I was reading something about this recently. The author was dismissive about the phases being important in planting, but pointed out that, since in the past working people worked dawn-to-dusk, then the only time they had in the busy seasons was at night, then much garden digging was done by moonlight, that being the only unemployed time that they had. It wouldn't be hard to see how a custom could grow from this, s/he said, as "My Dad always planted as the full moon was waning" is an obvious conclusion to draw.

Sorry, can't remember where I read it.