Mulching your perennial plants offers many benefits. For example, a well-decomposed compost applied as a mulch will provide much-needed plant nutrients and may even eliminate the need to feed the plants. Or, mulch can be used to moderate soil temperatures and help retain moisture in the soil. Used in this way, mulch can help plants survive drought periods. But for most people, mulching perennials is a key part of putting a garden to bed for winter.
Mulching for Winter Protection
When mulching is done as part of the garden winterizing ritual, it is done mostly to protect the roots of plants and help them survive the winter. Contrary to a common belief, though, a thick layer of mulch is not intended to insulate and prevent the roots from freezing. Instead, its purpose is to prevent the freeze-thaw cycles during the transitional seasons, which can destroy plant cells and kill the plant. Many plants will survive quite nicely if they freeze hard in December and stay frozen hard until a permanent spring thaw, but they can be destroyed if fall or spring swings back and forth between frigid and warm temperatures. Mulch helps to prevent these temperature swings and the resulting damage to plant roots.
In an effort to grow a species that is borderline hardy in a particular zone, some gardeners plant it in a sunny location, thinking that winter sun will somehow help the plant survive. This is actually the worst thing you can do, since this encourages thaw-freeze cycles usually kill borderline plants. Instead, these borderline hardy species often survive best if planted in shadier locations where the sun can't thaw the plant before spring fully arrives.
Mulching tends to be most necessary in regions that are subject to great swings in temperature during the transition periods between fall and winter, and between winter and spring. For example, some regions with long, cold winters may still experience sudden thaws—after which winter returns with a vengeance. If you garden in such a climate, you do not want your perennials being tricked into emerging from their sleep during one of these false starts. Mulch helps prevent tissue damage that may occur during freeze-thaw cycles.
Mulching is less necessary in regions where winter and spring arrive suddenly and without much vacillation in temperatures.
Not All Plants Need Winter Mulch
The plants that benefit most from winter mulch are those that are borderline hardy in your zone. If you live in the extreme northern part of zone 5, for example, plants rated for zone 5 may be tricky to grow. But mulch is not necessary—and may even be harmful—for plants that are very hardy in your region. Some types of Coreopsis, for example, are very cold-hardy, and a thick layer of mulch can encourage pests to nest and pathogens to lurk in the soil.
In general, native species often do just fine with no winter protection at all. Many hybrids or specialized cultivars, on the other hand, are less hardy and may need the extra protection offered by a layer of mulch.
Some plant species become much more able to tolerate freeze-thaw cycles after they are well-established. It's often a good idea to mulch new plants for the first winter or two, then allow them to fend for themselves once they are well established.
Finally, some perennials prone to crown rot (for example, delphinium) often die not from extreme cold but from excessive moisture. What they most need to survive the winter is not insulation, but good drainage. In fact, since mulch helps the soil retain moisture, it may actually be counterproductive for such plants if it is piled up over their crowns.
How to Apply Winter Mulch
If you are going to mulch plants in the fall, wait to do so until the soil has frozen hard. Avoid the temptation to smother mulch over the garden at the first light frost. Generally speaking (and this will vary depending on a number of factors), apply a layer that is about 4 inches thick once the ground freezes hard. Mulching too early, while the ground is still soft and wet, can encourage fungal spores and offer pests a place to nest over the winter.
When to Remove Winter Mulch
Some gardeners wonder, "When do I remove mulch from perennials in spring? Or should I just let them push up through it on their own?" There is no universal right or wrong answer to this question, but in general, if you have mulched with a coarse, thick layer of leaves, straw, or another dry material with the aim of protecting the roots, this mulch should be removed as soon as you are confident that "spring is here for good."
With some perennials (daylilies, for example) you can wait until the plants begin to confidently push up through the mulch with new shoots. Other plants, though, can be smothered and refuse to sprout at all unless you remove the mulch to give them air and light. To a large degree, getting the timing right for mulch removal requires you to be observant regarding your plants and the weather conditions where you live. If your memory is not good, it helps to keep a garden journal from year to year.
As spring begins to settle in, start checking the ground beneath the mulch. Leave the mulch in place as long as there are still icy spots in the soil. But if the ground is clearly thawed, it is time to remove the mulch and let your perennials breathe. Leaving mulch in place too long will encourage mold and other pathogens.
Once the perennials have achieved a bit of height and soil temperatures have fully warmed, then you can re-apply a thinner layer mulch around them to suppress weeds, moderate soil temperatures, and preserve soil moisture. Shredded leaves make for an excellent summer mulch because they are light and fluffy, and they break down readily to release valuable nutrients into the soil. An even better warm-season mulch is partially decayed compost, which will begin feeding your plants immediately.