The wreath can be both pagan and Christian. Pagans used wreaths for various things including crowning the winners of their athletic games. Christians adopted things like the Christmas wreath based on the crown of thorns worn by Christ. The crown of thorns was a corruption of the wreath placed on people who were given awards and honors after the manner of pagan customs. Since the Romans were pagan it stands to reason that what they were doing merged with what we believe, since the ultimate sacrifice was carried out by the hands of pagans.
Many people Confuse what Chritianity is and think it is the Catholic Church which in itself is from Pagan Religion and still practices it.
Flowers can be placed on graves for many reason, pagans did it for the reasons mentioned, but they were deceived that it did anything at all, so there is no evidence that Christians do it for Pagan reasons.