The moon calendar in the garden: an honest reckoning

There is a sentence almost everyone has heard over the garden fence at some point: “No wonder, you sowed on the wrong day.” It usually comes up when the radishes have turned woody or the row of carrots has only half come up. And the neighbour who says it usually has a small calendar in a kitchen drawer that tells you, for every single day of the year, whether it is a root day, a leaf day, a flower day or a fruit day.

These calendars are widespread in Germany, Austria and Switzerland. They hang in allotment sheds, sit in kitchen drawers, live as an app on the phone. Some gardeners organise half their year around them, others think the whole thing is nonsense. The two groups rarely talk to each other, and neither of them often gets a straight answer.

So let us take it in order: what the calendars actually say, what controlled trials have found, why the popular tidal comparison does not hold up physically, and why these rules persist so stubbornly anyway. At the end there is a proposal for settling the question in your own bed, without picking a fight with anyone.

What a moon calendar actually says

The best known version goes back to Maria Thun, who published annual sowing days from 1963 onwards. Her basic idea: the moon passes in front of the twelve constellations of the zodiac, and depending on which one it stands before, one particular part of the plant is favoured. The assignment runs through the four classical elements.

On top of that come secondary rules that vary from calendar to calendar. On a waning moon you are supposed to harvest whatever goes into store, and mow if you want the lawn to grow back more slowly. On a waxing moon you plant, water and feed. On a descending moon the energy is said to move into the root, which is why transplanting and spreading compost get put on those days. And there are barren days, when the advice is to do nothing at all.

Why two moon calendars disagree about the same day

There are, you see, two different zodiacs, and this point almost never comes up in the argument. The sidereal zodiac is tied to the stars, so it travels along with the constellations. That still does not tell you where its boundaries lie: sidereal astrology divides it into twelve equal sections of 30 degrees each, whereas Maria Thun works with the actual, unequally wide constellation boundaries. The tropical zodiac, used by classical astrology and by many popular moon calendars, is not anchored to stars at all, but to the turning points of the year, that is, to the spring equinox and the solstices.

And the real constellations in the sky are simply not equally wide. Scorpio takes up only about 7 degrees of the sun's path, Virgo about 44. Astronomically there are in fact thirteen constellations along the sun's path, because Ophiuchus belongs there too, and no moon calendar lists it. The moon itself, with its orbit tilted by five degrees, brushes past several more, reaching as far as Orion. So wherever a calendar draws the line between two signs, that line is always partly a decision made by the publisher.

The two zodiacs sat roughly on top of each other in antiquity. Since then the earth's axis has turned further, an effect called precession, which completes one full circuit in around 25,800 years. By now the two are about 24 degrees apart, a good three quarters of a sign. Where Thun has the moon in Taurus, a calendar calculated astrologically already puts the same moon in Gemini. That is how a root day turns into a flower day.

In practice this means: if you buy two moon calendars from different publishers, the same day can be a root day in one and a flower day in the other. Both appeal to the moon. Both cannot be right at the same time. This is not a detail for stargazers, it is a hard problem for any rule that rests on the constellation.

What the trials show

The question has been studied for over a hundred years, and it lends itself well to testing. Sowing date is a clean variable: same cultivar, same seed, same soil, same sowing depth, only the day differs. And that is exactly how it has been done.

As early as 1946, the forest entomologist C. F. C. Beeson reviewed the literature then available on lunar influence on plant growth in Nature and found no solid connection. Many trials have been added since, in commercial nurseries, at universities, and within the biodynamic world itself. The most persistent sowing and counting was done at the Institute for Biodynamic Research in Darmstadt. Hartmut Spieß sowed winter rye and radishes over several years and did find rhythms, just not the expected ones. Connections showed up in the synodic cycle, that is, in waxing and waning, and in the moon's proximity to the earth, and even those were only partly statistically significant. Of all things, the sidereal trigon positions on which the sowing days are based could not be confirmed in his trials. That is the most remarkable thing about this work: the scrutiny came from within the biodynamic world itself. What effects remain are small, they shift between years and crops, and they are swamped by soil temperature, moisture and cultivar. This is exactly the pattern you expect when you are looking at random variation rather than a stable effect.

A review by Mayoral and colleagues in the journal Agronomy (2020) went systematically through the physics and the biology behind the claims and reached the same conclusion: the sowing and yield rules have no reproducible basis whatsoever. What is striking is that the rules are passed on in textbooks and advisory literature all the same.

Honesty requires the other side too. There are measurement series showing fine, moon-synchronised fluctuations in plants, in leaf movements for instance, or in the trunk diameter of trees. Peter Barlow documented such lunisolar rhythms over many years, and in timber research the question of moon-dependent differences in water content is still being debated. That is not the same thing as a difference in yield in your bed. Between “fine sensors pick up a signal on a tree trunk in the range of micrometres” and “your radishes get fatter if you sow on a Tuesday” there are several orders of magnitude.

Why the tidal comparison does not hold up

The commonest argument runs: the moon moves the oceans, so it also moves the water in the soil and the sap in the plant. That sounds compelling and is nonetheless wrong, for a reason that fits into one sentence. Tides do not arise because the moon pulls on water. They arise because it pulls with different strength at different points of a very large body of water.

That difference is tiny. The moon's tidal acceleration at the earth's surface amounts to roughly one ten-millionth of the earth's gravity. The only reason visible tides come of it at all is that an ocean stretches thousands of kilometres and can resonate within its basin.

The proof of that is in every tide atlas. The same moon stands over the Baltic as over Canada. The western Baltic manages about 30 cm of tidal range, the Mediterranean usually less than half a metre, the North Sea on the German coast two to three metres, and the Bay of Fundy in Canada up to 15 metres. That is a factor of roughly fifty between the Baltic and Fundy, under precisely the same moon. It is not the moon that differs there, it is the basin.

A garden bed is 30 cm deep and a few metres wide. It has no tidal range you could measure. Earth tides do lift the ground along with everything else, by a few decimetres, but they lift soil, soil water and plant in the same rhythm and by the same amount. So no gradient forms inside the bed that could move water, much as the water in a glass stays flat inside a moving lift.

It is much the same with moonlight. A full moon delivers something like 0.1 to 0.3 lux. A dull rainy day outdoors sits at a few hundred to two thousand lux, a bright overcast day above ten thousand, direct summer sun at up to 100,000. For photosynthesis, then, moonlight is irrelevant. It does have effects on animals, on night-flying moths for example, and indirectly on pollination through them. The seedling in the dark soil notices none of it.

Why the rules persist anyway

If there is nothing to it, why does it work for so many people? Because it genuinely does work, just not for the stated reason. There are several good explanations for that, and none of them makes anyone who gardens by the moon a fool.

The calendar can hardly be wrong. On every single day of the year something is favourable. Sow lettuce on a leaf day and succeed, and you have your proof. Sow lettuce on a leaf day and fail, and the small print offers you a barren day, an unhelpful lunar node, or the suggestion to try again three days later next time. A set of rules that has an explanation for every outcome cannot be refuted by experience.

Confirmation bias does the rest, and it catches all of us. Hits stay in the memory, failures fade. The harvest that was splendid after a root day gets passed on. The three rows that stayed puny in the same year on the same day count as bad luck with the seed. After a few years a firm conviction stands there, built out of carefully filtered memory.

And then there is the part that genuinely works, which gets far too little airtime.

What actually decides germination

The good news: the quantities that really matter are ones you can take into your own hands. They are less romantic than a constellation, but they deliver the same result every year.

1. Measure the soil temperature, do not guess it

   A simple soil thermometer, pushed in 5 cm deep, read in the morning. Two numbers matter per crop: the point at which anything germinates at all, and the point at which it germinates briskly. Lettuce starts at around 4 °C, radishes at around 5 °C, and both come up within a few days from 10 to 12 °C. Carrots also start at around 5 °C, but below 10 °C they take two to three weeks and only come up evenly at 15 to 20 °C. Beans want at least 10 to 12 °C, cucumbers and squash at least 15 °C, reliably 18 to 20 °C. Below the threshold the seed just sits there and rots. There is a ceiling too: lettuce barely germinates above about 25 °C soil temperature, and corn salad becomes unreliable above 20 °C. That explains more failures than all the barren days put together.

2. Match sowing depth to seed size

   For coarse seed, as a rough guide: about two to three times as deep as the seed is long. Dwarf beans come out at 3 to 4 cm on that basis. For the flat squash seed the rule does not carry, and there 2 to 3 cm is the value that works. For fine seed it carries even less, otherwise the carrot would sit at 3 mm and the drill would be dry by midday. Use fixed values there: carrot about 1 cm, radish 1 to 2 cm, each with a fine covering layer that will not crust over. Light germinators such as celery and lettuce are only pressed down. Sowing too deep is one of the commonest reasons for patchy rows.

3. Keep it evenly moist until it comes up

   Not a lot of water, but reliable water. A seed drill that dries out twice gives you half a row. How long that takes depends on the crop and the soil temperature. At 12 to 18 °C radishes need four to six days, lettuce six to ten, carrot and celery two to three weeks. At 5 to 8 °C in March everything takes roughly twice as long, which is normal and no reason to write the row off. In summer a board or a sheet of fleece over the drill helps until the seedlings appear. There is more detail on that in our article on sowing and raising seedlings.

4. Sow in batches instead of hunting for the perfect day

   Two weeks between portions. With radishes and winter radish and with lettuce on a sowing rota that is the real trick. A staggered sowing beats any calendar, because it insures you against the weather as well.

5. Write down your own numbers

   Date, cultivar, soil temperature, sowing depth, result. After two years you will have better dates for your soil than any printed calendar can offer, because they come from your own garden.

If you want to garden by the moon

Then do. It costs nothing, it does no harm, and it brings structure to a gardening year. Just two requests. First: never let the moon overrule the weather. If the fruit day falls on a night at 2 °C, the tomatoes stay indoors. If the soil is sticky after three days of rain, nothing gets dug, whatever the calendar says.

Second: if the question genuinely interests you, settle it for yourself. That is surprisingly easy, because a sowing lends itself beautifully to being cut in half.

One more note on the rules that go beyond sowing. With fruit tree pruning you often hear that you should cut on a waning moon. What really counts there is the tree's dormancy, a frost-free day and dry weather so the wounds can dry out. The same principle applies to sowing carrots: when the first generation of carrot fly emerges depends on the soil temperature in spring. In mild regions it is about from late April · CW 18, otherwise from early May · CW 19. So the net belongs on the bed before the first warm days of May, not after them. The window sits in the calendar year, then, not in the moon's course.

In the end there is one observation all gardeners share: whoever writes their dates down gets better. That is exactly what our garden diary is built for. An entry with the date, the cultivar, the soil temperature and what came of it is worth more after three years than any printed calendar. And if you note which moon phase it was while you are at it, all the better. Then at some point you will have your own answer, out of your own bed.

Common questions about the moon calendar

Does sowing on a waxing moon give a bigger yield?

In controlled sowing trials this is not reproducible, neither across years nor across crops. Where differences were measured they were small, in part only marginally significant, and not repeatable the following year. What genuinely lifts the yield: soil temperature above the germination threshold, steady moisture until the seedlings appear, the right sowing depth, and nutrients matched to what the crop needs.

What is the difference between an ascending and a waxing moon?

Waxing means the lit surface is growing, from new moon to full moon. That cycle takes 29.5 days and it is the one you see in the sky. Ascending means the moon climbs a little higher above the horizon each day on its run through the constellations. That cycle takes 27.3 days. The two run independently of each other, which is why a day can be waxing and descending at the same time.

Why do two moon calendars say different things about the same day?

Because they use different zodiacs. Some calendars go by the real constellations, others by the astrological signs, which precession of the earth's axis has by now shifted about 24 degrees away from them, a good three quarters of a sign. As a result the same day lands on a root day in one calendar and a flower day in the other.

When should I mow the lawn so that it grows back more slowly?

The moon rule for this does not survive testing. What actually slows regrowth is little nitrogen, drought, shade and a slow-growing mixture, so red fescue rather than ryegrass. In high summer the lawn slows down on its own because it is short of water. Mowing higher, 5 to 6 cm, does not slow it down, but it helps it through the dry spell and keeps weeds in check. Mow in the late afternoon or early evening, when the grass is dry, not in the midday heat and not on dewy grass. And stay inside the quiet hours: in German residential areas a lawnmower may run Monday to Saturday from 7 am to 8 pm, and not at all on Sundays and public holidays (32nd Federal Immission Control Ordinance). Austria and Switzerland have similar windows, often with a midday quiet period on top.

When should I sow radishes?

Whenever the soil is at least about 5 °C and you can water evenly, so outdoors that usually means from mid-March · CW 12 to early September · CW 36. They germinate briskly from 10 °C. Sow a short row every two to three weeks. In the heat of high summer radishes turn sharp and spongy easily, and a spot in partial shade plus reliable water will help you more than the choice of day.

Is there any harm in gardening by the moon calendar?

No, as long as the calendar does not overrule the weather forecast. It only becomes a problem if a favourable date leads you to put young plants outdoors before the last frost night, or if a barren day makes you postpone a sowing by two weeks and you miss the crop's window. The calendar may set the rhythm, it must not set the limits.