I would guess the flowering plants would end up doing all right eventually; after all, they did OTL, even before the asteroid/comet impact. The bigger problem is the different climate and possibly atmospheric conditions of the time. I'm not really sure, but it's possible that the Cretaceous climate is too warm or too cold or the atmosphere is too oxygen-rich (or poor) and so they can't make a go of it--they die from heat stroke, frost bite, slowly suffocate, their crops can't grow, etc. Of course, the Native Americans are also really likely to suffer from diseases from the modern-day and 1830s populations, which can't possibly help.