Lunar calendar in the garden: should you garden by the moon?

Do you want to know whether sowing your carrots on a root day changes your harvest? In side-by-side trials, the gap goes one way one year and the other way the next. No known physical force explains it. The lunar calendar will neither ruin nor make your vegetable garden. The seasonal window matters far more. The calendar can still be read in two minutes.

A lunar calendar is four cycles, not one

The word "moon" covers four different things. Calendars overlay them, hence their complicated look.

Sources: Paris Observatory for the length of the lunation, Nature & Progrès Belgique for the rest of the table.

A rising moon and a waxing moon are therefore not the same thing. The two cycles do not have the same length and drift apart from one month to the next. To know whether the moon is rising, Nature & Progrès advises watching it two evenings in a row, at the same hour.

Root, leaf, flower and fruit days

This split comes from Maria Thun, a German woman who built her calendars on her own observations. The twelve constellations are sorted by four elements. When the moon passes in front of an earth sign (Taurus, Virgo, Capricorn), it is a root day. A water sign gives a leaf day, an air sign a flower day, a fire sign a fruit day.

Each vegetable is sorted by the part you eat. The classification holds a few surprises. At Nature & Progrès, leek and onion go with the roots, cauliflower with the leaves, broad bean and pea with the fruit.

This is not a sceptic's opinion. Nature & Progrès publishes the lunar calendar in its magazine. It writes in black and white that following it to the letter is "an ideal that is hard to achieve". The calendar specific to each plant "remains of prime importance".

Side-by-side trials

Charles Dowding is an English market gardener who publishes his trials year after year. He sowed the same vegetables a few days apart, on either side of the full moon.

The last figure is the most quoted and the most fragile. The two sowings were not made on the same day. Yet a week in spring changes the soil temperature, the rain and the slugs. You cannot separate the effect of the constellation from that of the date.

His beans tell another story. Sown under a waxing moon, they gave 15.41 kg, against 13.97 kg under a waning moon. The latter still produced more in September, when beans were getting scarce. His dwarf beans sown on a fruit day gave 9 % more than on a leaf day. Dowding concludes that the results are variable. Sowing on the "wrong" day still gives good harvests. He ignores the lunar rules that do not fit his work.

These are gardener's trials, on one bed or two per treatment. They are worth more than an impression, not as much as a repeated trial.

Researchers' trials

Hartmut Spiess tested lunar rhythms on winter rye and radishes, in biodynamic growing. He found weak effects. The constellation effects described by Maria Thun did not appear. Two other researchers, Kollerstrom and Staudenmaier, judged the trial well designed but the data analysis insufficient. No new trial has settled the debate.

In Nicaragua, a team sowed maize at each phase of the moon, with four repeats per phase. Germination was better at first quarter and at new moon. The height, length and weight of the cobs did not differ significantly. Maize sown at new moon had the highest average weight. The farmers of the region believed the opposite.

Why physicists doubt it

In 2020, a team from the University of Valencia reread the textbooks of botany, plant physiology and physics, then the published studies. It finds no reliable evidence of a link between the phases of the moon and plant physiology.

Its calculations give the order of magnitude.

• The moon's gravity at the surface of the Earth is about 300,000 times weaker than that of the Earth.
• The tide the moon exerts on a plant 2 m tall is of the order of 0.0000000000003 m/s². It is imperceptible.
• The light of a full moon on a clear night reaches 0.25 lux. An average day gives 32,000 to 100,000. Even at best, the moon lights 128,000 times less than the darkest day.

The argument of sap rising with the moon runs into a detail. Tides come back twice a day. If they made sap rise, it would be twice a day, not once a month. As for the rising or descending moon, it changes neither gravity nor light. Only its height in the sky varies.

Questions still open

Plants do have rhythms that follow the moon, but not those of the calendar. Two researchers, Barlow and Fisahn, observed Arabidopsis roots kept in continuous light. Their growth oscillated over 24.8 hours, the length of a lunar day, following the calculated tide. The authors acknowledge it. Their proof is a correlation, so indirect. Nobody can switch off the tide to check.

Other work has seen peat mosses grow faster near the new moon. These rhythms last a day or a month. They are tiny. Nobody has shown that they change a carrot harvest. We also do not know why so many gardeners say they do better with the moon. One hypothesis stays open. A calendar that makes you observe and plan perhaps makes you a better gardener, whatever the constellation.

So, do you follow the moon or not?

It is your choice. It costs nothing as long as you stay within the seasonal window. If the lunar calendar helps you get organised, keep it. If it makes you miss a sowing, drop it without regret.

In the app, the lunar calendar shows the root, leaf, flower and fruit days inside the sowing window of each of your crops. It is presented there as a tradition. See the dates for your crops.

To place the moon among the other methods, read the overview of natural gardening methods. Biodynamics, which popularised root days, has its own guide. Sowing dates month by month are in the sowing calendar for Belgium.

Photo: Gerald England, Wikimedia Commons (Geograph), licence CC BY-SA 2.0