Dr. Leonard Perry, Extension Professor
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 controlling weeds, in winter it acts as insulation for the soil and plant roots.
Failure to mulch landscape plants and evergreen shrubs in cold climates may lead to winter root injury. Alternate thawing and freezing of unmulched soil as temperatures warm during the day and drop at night may cause “frost heaving” in the spring. Freezing and thawing occurs mainly in the fall and spring, and in fall may keep perennials from hardening properly or cause injury to unhardened plants
Mulch retains soil heat and keeps it from escaping, which in turn protects root systems. Initially, the soil under mulch does not freeze as deeply, so plants will continue to absorb water. However, the soil will freeze eventually, so in the spring mulch doesn’t help much with desiccation (drying out) of foliage. Mulch helps evergreen shrubs go into winter with more moisture, so less damage will occur from drying out come spring.
While snow cover can provide good protection for plants, it’s not possible to predict when, or how much, snow, will fall each year. So, your best bet is to 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 swollen area at the base) with either compost or soil.
Keep in mind that you should avoid pine needles if you don’t want acidic soil. Avoid “deals” on weedy hay, using weed-free straw instead. Fresh wood chips might take up nitrogen when decomposing, so it’s better if they are already composted, or use chopped or milled pine bark. The coarser the material, the longer it will last before needing to be reapplied.
Regardless 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 may smother the root system, and possibly kill the plant, especially shallow-rooted perennials such as yarrow or bee balm. By adding only a couple inches of mulch in the fall, you can prevent soil temperatures from fluctuating wildly. So if air temperatures drop to below freezing overnight, soil temperatures might 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 winter progresses. If you use straw such as around strawberries, start with a 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 maintain that three-inch depth, especially if snow cover is sparse or nonexistent and the site is windy.
After applying mulch around woody plants, gently pull it away from their stems or trunks. Mulching too close to the trunk may provide optimum conditions for the development of cankers on the lower trunk or stems. If this occurs, the damage cannot be reversed, and the plants die in a matter of seasons.
A “volcano” mound of mulch around trunks also will provide a home for mice, which can chew the bark and “girdle” the stems, resulting in the plant’s death. 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 water between tops and roots.
Mulch fall-transplanted trees and landscape plants as soon as you’ve planted them. That’s 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 around 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.)
For established landscape plants, although many gardening books recommend mulching when the soil cools or is slightly frozen, I prefer to mulch earlier in the fall. This helps retain soil warmth, so roots continue to grow for a longer period, and plants can absorb more moisture to head into winter in better shape against drying out. If temperatures drop really low overnight, such as to 10 degrees, soils might remain just below freezing or around it, so less-hardened roots will not suffer damage in the fall as they otherwise might without mulch.
A rule of thumb with strawberries, which works for many other perennials, is to mulch when there have been three consecutive days of soil temperatures below 40 degrees (F). These might occur in mid-November in northern New England, and late-November in southern New England. If temperatures remain low in the fall for a few days, soil temperatures will drop more slowly on mulched soils, so wild swings in temperature are less apt to occur.
Then, in spring, you will need to remove the mulch from herbaceous perennials, or pull it away as soon as snow disappears and plants start to grow. For woody plants, you can leave it on for some weed control in summer.
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