Mulching is a great way to protect garden plants and help them survive until spring.
By December, we tend to think our work is done and the garden’s been put to bed until spring, but winter mulch is just reaching its season.
We’re accustomed to summer mulches, used to suppress weed growth and hold moisture in the soil. A layer of aged wood chips, shredded bark, shredded leaves or compost 2 to 3 inches deep generally does the job.
Shallow-rooted plants such as Japanese maples and rhododendrons benefit from this same mulching through the winter, which continues to keep the soil and roots moist, preventing desiccation. In winter weeds aren’t a problem, but the roots are further protected from extremes of temperature.
Hardy plants can tolerate having their roots frozen.
The risk lies in cycles of repeated freezing and thawing, which will cause the roots of vulnerable plants to become mushy, much like food that has been in and out of the freezer too many times — the cell structure is damaged beyond repair. The soil is a great moderator of the extremes, and mulching enhances the effect.
Late-planted perennials are at risk for frost heave.
Because the plant hasn’t had time to develop a good network of roots to anchor it into the soil, the freeze/thaw cycles can push those new plants up out of the ground, similar to the damage we see on road surfaces during thaws. The exposed roots and plant crowns are then damaged by wind and further temperature swings.
For these plants, it is best to wait until the ground has frozen, then apply a loose, coarse-textured mulch that will not mat down under the weight of snow.
This will hold the ground frozen, keeping the plant in deep freeze until the weather warms in spring.
Grafted hybrid roses and semi-woody plants such as butterfly bush (Buddleia), lavender, Russian sage (Perovskia) and bluemist spirea (Caryopteris) benefit from a 4- to 6-inch layer of coarse loose mulch or cut evergreen boughs. This will protect the graft union on the roses, preserving the premium hybrid that was grafted onto the root stock. The other semi-woody plants tend to sprout their new spring growth from buds at the base of the plant, rather than the root, and the mulch helps more of those buds to survive until spring.
Gardeners have a natural tendency to push the limits of plants and our climate, especially when tempted by the new and exotic. We are considered USDA zone 5A to 6B (depending on your address), with a minimum low temperature of 10 to 15 degrees below zero.
In winters with good snow cover that starts before the ground freezes and lasts until spring thaw, I have had plants survive that are rated to Zone 7, minimum zero degrees.
By taking advantage of protected corners or other microclimates, we can often overwinter plants that are not considered hardy. A thick layer of coarse mulch before the ground freezes can mimic the effect of a blanket of snow, and for plants that need good soil drainage, can actually be more effective than the snow.
Evergreen boughs are very effective as winter mulch, whether before or after the ground has frozen.
This is an excellent — and earth-friendly — use for fresh cut Christmas trees after the holidays. Simply cut branches from the main trunk and place a few over each of your late-planted perennials or other treasures that you’re hoping to carry through the winter. If you need extra, or have an artificial tree, offer to rescue your neighbors’ castoffs.