Our last frost date is also May 15, and we removed the dead
foliage near the end of February. Here, evergreen or semi-evergreen
plants can't seem to unfurl and push the new foliage beyond the dead stuff
without removal, and could be subject to rot. We also help the plants
to unfurl by pinching the tops off where they are stuck together.
Dormant type foliage needs no help at all, but we do remove the
dead stuff on them also.
Each year is different in the timing though. It depends on the
number of warm days and moisture the plants get and how fast
they seem to want to grow.
