The moon and the mushroom

The rule has as many versions as it has tellers. Some people go out on the waxing moon and leave the waning alone. Some hold that the days either side of full moon are the ones worth walking. Plenty inherited a flat instruction — i nedan — and never had cause to ask why. It is not a fringe belief and it is not a joke. It is one of the oldest pieces of timing advice in European agriculture, and it reached Swedish kitchens through the same channels as everything else people knew about the farming year.

It is worth being precise about what is actually being claimed, because the claim is not absurd. Nobody argues that moonlight builds mycelium. The argument is about the trigger: the mycelium is down there all season, fruiting is episodic, and something decides when. If that something ran on a roughly monthly period, the moon would be the obvious suspect. That is a reasonable hypothesis. It has been tested far less than you would expect, and where it has been tested carefully it has not held. The more interesting question is why the observation feels so solid anyway.

Where the rule comes from

It is genuinely old. Book 18 of Pliny’s Natural History — his book on grain, sowing dates and the farming calendar — carries a chapter headed “The revolutions of the moon,” sitting among chapters on the proper time for winter sowing and the epochs of the seasons. Lunar timing was ordinary Roman agronomy, filed next to the star calendar, not off at its edge.

The route into Nordic practice runs through print. Institutet för språk och folkminnen records that printed almanacs reached peasant households from the mid-1700s as literacy spread, carrying astrological forecasts alongside postal timetables and currency tables, and that from 1749 the Kungliga Vetenskapsakademien held the exclusive right to publish them in Sweden — a monopoly that ran until 1973. For two centuries, the most widely distributed printed object in the Swedish countryside was a moon calendar with the Academy’s name on the cover. The working distinction — ny for what grows upward, nedan for what swells below ground — is documented across Nordic folk tradition (Ajanti 2022, drawing on Pöyhönen’s study of lunar timekeeping in Finnish folk practice). Once potatoes and turnips belong to nedan, it is not a long step to put the mushrooms there too.

The twentieth century gave it a second life rather than retiring it. Steiner’s 1924 agriculture lectures put cosmic rhythm into the founding text of biodynamics, and Maria Thun’s sowing calendar, published annually from the early 1960s, made it operational — keyed to the moon’s sidereal position against the zodiac rather than to the phase you can see. Turinek et al. (2009) survey where biodynamic research actually stands. The strongest citation the lunar-planting side offers is a review by Kollerstrom and Staudenmaier (2001) in Biological Agriculture & Horticulture arguing for lunar-sidereal rhythms in crop yield; Mayoral et al. (2020), reviewing agronomy, botany and plant-physiology textbooks together with the peer-reviewed literature, reported finding no reliable science-based evidence for any lunar–plant relationship.

The gravity argument, done with numbers

The mechanism people reach for first is gravity. The moon moves the sea; the sea is water; soil holds water. The first two steps are right. The third is where it fails, and it fails by a margin worth seeing written down.

Tidal force is not the moon’s pull on you — it is the difference in that pull across the width of the body it acts on. The standard calculation puts the lunar tidal acceleration at Earth’s surface at roughly 1.1 × 10⁻⁶ m/s², about one ten-millionth of the 9.8 m/s² the Earth exerts on the same patch of ground. That is arithmetic from published constants, not a study; anyone can redo it. The ocean responds to a differential that small because it is a continuous, low-friction basin thousands of kilometres across, integrating a minute force over an enormous mass. A hectare of forest soil in Blekinge is not a basin. Water in the top thirty centimetres is held by matric and capillary forces orders of magnitude stronger than anything the moon brings.

The honest concession is that solid ground does flex, and people have looked for the consequence in living tissue. Zürcher et al. (1998), in Nature, reported that the stem diameters of two containerised Norway spruce (Picea abies), held in continuous darkness at controlled temperature, fluctuated in step with the tides. Vesala et al. (2000) went looking for the same signal in trees growing outdoors in Finland and Scotland, found no correlation, and concluded a gravitational cause was unlikely. And set against all of it: a single 15 mm rain event changes the water balance of the litter layer by an amount no plausible lunar signal approaches. On the variable that actually governs fruiting, the moon is competing with rainfall, and it is not close.

The light argument is better, and still thin

Moonlight is real light, and fungi are not indifferent to light. This is the stronger version of the belief. The problem is the size of the number. Kyba, Mohar and Posch (2017) measured it and wrote their note specifically to correct the inflated figures then circulating: most full moons at mid-latitudes deliver only 0.05–0.2 lux at the ground, with about 0.3 lux the extreme upper case. Full daylight is conventionally put at 10,000–100,000 lux. That is five to six orders of magnitude, in a country where an August day runs to fifteen hours of it — and Swedish autumn nights are not reliably clear, so the realised figure is a fraction of the small fraction.

Small is not zero, though, and this is the one place the research has genuinely moved. Tansey and Jack raised the question in the Journal of Theoretical Biology in 1975. More recently, Priyanka et al. (2025) reported that three nights’ exposure to full moonlight in Indian mustard (Brassica juncea) increased nuclear size and shifted protein and metabolite profiles in ways still detectable fifteen days after exposure; Sannidhi et al. (2025) revisit the whole subject in Plant Science. Read those carefully before you carry them across. They concern a green plant with photoreceptors that evolved for light, in a controlled experiment, exposed against a dark control. A chanterelle (Cantharellus cibarius) primordium under spruce litter is a different problem, and nobody has run the equivalent experiment on a forest fungus.

What has actually been tested

The plain answer is smaller than anyone on either side would like. On lunar phase against fungal fruiting in the field, there is essentially one careful study.

Egli, Ayer and Merlini (2011), at the Swiss Federal Institute WSL, analysed 1,715 dated mycological records collected between 1990 and 2007 on five permanent plots in western Switzerland, under Norway spruce, Scots pine (Pinus sylvestris) and mixed forest. One mycologist, one inventory per week from May to December, every fruiting body marked with methylene blue so nothing was counted twice. They tested two ways — waxing half against waning half, and a full-moon window (days 11–20) against the rest — for each site, for all sites pooled, and for mycorrhizal and saprobic species separately. Then they shifted the entire series back five days, on the reasoning that if the moon acts at all it acts on initiation rather than on the mushroom you can see. Nothing, anywhere: p-values running from 0.24 to 0.99, none near significance.

The other episode is more instructive. Hirschmann and Hirschmann (2000), a pair of mushroom advisers, compiled around 1,800 consultation records spanning 32 years intending to put the question to bed, and were startled to find a sinusoidal curve peaking near first quarter. Guiard (2002) reanalysed their original data with generalised additive and generalised linear models. The lunar term was not significant, and the apparent effects were not stable from decade to decade. What was significant, at p < 0.0001, was the day of the week.

The confound you can feel

Day of the week is the tell, because mushrooms do not know it is Saturday. Two things are happening, and neither implies carelessness.

The first is that you only know the phase if you looked, and you look on clear nights. Clear nights sit in particular positions in a weather sequence — behind the front, inside the settled high — and those positions carry their own moisture and temperature history into the following week. The moon is not causing the flush; it is visible from the same synoptic situation that produced it. The second is that the memory is asymmetric. A heavy basket under a bright moon is a story you tell. A heavy basket on a night you could not have named the phase is just a good day. Decades of honest observation, filtered that way, will assemble a rule. The Hirschmanns were experienced and were actively trying to debunk the thing, and they still produced a curve that dissolved under a proper model.

Where that leaves it

Not at “the moon does nothing.” The tested claim is narrower than that: across 1,715 field observations in one country, fruiting did not track lunar phase, and across 1,800 consultations in another, an apparent lunar signal turned out to be human habit. That is a thin literature, and Egli’s team say as much themselves — most of what surrounds it is, in their words, personal opinion or unsystematic observation. It is what we have, and it points one way.

So keep an eye on the moon if you want to. It is a decent clock, it gets you out of the house in September, and it connects what you are doing to several centuries of people who did the same thing on the same ground. The tradition is worth considerably more than the mechanism. What should not happen is anyone — a book, a forum, a piece of software — handing it back to you as measurement. The distance between “my grandmother went out i nedan” and “lunar phase is a term in the model” is the whole distance between a practice and a claim.

Sources

• Egli, S., Ayer, F. & Merlini, M. (2011). More mushrooms under a full moon — myth or reality? Sydowia 63(1): 23–33.

• Guiard, V. (2002). Beeinflusst der Mond das Pilzwachstum? Eine Reanalyse. Zeitschrift für Anomalistik 2(3): 292–307.

• Hirschmann, U. & Hirschmann, F. (2000). Unser Mond und sein Einfluss auf das Pilzwachstum — Lassen sich Zusammenhänge nachweisen? Natur und Mensch (Jahresmitteilungen der Naturhistorischen Gesellschaft Nürnberg) 1999: 79–82. (Not read directly; cited as reported by Guiard 2002 and Egli et al. 2011.)

• Mayoral, O., Solbes, J., Cantó, J. & Pina, T. (2020). What Has Been Thought and Taught on the Lunar Influence on Plants in Agriculture? Perspective from Physics and Biology. Agronomy 10(7): 955.

• Zürcher, E., Cantiani, M.-G., Sorbetti-Guerri, F. & Michel, D. (1998). Tree stem diameters fluctuate with tide. Nature 392: 665–666.

• Vesala, T. et al. (2000). Do tree stems shrink and swell with the tides? Tree Physiology 20(9): 633–635.

• Kyba, C. C. M., Mohar, A. & Posch, T. (2017). How bright is moonlight? Astronomy & Geophysics 58(1): 1.31–1.32.

• Tansey, M. R. & Jack, M. A. (1975). Moonlight, mushrooms and moulds. Journal of Theoretical Biology 51(2): 403–407.

• Priyanka, G., Singiri, J. R., Novoplansky, N. & Grafi, G. (2025). Short Exposure to Full Moonlight Has a Long-Term Impact on Brassica juncea Cell Activity and Growth. Plant, Cell & Environment 48(5): 3038–3051.

• Sannidhi, S., Priyanka, G., Singiri, J. R., Novoplansky, N. & Grafi, G. (2025). Plants and the moonlight: A controversial subject revisited. Plant Science 362: 112841.

• Kollerstrom, N. & Staudenmaier, G. (2001). Evidence for Lunar-Sidereal Rhythms in Crop Yield: A Review. Biological Agriculture & Horticulture 19(3): 247–259.

• Turinek, M., Grobelnik-Mlakar, S., Bavec, M. & Bavec, F. (2009). Biodynamic agriculture research progress and priorities. Renewable Agriculture and Food Systems 24(2): 146–154.

• Institutet för språk och folkminnen. Tideräkning och almanackor (kunskapsbank).

• Ajanti, C. (2022). The Whole of The Moon: folktro och månen. Kulturanalyser, Åbo Akademi — citing Pöyhönen, A. (2010), Månen i ny och nedan: konsten att bestämma tid i finsk folktradition.

• Pliny the Elder, Natural History, Book 18, ch. 75 — “The revolutions of the moon” (Bostock & Riley translation, Perseus Digital Library).