Folklore says the moon can affect plants growing in the field and garden. The reason is simple: Just as the gravitational pull of the moon affects the ebb and flow of the tides, so also does it affect the moisture levels in the earth.
That said, it is believed that you should plant things that produce their fruit above ground when the moon is waxing, or getting fuller. You plant things that produce their fruit below ground, such as potatoes, turnips, and carrots, when the moon is waning.
The Old Farmer’s Almanac has made a useful little video explaining why the moon can have an effect on your crops.
While it’s not necessary to follow these dates as steadfast rules for planting, you may find that the old-timey practice of planting by the moon’s cycles will benefit your garden.