I'm finding a lot of recent evidence... So for VEGetative stage, USE A MAGNET (south end of the magnet)

check this out:
CORDIS : News

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Cryptochromes are photoreceptors which are sensitive to blue light, and they are involved in a number of processes linked to the circadian cycle, such as growth and development.

To determine whether the magnetic field influences the function of the cryptochromes, researchers from France's National Centre for Scientific Research (CNRS) and universities in Frankfurt and Marbourg grew the plants in the presence of blue and red light and magnetic fields of varying strengths. They found that increasing the magnetic field only increases the inhibition of the growth of the hypocotyle in the presence of blue light. When red light is used, the plant uses other photoreceptors called phytochromes, and the growth of the hypocotyle (stem) is not affected by changes in the magnetic field. Furthermore, mutant plants which have no cryptochromes are also insensitive to changes in the magnetic field.

The study shows for the first time that in plants, the work of the cryptochromes is affected by magnetic fields and suggests that the mechanisms of magnetic field perception in plants, and by extension in migratory birds, use the same photosensitive molecules. The researchers also suggest that, as cryptochromes have been strongly conserved throughout evolution, all biological organisms could have the ability to detect magnetic fields, even if they do not use them. For more information, please visit:
CNRS...