I was able to farm 5 in about an hour. Maybe I was lucky but i definitely noticed a difference in foxgloves and periwinkle.Farmed cobalt foxgloves. noticed no difference on the spawn rate.
One problem is if you don't pick it up right away on spawn someone sees a decent one and picks it up leaving all the undesirable ones. It's like fettlecaps - they are actually the most common spawn but people pick them until all you are left with is milkstalks and a few deathcaps.