Gardeners can prevent winter damage in their landscapes, but first they need to recognize which plants are at greatest risk. Plant damage during winter is often caused by frost heaving, desiccation or cold temperature injury to roots and crowns.
Plants most at risk of winter injury include:
- late fall-planted trees, shrubs and perennials,
- broadleaf evergreens, and
- plants in containers.
Understanding Winter Damage
Late Season Plantings & Frost Heaving
Fall-planted trees, shrubs and perennials haven’t had much time to establish their root system, which puts them at risk of frost heaving. Frost heaving occurs when soil alternately freezes and thaws. Plants with limited rooting can be pushed up out of the ground so the crown or roots are directly exposed to cold winter temperatures. Exposed roots dry out and die.
Winter desiccation occurs when the amount of water lost by a plant’s foliage exceeds the amount picked up by the roots. All plants lose water during normal metabolic processes and, even though these processes are much slower in winter, they still occur. Broadleaf evergreens, such as boxwood, mahonia and holly, hold onto their foliage all season making them very prone to dormant-season desiccation due to the higher amounts of water lost through their foliage during the winter months. But winter desiccation can also be seen in needle evergreens and deciduous plants.
Contributing factors include the following
- Plants not well watered during fall, which go into winter with low amounts of internal water.
- Warm, sunny days or windy conditions which increases plant water losses. Wind affects often result in more severe plant damage or death on the side of the tree facing the prevailing wind.
- Frozen soil or low soil moisture due to dry winter conditions. Plant roots are unable to pick up enough water to meet plant needs.
Symptoms in evergreen needles include browning from the tip, either partially or the entire needle. But evergreen needles often hold their green color until warmer temperatures arrive in spring, when the chlorophyll molecules in the dead needle sections finally loses it’s color. This delays the onset of desiccation symptoms, making gardeners think the problem occurred recently when in reality the damage could have happened or been accumulating all winter.
Cold Temperature Injury in Container Plants
Woody plants or perennials grown in containers are another group of plants particularly at risk of winter damage. Root systems of container plants are elevated above ground level and therefore are highly exposed to cold winter temperatures. Because these plant’s root systems don’t benefit from the soil’s temperature modifying effects, like ground plantings do, the planting media around their roots often gets much colder than normal winter soil temperature.
For example, if air temperature is 15° F, the soil around a container plant’s roots will be very close to air temperature. Most tree and shrub roots are killed at temperatures at or below 0 to +10° F.
Protecting Your Plants
Protection for In-Ground Plants
Apply a 4-inch layer of loose organic mulch in fall around the crown and over the root system of susceptible plants. Suitable mulches include wood chips, pine straw, evergreen boughs, straw, clean hay or any loose mulch that will not compact heavily.
Plant protection provided by mulch includes the following
- Holds moisture in the soil and minimizes the effects of dry winter conditions.
- Reduces the potential for frost heaving by keeping the soil cold and reducing soil temperature swings.
Winter watering during dry periods, when little snow cover is present and the ground is not frozen, is beneficial.
Protection for Container Plantings
Extra care is necessary for container plantings to provide adequate winter protection.
- Ideally, containerized plants should be moved to an unheated garage for the winter. Winter watering is necessary to keep plants roots from desiccating. One good watering every time it snows or rains is a good place to start – and the rain or snow can be your reminder! But check the soil periodically and water whenever it gets dry.
- Alternatively, if the container cannot be moved, protect plant stems, crown and roots by wrapping the container with burlap and stuffing it full of straw. This may provide a few degrees temperature buffering, but plant jury can still occur if winter weather conditions are severe.