i've used colonial knots with tails for these lighter green stems (2 strands) but you could also just use a straight stitch with a knot on top.
for the yellow seed stitches, i threaded 1 strand of each yellow (light and dark) together, and stitched the seeds with 2 strands. the yellow variation wasn't quite as dramatic as i hoped, but i think it still adds some nice subtle visual interest, even if it isn't very obvious.
i also added some small straight stitches (in light green, using 1 single strand) to the dark green fern stitches.
i feel a little like a broken record, but i always say that the tiniest details make the biggest impact.
- these tiny green stitches lighten the entire the piece as a whole and it really adds a lot to the piece, even though the stitches are small.
- i believe it's worth it to take the time to add special touches here and there. so often it is the tiny details that give a design it's unique personality.