A winter mulching mistake could cost you your $200 investment in lovely perennials. Although mulch protects some plants from harsh freeze-thaw cycles, it can literally suffocate other plants that require wintertime air circulation in order to survive.
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Check PriceUnderstanding the specific needs of each perennial can mean the difference between the survival or death of the plant. Some perennials need protection while others will die if covered with mulch too soon. I'm going to show you which plants can be mulched and when, plus the specific methods that differentiate successful gardeners from those who will be mourning the death of their plants next spring.
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The Perennials That Need Winter Mulching
Hostas are #1 because their shallow crowns are at soil level where they are susceptible to freeze damage. Protective domes can be created over the crowns by applying 3-4 inches of shredded leaves or straw following the first hard frost.
Daylilies in zones 5 and colder need mulch magic around their roots. Aged compost 2-3 inches thick prevents ground 'heaving' and breaks root connections from the frost. Their fleshy rooting systems will expand and contract with temperature changes.
Coral Bells (Heuchera), have shallow roots that over time tend to push up out of the soil. Winter mulch will keep these exposed crowns from freezing completely, but wait until the ground begins to freeze before applying. Mulching too early can invite crown rot.
Chrysanthemums require varying mulch treatment depending on the type. Hardy mums benefit from a light mulch treatment after stems are cut to 6 inches, whereas tender variety chrysanthemums require heavy 6-inch mulch protection in zone 6 and colder.
When is the best time to buy and spread mulch?
Most gardeners are getting it wrong by mulching too early. Winter mulch should be put on **AFTER** the ground starts to freeze. Mulching too early keeps soil warmer for longer, preventing plants from going into dormancy, and putting them at greater risk of winter damage.
Peonies are a perfect example. If you mulch them too early, the soil-dwelling fungi will flourish in the warm, moist conditions. Instead, wait until late November or December to apply just 2 inches – more invites fungal problems that will kill these long-lived beauties.
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Since Astilbes have shallow and fibrous roots, they need mulch to retain moisture because their roots can dry out quickly in frozen soils. Apply a mulch layer of about 4 inches deep after several consecutive nights below 25 degrees Fahrenheit to avoid encouraging premature spring growth.
The Perennials That Mulch Kills
Some perennials die if they are covered with mulched because they need circulation from winter winds in order to survive. Winter mulching is detrimental to Mediterranean herbs such as lavender, rosemary, and sage. These plants need free draining soil and their tops to be exposed to air. Covering them with mulch will keep soil moist, increasing the fungal conditions that will rot these drought-adapted plants.
Crown rot can affect sedum and other succulents because their leaves and stems store water and moisture-retaining mulch can hold too much moisture at the base of the plants. Although these plants can survive very cold temperatures, they can die rapidly when conditions are too wet.
Ornamental grasses should have their top growth left throughout the winter for insulation. Leaving 12 inches of stem while mulching around the base allows wind protection and does not create moisture problems.
Effective Mulch Materials
Shredded leaves are the best insulating layer because they never mat down solid. The best leaves to use are oak and maple since they also breakdown slowly and are not blown away by winter storms.
Straw or salt hay insulates the roots and provides good air circulation. Do not use fresh grass clippings, as they form layers that mat and do not permit air circulation; and do not use regular hay because it has seeds that will germinate and create weed problems.
While decomposing, pine needles insulate and gradually cause soil to become more acidic. This makes them great for use with astilbes and coral bells!
The Critical Depth Formula
How deep your mulch is can be the determining factor when it comes to winter protection. Perennials that have shallow roots need 3-4 inches of mulch to help insulate them from drastic changes in temperature. On the other hand, deep-rooted plants only need 2-3 inches of mulch to help prevent soil from heaving. If you have crown-sensitive plants such as delphiniums, then you will need to pull back 6 inches of mulch from the center of the plant to prevent rot, but you will still be protecting the root zone.
The ideal amount for most perennials is three inches of loose organic mulch. It should be placed in a donut shape – covering their root zone and leaving space around the plant's crown for airflow.
The cone flowers (Echinacea) show us the right way to apply mulch. Around each cone flower, apply 3 inches of shredded leaves within a 2-foot radius, keeping at least 4 inches away from the crown (to avoid root rot which is the leading cause of dead coneflowers over winter).
Spring Removal: The Moment of Truth
The timing of spring mulch removal is important for winter mulching to be successful. As soil temperature increases, you should remove mulch gradually, not all at once. Coral bells and astilbes must have mulch removed at the earliest signs of warming soil. However, daylilies and hostas can tolerate mulch until new growth emerges.
Focus on soil temperatures instead of air temperatures. If soil stays above 40°F, begin pulling mulch back about 2-3 inches. This will allow air circulation while still insulating roots during late-winter temperature changes.
Your long-term investments deserve protection that works WITH each plant's natural survival strategy, not against it. If you learn the rules of effective mulching, you'll be among the few gardeners whose perennials grow back bigger and stronger each spring while those around you are left to guess what went wrong.
