As to what influences the pendulum, many people would be sceptical that anything exerted an influence. Some Christians would no doubt ascribe a negative spiritual influence of some sort, because that’s often the sort of thing that happens.

Having said that, many of the prohibitions against ‘magic’ were culture-specific in a world that was often dominated by shamans and magicians. Much of the tribal religion around the Hebrews while in exile, and later the nation of Israel, was oriented towards magical religious practices.

Things are different these days. However there is still an aversion among Christians to practices like this because it seems occult, which conflicts with the idea of only worshipping or giving glory to God. (Although whether you could use a pendulum in a similar way to the urim and thummim or how lots were cast to divine God’s will in the Old Testament is worth thinking about.)

I’m not sure how effective using a pendulum in your gardening is. I don’t know anyone who prays before planting, but I guess there probably are some who feel ‘guided’ to plant certain plants in certain places. Certainly Jesus used plants in his parables a few times. Maybe there’s scope for an article on Bible-inspired gardening.