This is not enough to disregard other sources of D3, but it is an interesting anecdote regarding this all so important vitamin.

Arch Biochem Biophys. 1984 May 15;231(1):67-71.
The isolation and identification of vitamin D2 and vitamin D3 from Medicago sativa (alfalfa plant).
Horst RLReinhardt TARussell JRNapoli JL.
Vitamin D2 and vitamin D3 were isolated from Medicago sativa (alfalfa) grown under field and laboratory conditions and then irradiated with ultraviolet light. The vitamins were identified by ultraviolet absorption, mass spectroscopy, and comparison with synthetic standards on several chromatographic systems. Sun-cured, field-grown alfalfa contained vitamin D2 at a concentration of 48 ng/g (1920 IU/kg) and vitamin D3 at 0.63 ng/g (25 IU/kg). Laboratory-grown alfalfa, artificially irradiated, contained vitamin D2 at a concentration of 80 ng/g and vitamin D3 at 1.0 ng/g. Therefore, the presence of vitamin D2, as well as vitamin D3, has unequivocally been demonstrated in alfalfa plant tissue.