Planting by the Moon: What the Evidence Says

What moon planting and “best days” are

There are two related systems. The simpler one follows the phase of the moon. University of Arkansas Cooperative Extension explains it this way: crops that grow below ground, such as potatoes, onions, carrots and radishes, are planted in the dark of the moon, the waning phase from full moon to new moon. Crops grown for what is above ground, such as broccoli, lettuce, spinach, beans and tomatoes, are planted in the light of the moon, the waxing phase from new moon to full.

The fuller system, often called planting by the signs, adds the sign of the zodiac the moon is passing through. Arkansas says the twelve signs are grouped as water, earth, air and fire, and water and earth signs are counted fertile and good for planting, air and fire signs barren. NC State Extension in Henderson County lists Cancer, Scorpio, Pisces, Taurus, Capricorn and Libra as the fruitful signs in that tradition. The “best planting days” printed in almanac planting calendars are worked out from these rules for each month.

Where the idea comes from

NC State’s Steve Pettis points out that ancient peoples relied on the sun and the phases of the moon for keeping time, and after thousands of years of watching the sky it made sense to believe its rhythms ruled much of what happens on Earth. University of Illinois Extension’s Ryan Pankau says the same: before modern timekeeping, the moon’s phases were how people tracked the season and planned the work of the year. Planting by the signs rests on the idea that the moon, which plainly moves the tides, affects water on Earth and so affects how plants grow.

It is also family knowledge. Pettis writes that gardeners who plant by the signs learned it from their ancestors, and that his own grandparents, who planted that way, were better gardeners than he is. Arkansas notes that Vance Randolph, in his 1947 Ozark Superstitions, recorded that even a hill farmer who could not read or write could glance at a calendar, tell whether the moon was waxing or waning, and plant accordingly.

What extension services and research say

University of Illinois Extension describes a 2020 review in the journal Agronomy (Mayoral and colleagues, “What Has Been Thought and Taught on the Lunar Influence on Plants in Agriculture?”) whose authors went through over 120 references to ask whether the moon affects plants beyond moving the tides. As Illinois summarizes it:

• Gravity. Plants do respond to the Earth’s gravity, but the moon’s gravity as it acts on Earth is about 300,000 times weaker than the Earth’s own, and the reviewers found it imperceptible beside the force that moves sap up a stem.
• Moonlight. A full moon gives about 128,000 times less light than the sun, not enough to play a meaningful part in photosynthesis, even for the most sensitive plants.
• Overall. The review found no consistent relationship supporting a lunar influence on plant life.

NC State Extension in Henderson County quotes the same conclusion and adds that other studies have similarly found that planting by the signs has little basis in fact. University of Arkansas Cooperative Extension says a subtle effect on plants is easy to speculate about but hard to prove.

Where the evidence is weak or missing

Honesty runs both ways, so here is what the evidence does not settle.

• The question is not closed. Illinois reports that the review’s authors themselves called for more research, given how widespread these traditions are, and that the review turned up interesting side findings: a Middle Eastern shrub that produced more pollen at full moon, when its pollinators could find it more easily by moonlight.
• We have not read the paper itself. The publisher’s site and the university repository could not be reached when we wrote this, so everything above about the review is as University of Illinois Extension and NC State Extension describe it.
• Personal observation is not nothing, but it is not data. Arkansas’s columnist writes that he judges the last killing frost by the April full moon, and in the same breath calls it an unscientific observation with no data to back it.
• Good results may come from good gardening. NC State’s Pettis suggests that if moon planters do get better crops, it may be because the practice keeps them paying close attention to their gardens. His view: if it works for you, do it.

What reliably decides planting dates

Arkansas’s columnist recalls that on the Oklahoma farm where he grew up, work schedules and the weather decided planting time, not the moon. That is still where extension planting advice starts: from your last spring frost and your first fall frost, and from whether the soil has warmed.

• Frost dates. A frost-tender crop planted before the last spring frost can be lost in one night, whatever the moon is doing. Frost dates differ a great deal from place to place, even within one USDA zone (see your hardiness zone is not a planting date).
• Soil warmth. University of Minnesota Extension says squash seed will not germinate in cold soil, and sows after the last frost once the soil has warmed. University of Illinois Extension plants tomatoes when the soil is warm, soon after the frost-free date, and warns that a plant exposed to cold nights may take up to two weeks to resume normal growth. We do not publish soil temperatures; a soil thermometer in your own bed is the way to check.

Three free tools here date planting from the NOAA weather station nearest your ZIP code:

• ZIP zone finder: your USDA zone and your freeze dates.
• When to start seeds: a dated calendar for sowing indoors, setting out and sowing outdoors, counted from your last spring freeze.
• What can I still plant: which crops still have time to mature before your first fall freeze.

If you plant by the moon

There is no need to choose. Use your frost dates to find the safe window for each crop, then plant on the moon days you prefer inside it. The frost dates decide whether a tender crop survives; inside the safe window, the day is yours. For indoor sowing dates counted back from your zone’s last spring freeze, see when to start seeds indoors by zone, and for putting a whole season together, how to plan next year’s garden.