The decision to cover a garden for winter relies on whether the potential damage from cold, wind, and the freeze-thaw cycle outweighs the risks of improper covering. Winter protection mitigates the physical stresses extreme weather places on plants, especially their root systems and exposed structures. The goal is not to keep the garden warm, but to stabilize the environment and prevent desiccation and temperature shock. This proactive step can mean the difference between plant survival and loss, setting the stage for a healthy return in spring.
Determining If Your Garden Needs Winter Protection
The need for garden covering is determined by a few key environmental and horticultural factors. The primary consideration is your garden’s USDA Plant Hardiness Zone, based on the average annual minimum winter temperature in your region. Plants rated for a zone colder than yours generally do not require covering, while tender perennials or plants rated for a warmer zone often need significant protection to survive.
A plant’s cold tolerance also depends on its specific type; hardy annuals may be left alone, but broadleaf evergreens and young, newly planted trees are vulnerable to winter burn and wind damage. Beyond the general zone, local variations create a microclimate. Structures, fences, and south-facing walls can absorb heat, making a localized area up to a full zone warmer, while low-lying areas may be colder frost pockets where protection is necessary.
Insulating the Soil and Root Systems
Protecting the soil and the dormant root zone addresses the damaging freeze-thaw cycle. This cycle, known as frost heaving, occurs when repeated freezing and thawing physically pushes shallow-rooted plants out of the ground, exposing and damaging the roots. A thick layer of organic mulch acts as an insulating blanket, moderating the soil temperature and preventing the constant fluctuation that leads to heaving.
Suitable organic materials for this purpose include:
- Shredded leaves
- Straw
- Wood chips
- Pine needles
These materials are applied once the soil is cool or lightly frozen. Applying the mulch too early can trap warmth, which may encourage plants to break dormancy prematurely. The recommended depth for this insulating layer is between two to four inches, spread over the root zone but kept a few inches away from the crown or stem to prevent rot and discourage pests. This protective layer also helps retain soil moisture, which is important because well-hydrated soil holds heat better than dry soil, guarding against cold damage.
Protecting Above-Ground Plant Structures
Above-ground plant structures, particularly the foliage and stems of evergreens and shrubs, require protection from windburn, heavy snow load, and direct frost. Harsh, drying winter winds can desiccate evergreen foliage, causing winter burn, where leaves turn brown due to moisture loss that frozen roots cannot replenish. Covering materials must be breathable to allow for air circulation and prevent excessive moisture buildup, which promotes fungal diseases.
Floating row covers, typically lightweight, permeable fabric, create a warmer microclimate around vegetables and tender perennials. These covers can raise the temperature by six to eight degrees Fahrenheit by trapping heat radiating from the soil and shielding plants from cold wind. For larger shrubs and young trees, burlap is an effective material, often erected using wooden stakes to form a screen or tent. Ensure the burlap does not directly touch the foliage, as frozen, wet material can damage the plant tissue. Cold frames, small, unheated greenhouses with transparent lids, offer robust protection for entire beds, allowing for winter harvesting of cold-tolerant greens. These structures require ventilation on sunny days to prevent internal temperatures from scorching the plants.
Timing and Avoiding Common Winter Covering Errors
The timing of winter protection involves waiting until plants have properly entered dormancy. A common error is covering plants too early, which insulates the ground and foliage from the cold exposure needed for hardening off. Covers should be applied once nighttime temperatures consistently drop below freezing, and the ground has begun to cool or lightly freeze.
When applying covers, avoid using non-breathable plastic sheeting directly on foliage, as it traps moisture and heat, leading to rot or “cooking” the plant on sunny days. Any covering material, such as a frost blanket or burlap, must be secured to the ground to effectively trap the earth’s residual heat. In the spring, covers must be removed gradually as the weather moderates, ideally before the plant buds begin to swell and turn green. Leaving covers on too long can hinder new growth or expose the plants to overheating.