Planting by the phases of the moon is an intriguing concept. Gardeners and farmers have been drawn to the idea that the moon affects the growth of plants for thousands of years. The effects of the moon on the waters of the earth were noted in Roman times, and by a number of ancient peoples around the world. The moon creates the tides, and likewise more subtly draws the waters of the soil upwards. Many argue that this means that certain plants will grow better in certain phases of the moon than in others. Unfortunately, there is little to no scientific evidence to back up this theory.
While there is no doubt that the moon acts on our planet in many different ways, and does obviously affect all living things on Earth, few credible scientific studies have been undertaken to discover the impact that the moon has on plants, and no credible studies that have managed to conclusively prove the efficacy of planting certain types of plant during certain moon phases. There is certainly no reason to give any credence to the idea that there is anything scientific about planting according to where the moon falls within the signs of the zodiac.
Though those talking about lunar phase planting or moon sowing often wander off into the realms of fantasy when they begin to talk of astrology, it certainly does no good to throw the baby out with the bathwater. The moon has a strong impact on many elements of human and animal life and there is no reason to assume that plants are not similarly effected by its progressions. The fact that science cannot tell us exactly how the moon influences plant growth is reason enough to keep an open, though sceptical, mind.
In the lunar planting calendar, certain plants are best sown during a new moon, or around a full moon, for example. Many gardeners adhere to this calendar rigorously. Many feel that, anecdotally, their efforts to garden according to the phases of the moon have borne fruit. Many of the success stories, however, can be explained away by a lucky coincidence of timing. By waiting for a propitious phase of the moon, for example, a gardener may have avoided planting too early and losing seedlings to a late frost.
But this could also work the other way. By rigidly adhering to lunar planting calendars, a gardener is ignoring all the other factors such as local temperature, weather and other variables. Whether or not the moon has a noticeable impact on plant growth, it is clear that other factors, much closer to home, are more likely to lead to the success or failure of your gardening endeavours. By all means be open minded about the possible impact of the moon on plants in your polytunnel, but don't be ruled by it.