This snippet of Barbara Allen seems to me to be a pagan reference. I don't know of anything in any modern orthodox religion that teaches of such a relationship between plants and the character of the interred.

They buried her in the old churchyard,
Sweet William's grave was nigh her,
And from his heart grew a red, red rose,
And from her heart a brier.


They grew and they grew o'er the old church wall,
Till they couldn't grow no higher,
Until they tied a true lover's knot,
The red rose and the brier.