Answer: Mulch should be applied after the ground freezes in late fall. This keeps the ground frozen, avoiding the freeze-thaw cycle that can often happen repeatedly, especially as winter comes into spring. Plants that are subject to repeated freezing and thawing can suffer root and crown damage, which can weaken them and leave them open to disease. The best kinds of organic mulch are those that don't mat down when wet -- straw, shredded leaves (whole leaves form matted layers), and evergreen boughs are good examples.
Congratulations on your perennial garden -- hopefully your success will also be perennial!
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