University of Vermont Extension
Department of Plant and Soil Science
MULCH TO PREVENT WINTER INJURY
Dr. Leonard Perry, Extension
University of Vermont
Mulch is a standard form of winter protection for many
shallow-rooted plants, and young or tender perennials. While in
summer it is
effective in retaining soil moisture, preventing erosion, and
weeds, in winter it acts as insulation for the soil and plant roots.
to mulch landscape plants and evergreen shrubs in cold climates may
winter root injury. Alternate thawing and freezing of unmulched
temperatures warm during the day and drop at night may cause “frost
the spring. Freezing and thawing occurs mainly in the fall and
and in fall may keep perennials from hardening properly or cause
retains soil heat and keeps it from escaping, which in turn protects
systems. Initially, the soil under mulch does not freeze as deeply,
plants will continue to absorb water. However, the soil will freeze
so in the spring mulch doesn't help much with desiccation (drying
foliage. Mulch helps evergreen shrubs go into winter with more
so less damage will occur from drying out come spring.
snow cover can provide good protection for plants, it's not possible
when, or how much, snow, will fall each year. So, your best bet is
spread pine needles, straw, chopped leaves, wood chips, shredded
bark, or other
organic materials around the base of your landscape plants. All are
effective, although availability and cost may influence your
decision as to
what to use. For roses, which are
usually grafted, make sure and cover the less hardy graft union (the
area at the base) with either compost or soil.
mind that you should avoid pine needles if you don't want acidic
Avoid “deals” on weedy hay, using weed-free straw instead. Fresh
chips might take up nitrogen when decomposing, so it's better if
already composted, or use chopped or milled pine bark. The coarser
material, the longer it will last before needing to be reapplied.
of the material you select, the rule of thumb for winter mulches is
to apply a
two or three-inch layer. Adding more not only wastes money but also
smother the root system, and possibly kill the plant, especially
perennials such as yarrow or bee balm. By adding only a couple
mulch in the fall, you can prevent soil temperatures from
fluctuating wildly. So
if air temperatures drop to below freezing overnight, soil
remain at 40 degrees or above and roots will continue to grow.
Pine bark and pine needles will pack
down very little, so you probably won't need to add any more as the
progresses. If you use straw such as around strawberries, start
four- to six-inch layer, as it will pack down to the desired final
depth of two
to three inches. You may need to add more throughout the winter to
that three-inch depth, especially if snow cover is sparse or
the site is windy.
applying mulch around woody plants, gently pull it away from their
trunks. Mulching too close to the trunk
may provide optimum conditions for the development of cankers on the
trunk or stems. If this occurs, the damage cannot be reversed, and
plants die in a matter of seasons.
“volcano” mound of mulch around trunks also will provide a home for
can chew the bark and “girdle” the stems, resulting in the plant's
Girdle means to remove a band of bark and living tissue from around
a tree or
plant. This usually kills the plant as
it has no way without this conductive tissue to move nutrients and
between tops and roots.
fall-transplanted trees and landscape plants as soon as you've
because the mulch is needed to keep the soil warmer and
moister for as long as possible before the ground freezes to help
the roots become
established. For these plants, you might want to use mouse guards
trunks to prevent injury. (Tree wrapping
materials and plastic guards also protect against sunscald in
winter. This is caused by the rapid warming and then
freezing of bark on sunny winter days and cold nights.)
established landscape plants, although many gardening books
recommend mulching when
the soil cools or is slightly frozen, I prefer to mulch earlier in
fall. This helps retain soil warmth, so roots continue to grow for
longer period, and plants can absorb more moisture to head into
better shape against drying out. If
temperatures drop really low overnight, such as to 10 degrees, soils
remain just below freezing or around it, so less-hardened roots will
damage in the fall as they otherwise might without mulch.
of thumb with strawberries, which works for many other perennials,
is to mulch
when there have been three consecutive days of soil temperatures
degrees (F). These might occur in
mid-November in northern New England, and late-November in southern
England. If temperatures remain low in
the fall for a few days, soil temperatures will drop more slowly on
soils, so wild swings in temperature are less apt to occur.
in spring, you will need to remove the mulch from herbaceous
pull it away as soon as snow disappears and plants start to grow.
woody plants, you can leave it on for some weed control in summer.