He graduated in 1887 at the University of Würzburg on a thesis entitled "Geschichte der Assimilation und Chlorophyllfunktion" (supervised by Julius Sachs).
However, he always worked alone, supervised very few doctoral students (4 in 39 years), and stood outside the development of experimental physiological botany among his contemporaries.
[1][2] Hansen wrote scholarly works on Goethe's Metamorphosis of Plants and had an aggressive dispute over this hypothesis with Houston Stewart Chamberlain.
[3] In 1901, Hansen wrote a treatise of the dune vegetation of the East Frisian Islands,[4] in which he proposed salt as the main plant-distributing factor.
He thereby neglected previous work by Warming[5] and Raunkiær[6] contending the importance of the wind.