It's pretty curious complex and awesome to read Rudolf Steiner lectures.
He makes so fantastic correlations between the life force of a plant, the overall ecosystem, relationship between plants and animals, their fruits and food purposes and qualities, their state of health and how to prevent plant diseases and, most amazingly of all, the relationship between metaphysics and the spiritual with the material world. How the tales from above, make their influences to our green world below.
This wisdom is quite present in the old peasant lore, that is even reported in Portugal by old practicing rural farmers, the almanaques Borda d'Água and Seringador. The Maria Thun calendar takes some insight from it, but there are some misinterpretations to my belief.
Today I've tried all biodynamic preparations such as yarrow (Milefólio), chamomile (Camomila), horsetail (Cavalinha), nettle (Urtiga) and valerian (Valeriana), except for dandelion (Dente-leão) and oak bark (Casca de Carvalho). Let's see what happens. I had remarkable insights from following the moon-calendar since January, yet still under trying.
Please have a look at http://www.garudabd.org/Agriccourse/contents.html for Steiner lectures,
http://en.wikipedia.org/wiki/Biodynamic_agriculture for Wikipedia article, and http://mulliganstew.wordpress.com/2006/10/31/biodynamics-the-original-and-future-organics/ for a short introduction on Biodynamics.