Although there is a tremendous popular interest in magnetic fields...and the health 'risks' or electromagnetic fields, I have never heard of a study that conclusively shows that magnetic fields..or changing magnetic fields...affect plant growth AT THE LEVELS FOUND IN THE NATURAL ENVIRONMENT. Powerful local magnetic fields created artificially can suck car keys out of your pockets, and these kinds of fields can probably cause all kinds of biological changes. But the fields found in nature at the scale of your backyard garden, are way too weak to do much at all that makes a difference. If you know of any study that says otherwise, let me know!!!