Fall Article
MULCH TO PREVENT WINTER INJURY
By Dr. Leonard Perry. Extension Professor
Mulch is a standard form of winter protection for many shallow-rooted plants. 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 may lead to serious 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 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, we will get each year. So, your best bet is to spread pine needles, straw, chopped leaves, wood chips, corn cobs, 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.
Keep in mind that you should avoid pine needles if you don't want acidic soil. Avoid weedy hay, using weed-free straw instead. Wood chips might take up lots of nitrogen when decomposed, so it's better if they are already composted, or use chopped or milled pine bark.
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. Some plants can withstand such abuse, but overmulching is often a leading cause of death to azaleas and rhododendrons, as well as ash, linden, maple, and narrow-leaf evergreen trees.
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, 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.
After applying the mulch, gently pull it away from the stems or trunks of the plants. Mulching too close to the trunk may provide optimum conditions for the development of cankers on the lower trunk or stems of woody plants. If this occurs, the damage cannot be reversed, and the plants die in a matter of seasons. It 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 cambium from the circumference of a tree or plant, which usually kills it.
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.
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. Then in spring you will need to remove the mulch from perennials or pull it away soon as snow goes and plants start to grow. For woody plants you can leave it on.
For years I've monitored soil temperatures under various mulches and of soil without mulch. I have found that by adding only a couple inches of bark mulch in the fall, you can prevent soil temperatures from moderating or fluctuating wildly by as much as 10 degrees F. So if air temperatures drop to below freezing overnight, soil temperatures might remain at 40 degrees F or above and roots will continue to grow.
If temperatures drop really low, like 10 degrees F overnight, soils might remain just below freezing or around it, so less hardened roots will not suffer damage in the fall as they otherwise would. Mulch is key in fall as perennials harden as the season progresses. So while they might take 20-degree F soil temperatures in midwinter, if such occurred in mid-fall it might kill them as they aren't fully hardened. That's why it is so critical to mulch early. 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.