Similar to the way motorists are urged to "winterize" their cars at this time of the year, gardeners should do the same for their more tender garden plants. Many of our perennial garden flowers have been selected for characteristics such as flower size, color and form, rather than for their ability to endure severe winter conditions. Additionally, in an attempt to have something unusual in their landscape, gardeners often attempt to coax perennial plants further north than their zone of natural hardiness. While nature promotes the survival of the fittest, many gardeners are upset when nature has not chosen their plants for survival. Therefore, winter protection becomes important to ensure that certain plants will survive our midwestern winters.
One way to protect our plants is through proper site selection. Plants that have been placed in sites poorly suited to their needs are more susceptible to winter damage. Too much shade for some, too little shade for others, poor soil drainage, or other environmental problems can lead to winter damage.
A goodly number of our garden plants would disappear, if grown in the wild without any help. In other cases, nature provides a protective cover of leaves for tender plants growing near trees, thus allowing them to survive. In the garden, clean appearance is most often desired. Thus, leaves usually are removed. Additionally, leaves allowed to remain as mulch often will not stay where we want them and will blow throughout the yard.
Mulches provide the best winter protection for most flowering perennial plants. As soon as growth has ceased in late fall and temperatures are cold enough for slight soil freezing, mulches should be applied. Not all plants need covering; durable perennials such as peony, daylily and iris will survive without it. Nevertheless, mulches still are beneficial for moisture retention and weed control.
The main purpose of a winter covering on flowering perennial plants or marginal woody plants is not to conserve soil heat. Instead, coverings prevent abrupt, wide temperature fluctuations at the base of the plant and in the upper root zone. For younger plants that might not yet be well established, an important function of mulch comes in late winter and early spring. During that time, unprotected grounds alternately freezes and thaws.
As soil freezes in expands; when soil thaws in contracts. When this happens, young plants or perennials with fleshy root systems may be heaved out of the ground so roots become exposed and dry out. These plants with exposed roots are killed or weakened greatly. This problem is very common with shallow-rooted species such as chrysanthemum, Shasta daisy or certain small bulbs such as crocus. The problem of frost heaving is most common in heavy clay soils which have not been improved with organic matter. It normally is not a problem in loose, sandy soils.
Pine needles, wood chips, sawdust, leaf mold and compost are all useful materials for mulching for winter protection. Most of these items are too dense to put directly over the crowns of herbaceous perennials that maintain green growth during the winter such as oriental poppy and tritoma. Straw or evergreen branches placed directly over the latter type of plants provide some protection, yet allow light and air to penetrate. Avoid the use of hay, which often is laden with weed seeds.
Other, heavier materials might be placed on the soil around tender perennials. If they can be kept in place, stiff leaves such as those from oak trees make a good mulch because of their slow rate of decomposition. Soft leaves, such as maple or ash, often become too soggy during the course of winter and can suffocate small shoots.
Coverings of loose materials for winter protection should be at least two inches deep. Check the mulch and condition of the plants it covers on mild winter days. Even after the holidays, it is not too late to add an additional covering of leftover holiday greens to tender plants that need them.
Finally, in the spring when perennials break dormancy, mulches need to be pulled back. The objective is to expose the plant's crown (growing point) to sunlight to promote new growth. Surrounding the base of the plant with the mulch that was protecting its crown, is a good way to deter weed growth and conserve water during the coming growing season.
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REVISED: December 8, 2020