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Lunar farmers' almanacs are calendars which give advice on agriculture based on the phase of the moon.

One such example is this site:

The Farmers Almanac Gardening by the Moon Calendar is determined by our age-old formula and applies generally to regions where the climate is favourable.

Another Italian example.

A more serious site (with "scientific" explanations) is available here

I approached this subject initially as a benign sceptic, experimented a little, compared some folklore traditions, chased journalists for scientific references and sifted some hard science out of a morass of junk science, rumour and hokum.

I am now fairly sure from my own experience gardening that the cycles of the moon are significant factor in plant growth (and I am 100% sure after many moonlit winter nights spent parting my donkey from her swain that the cycles of the moon significantly affects animal behaviour!) "Significant" here means statistically significant, it doesn't mean "big" and it doesn't mean "most important".

Is there any evidence that the phase of the moon has any statistically significant effect on plant growth?

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TL;DR; There are studies that do show at least some correlation between lunar cycles and germination and plant life cycles. However, the correlation is rather complex, and apparently species-dependent.

A quick google search for this finds many references to university studies on the topic, allegedly done by Wichita State University and Tulane University. The oft-quoted paragraph reads:

Other experiments have been conducted at Wichita State University and at Tulane University. All have achieved the same results. Experimentation indicates that seeds sown just before or around the full moon have a higher rate and speed of germination than those sown at the new moon because seeds are able to absorb more water at the full moon.

I have yet to find any specific reference to the Wichita State study, however the Tulane study is easier to come by, although I have yet to find an online copy. (source)

A few other referenced (but unconfirmed) claims include:

Metabolism, water absorption, fertility and germination all are reported to respond to this synodic cycle (Graviou, E.. 'Analogies between Rhythm, in Plant Material in Atmospheric Pressure and Solar-Lunar Periodicities'. International Journal of Biometeorology, 1978. Vol. 22, No.2.). North-western University in Illinois demonstrated 35% higher water absorption in beans just before full Moon compared to new Moon. (Brown, F., & Chow, C., 'Lunar-correlated Variations in water uptake by Bean seeds', Biological Bulletin, Oct., 1973, 145, 265-278. ) Confirmation of this was attested by Dr Jane Panzer of Tulane University. (Panzer. J. J., 'Lunar Correlated Variations in Water Uptake and Germination in 3 Species of Seeds', PhD. U. of Tulane. 1976) Interestingly these remained in a diminished form if the pinto beans were sterilised, and further diminished by pasteurisation. Germination also showed this monthly rhythm in her studies (source)

I have thus far found only one original study on this topic, entitled Chronobiology of trees: Synthesis of traditional phytopractices and scientific research as tools of future forestry, and it details experiments with germination of seeds at various times of the lunar calendar. The study was conducted in 1990 and 1991 (with a brief preliminary study in 1989). A few relevant excerpts (emphasis added):

The speed of germination or beginning of emergence already shows a significant difference between the [full moon] and [new moon] sowings for all the sowings and especially for those corresponding to the dry season, which took place in the middle of the trial. In mean values for the whole trial, the FM seeds germinated after 47.5 days, 19 per cent faster than the NM seeds, which appeared after 58.5 days. Experiments on the radish (Raphanus sativus) by Fritz (1994) show the same tendency: faster germination for sowings shortly before FM.

These studies seem to agree with the hypothesis that the cytokinine content of plants is linked to the synodic lunar rhythm, with a maximum at full moon (this had been shown by Hofman, Featonby-Smith and Van Staden on algae in 1986 – cited in Fritz, 1994). Cytokinine also plays a role in the model proposed by Rossignol et al (1990) to explain the variations in the relative frequency of three forms of DNA according to lunar phases.

These differences in the speed of germination are probably also partly linked to cyclic variations in the absorption of water by seeds, as shown by Brown and Chow (1973) working on a large scale: 7,931 series of 20 beans. One of the absorption maxima coincides with the FM or shortly before (see Figure 1).

Water absorption by bean seeds and lunar phases

Water absorption by bean seeds (Phaseolus vulgaris) and lunar phases. Note: A, B and C are the three different periods in 1972 and 1973. 3-day mobile means. Source: Brown and Chow, 1973

And from the report's Conclusion and outlook:

These trials make clear, for the first time in trees or shrubs, the existence of a real phenomenon, often mentioned in traditions or issuing from empirical experience, consisting of a link between the lunar phases (synodic rhythm) and behaviour at germination and during initial growth. They demonstrate that the phenomenon is not as simple as it might seem at the outset, going beyond the general ‘cause and effect’ model and calling on predispositions or types of reaction specific to plants themselves. These trials in turn raise questions about the exact nature of this phenomenon and of the physiological processes involved.

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Many plants are photosensitive and the phases of the moon marginally affect light (though by an extremely minor amount compared with solar seasonal effects, cloud cover or shading), so you might expect a very small effect in some plants.

A principle of statistical sampling is that with a large enough sample size it is possible to find statistically significant results which are robust to repeat testing (as opposed to the 5% of experiments which produce apparently statistically significant results simply by chance) even if the effect is not substantial enough to affect behaviour in the real world.

There was a study "Effects of Moonlight on Flower Induction in Pharbitis nil, Using a Single Dark Period" by Kadman-Zahavi and Peiper in 1987. The abstract says

Flowering of Pharbitis nil plants was slightly inhibited by exposure to the light of the full moon for 8 or more hours with a single dark period of 16, 14 or 13 h. It is suggested that in the natural environment moonlight may have at most only a slight delaying effect on the time of flower induction in short-day plants.

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    @Flimzy: except perhaps for plants growing on the seashore close to the high or low water marks, that looks like an even smaller effect given the daily rotation of the Earth averaging out the gravitational impact of the Moon. – Henry Feb 26 '12 at 10:44
  • @Flimzy - And at one time people claimed that if you sailed over the horizon you would fall off the side of the earth. Then we understood what really happened. – Chad Feb 27 '12 at 13:41
  • @Flimzy - I agree but the comments of an answer to a related question are probably not the place to ask it :) – Chad Feb 27 '12 at 20:48

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