Moonlight is the connecting link between living processes and the lunar cycle, suggesting that its elimination can
seriously weaken the biological effect of the lunar cycle. Cloud cover, especially its lower level, effectively absorbs
moonlight under natural conditions. However, it is unclear how this process affects the sensitivity of plants
to the lunar cycle. To address this, I analyzed the change in the growth response of the peat moss Sphagnum
riparium Ångstr. (Sphagnaceae, Bryophyta) to the illumination of the moon due a gradual increase in the low
cloud cover. The data on the growth of this species were obtained as a result of extensive six-year monitoring
carried out over the full growing season in the mires of Karelia (Russia). In total, 1081 days of the Sphagnum
growing were covered, 161190 growth increment of shoots were measured and 1075 values of the Sphagnum
growth rate were obtained. As a result, it was found that from certain values, low cloud cover disrupts the influence
of the lunar cycle on the growth of S. riparium. Under clear and little cloudy sky conditions, the illumination
of the moon most strongly inhibits the growth processes (r varies from 􀀀 0.434 to 􀀀 0.366; p < 0.001).
With the cloud cover from about 40%, this inhibitory effect begins to dramatically weaken. With the cloud cover
of about 60%, it reaches a significance level (r = 􀀀 0.182; p = 0.01), and with a further increase in cloudiness it
becomes insignificant. This finding confirms the influence of the lunar cycle on plants through variations in the
moonlight and indicates that cloud cover can strongly control the plant response to the lunar cycle.