The idea that moonlight helps plants grow has been a persistent belief in folklore and traditional gardening for centuries. This query often stems from observing the moon’s powerful influence on ocean tides, leading many to assume a similar effect on the water within plants and soil. To determine the truth, we must examine the physical characteristics of moonlight and compare them against the measurable biological requirements of plant life.
The Physical Properties of Moonlight
Moonlight is not a unique light source but reflected sunlight, sharing the same spectrum of wavelengths but vastly reduced in intensity. The moon’s surface, composed of dark rock and dust, is a poor reflector, with an average albedo (reflectivity) of only about 12%. At its brightest during a full moon, the light intensity reaching Earth is extremely low, typically providing only 0.05 to 0.1 lux of illumination.
For comparison, a sunny day provides roughly 100,000 lux, making peak moonlight approximately one-five hundred thousandth the intensity of direct sunlight. Moonlight also has a slightly warmer or “redder” color temperature, around 4000 to 4500 Kelvin, because the lunar regolith reflects longer wavelengths more efficiently.
Plant Light Requirements for Growth
Plant growth is fundamentally driven by photosynthesis, which converts light energy into chemical energy (sugars). The light spectrum plants use for this conversion is Photosynthetically Active Radiation (PAR), spanning the 400 to 700 nanometer range. Chlorophyll pigments absorb blue light (400–500 nm) for vegetative growth and red light (600–700 nm) for processes like flowering.
For a plant to achieve net positive growth, photosynthesis must exceed respiration, a threshold known as the light compensation point. This minimum requirement is measured in photosynthetic photon flux density (PPFD) units. Even seedlings require around 100 to 200 \(\mu \text{mol m}^{-2} \text{s}^{-1}\) of light for healthy development. A mature plant needs a significantly higher intensity, often in the range of 400 to 600 \(\mu \text{mol m}^{-2} \text{s}^{-1}\), to sustain robust growth.
Moonlight’s Specific Effects on Plant Biology
When the extremely low intensity of moonlight is compared to plant requirements, it is evident that the moon cannot serve as a significant energy source for growth. Peak light from a full moon measures only about \(0.0059 \mu \text{mol m}^{-2} \text{s}^{-1}\) in PAR units, which is a fraction of the minimum light compensation point for most species. Therefore, moonlight contributes no measurable energy to a plant’s overall photosynthetic production.
However, light also functions as a signal to regulate plant behavior. The subtle light from the moon is perceived by highly sensitive photoreceptors within plant cells, which detect minor changes in the light environment. This signaling effect relates primarily to photoperiodism, the plant’s mechanism for measuring the length of the dark period to time activities like flowering.
In short-day plants, which flower only when the night is long, the low light of a full moon can interrupt their perception of a continuous dark period. Moonlight can also trigger changes in gene expression, including those associated with the plant’s internal circadian clock.
Furthermore, moonlight possesses a low Red:Far Red light ratio, a common signal for shade avoidance. This specific spectral quality is perceived by seedlings, causing them to respond by unfolding their apical hook, a behavior typically seen when sensing a competitor nearby.
Lunar Cycles and Traditional Planting Practices
The idea of planting according to lunar phases, often called lunisolar gardening, has deep historical and cultural roots worldwide. These traditional methods are based on the belief that the moon’s gravitational pull affects water movement in the earth and within plants, rather than its light intensity.
The theory suggests that as the moon waxes (grows brighter), its gravity pulls groundwater closer to the soil surface, encouraging seed germination and sap flow. Conversely, the waning phase is believed to be the best time for planting root crops and pruning, as sap flow recedes towards the roots.
While the moon’s gravity creates ocean tides, its effect on localized bodies of water like soil moisture or capillary action in plants is considered negligible. The gravitational force of the moon is approximately 300,000 times weaker than Earth’s own gravity. Furthermore, the cohesive forces within a plant are significantly stronger than the moon’s pull. There is no consistent scientific evidence to support the claim that planting by the moon leads to a measurable increase in crop yield.