Anne Greer, of Caversham, asks :-

Does the moon affect the germination of seeds?

Jeff Reid, a crop physiologist and soil scientist at the New Zealand Institute for Plant & Food Research Limited, responded.

Moonlight could influence plant growth and development, but there is no irrefutable evidence that the moon affects seed germination.

Seeds are normally in a dormant state. Different species vary in what trigger is needed to break that dormancy. Some require some time in cold conditions, some need to be very dry and then wet, and some seem to require nothing at all – they are ready to germinate as soon as they are mature on the parent plant.

Once they have received that trigger, the germination rate depends on there being enough water and oxygen around the seeds. It also depends strongly on temperature, provided that is above a minimum value that varies between species.

Observations on carrots prompted your question. Carrot seeds don't need a strong trigger to break dormancy, they don't need the soil to be especially wet, and germination can start at temperatures just above 0 C. But at such low temperatures the germination rate is dreadfully slow even though they readily absorb the water they need. Soil organisms and diseases that attack germinating seeds are often better able to cope with low temperatures, and so you may never see those seeds emerge. Usually at temperatures above 10 C germination (and emergence) is fast enough for the plants to have an advantage over the organisms that might attack them.

So – how could the moon affect germination? In the books and internet articles that say the moon does this, it's very hard for us to find the detail of any experiments and so how reliable they are. It seems that they did not separate out the effects of different planting times on soil temperature and other biological activities that vary with it. Usually planting later in spring improves carrot germination because the soil is warmer (irrespective of the phase of the moon). I have heard explanations that the moon improves soil water content near the seeds by pulling it up gravitationally from deeper in the soil. There is no evidence for this – the moon's gravitational effect is miniscule compared to the forces that move water up, down and sideways around germinating seeds. The moon's effects on ocean tides are without question of course, but there the body of water is large and effectively unbounded, so it is not subject to the sorts of restrictions to movement that water is in the soil or even quite large lakes.