Yesterday one of my dear skeptical colleagues sent me a link to a new article on lunar influences on plants (you can find it here). Briefly, the authors argue that scientific evidence supports the concept of a lunar cycle influence on plants. Interspersed within the discussion are references to seasonal and daily plant cycles, along with legitimate references to these verifiable phenomena. (Had these references to circadian and diurnal rhythms been left out, the literature citations would have been rather paltry.) Plants depend on these daily and seasonal cues for a variety of physiological and behavioral activities; lunar cycles have little obvious relevance to plants. Nevertheless, “planting by the moon” is a belief system that has existed since ancient times.
This article is a great example of how pseudoscience insinuates itself with legitimate science. Many of the references used as evidence for lunar effects on plants are of nebulous quality as they haven’t been reviewed by the scientific community; these include self-published books or lectures. Furthermore, for every article that claims a lunar effect, I can find another discounting it entirely. That being said, there are some legitimate papers indirectly linking lunar cycles with plant biochemistry. Coincidentally, the lead author of one of these articles is a close friend and colleague whose research credentials are impeccable.
Here’s where the fascinating and complex nature of species interactions helps explain conflicting data. Lunar cycles do affect certain species, including some herbivorous insects which are dependent on moonlight for feeding. During the full moon, such insects feed more heavily and affected plant populations retaliate by altering the digestibility of their tissues. It’s likely that these biochemical changes have been erroneously attributed to direct lunar influence rather than herbivore defense.
To demonstrate direct lunar influence, one would need to study plants in an herbivore-free, controlled environment so that the only variable under consideration was lunar cycle. Under such controlled conditions, would the same changes be noted over time if plants weren’t eaten by moon-managed insects? Would you see changes if you modified the lunar cycle to make it longer or shorter (again without insects)? Positive and repeated results would be necessary to establishing a role for lunar control.
As with so many other mystical explanations of natural phenomena, the real story is infinitely richer and more satisfying.
UPDATE: A peer-reviewed literature review on this topic has just been published. It’s well worth reading.