten allergies, Andrew’s oldest daughter developed a full body rash. Convinced it was gluten, his wife at the time removed all bread from their daughter’s diet while also giving her an over-the-counter antihistamine. The rash quickly disappeared with the blame placed squarely on the wheat products. Compelled by a love of carbs and science, Andrew explained the error in logic, winning bread back in his life though somehow still losing when all was said and done.
A Followup Study
A more recent study used dendrometers (i.e., precision sensors that measure stem/root growth and shrinkage) to collect over 2 million hourly stem radius measurements from 62 trees across six species over six years. During this time, they captured 74 consecutive lunar cycles. The researchers found diameter changes related to key meteorological control measures like air temperature and vapor pressure deficit, but found no significant relationship between lunar cycle and stem water content or growth rate.
Conclusions
The moon controls both the tides and the transformation of werewolves, but there is little evidence in these studies that it affects tree growth.
About this Blog
Rooted in Tree Research is a joint effort by Andrew Koeser and Alyssa Vinson. Andrew is a research and extension professor at the University of Florida Gulf Coast Research and Education Center near Tampa, Florida. Alyssa Vinson is the Urban Forestry Extension Specialist for Hillsborough County, Florida.
The mission of this blog is to highlight new, exciting, and overlooked research findings (tagged Tree Research Journal Club) while also examining many arboricultural and horticultural “truths” that have never been empirically studied—until now (tagged Show Us the Data!).
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