Answer: Fall mulching of hardy perennials isn't always necessary, but it's a good insurance policy. The idea behind it is to keep the ground frozen through the winter, since winter thaws followed by cold weather can cause frost heaves that damage roots and crowns of perennials. If you mulch too soon, you can keep delay freezing, and plants can be heaved when it gets really cold out. You and your plants will be safe if you mulch once the topsoil is hard and doesn't thaw in the warmth of the day.
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