The lone voice of horticultural reason
“You cover perennials with mulch in the fall to keep the ground warm”
Wrong, you cover your perennials with mulch (marsh hay, bags of leaves, etc.) in late fall/early winter, after the ground has frozen, to keep the ground FROZEN.
If the perennials you grow are hardy to your zone, they’ll handle the frost. What they often won’t handle is freeze-thaw conditions that occur most winters, particularly in March and early April, especially since we’ve been lacking snow cover in recent winters.
So wait until your ground freezes—often as late at mid-November or even early December, depending on your northern zone—THEN cover your garden beds.
The Renegade Gardener