Perennial plants, which live for more than two years, often possess natural mechanisms to survive cold temperatures. Whether a perennial requires protection from frost depends entirely on its inherent hardiness and the specific conditions of the cold event. Many common garden perennials are rated as “hardy,” meaning they have adapted to survive the typical winter low temperatures in a specific region. However, “tender” or newly planted perennials, or those exposed to unseasonably early or late freezes, may suffer damage without intervention.
How Cold Temperatures Affect Perennial Plants
Freezing temperatures threaten plant health through two main physical processes: cell damage and soil movement. When the temperature drops below freezing, water within the plant’s tissues forms ice crystals, particularly in the spaces outside the cells. This process draws water out of the cells, and the resulting ice crystals can physically puncture and rupture the cell membranes. Damaged tissue will initially appear dark green and water-soaked, eventually turning black and necrotic as the cells die.
Plants naturally prepare for cold through a process called cold acclimation, triggered by shorter days and cooler temperatures in the fall. This adaptation involves moving water out of vulnerable cells and concentrating internal sugars, which act as a form of natural antifreeze. A sudden, sharp freeze that occurs before the plant has fully acclimated can cause significant damage, even to species that are normally cold-tolerant.
The second major threat comes from the cycle of freezing and thawing, which causes a phenomenon known as frost heaving. As soil water freezes, it expands, pushing the soil and any shallowly rooted plants upward. When the soil thaws and settles again, the plant’s crown and roots can be left exposed above the ground, making them susceptible to drying winds and severe cold.
Long-Term Strategies for Winter Survival
Preparing perennials for winter involves proactive steps taken in late autumn. One of the most effective methods is the strategic application of an insulating layer of mulch. This organic material, such as straw or shredded leaves, must only be applied after the ground has fully frozen or after the first few hard frosts. Mulching too early can retain warmth, delaying the necessary dormancy period for the plant.
The mulch layer should be approximately two to four inches deep to effectively moderate the soil temperature and prevent damaging freeze-thaw cycles that cause heaving. It is important to keep the mulch material a few inches away from the plant’s crown, or the point where the stems meet the roots. Placing mulch directly against the crown can trap excessive moisture, leading to rot and encouraging pests.
Cutting back foliage in the fall is a selective process that depends on the plant type. Herbaceous perennials that have died back can be cut to the ground to remove potentially diseased or pest-ridden foliage, tidying the garden and reducing disease risk. However, plants with semi-woody stems or ornamental grasses should be left standing, as their existing foliage traps snow and provides a natural layer of insulation for the crown and roots.
Before the soil freezes solid, deep watering is essential. Roots in dry soil are much more vulnerable to freeze damage and desiccation during the winter. Watering thoroughly in late fall ensures the roots are hydrated, and moist soil retains several degrees more heat than dry soil, adding protection against sudden, deep cold.
Immediate Protection During Sudden Frost Events
Immediate protection is necessary when an unexpected cold snap threatens plants that are not yet dormant in the fall or have begun to emerge in the spring. The goal is to trap the radiant heat stored in the soil beneath a protective layer. Breathable materials like old sheets, blankets, or horticultural fleece are suitable for this purpose.
The cover should be placed over the plants before sunset to capture the day’s warmth and must be secured to the ground using rocks or soil to prevent heat from escaping. Ensure the covering material does not directly touch the foliage, as cold can transfer through the material and freeze the leaves on contact. Using stakes or tomato cages to create a temporary tent-like structure can maintain an insulating air pocket around the plant.
Potted perennials are at a higher risk of root damage because their roots lack the insulation of the surrounding ground. These containers should be moved to a protected location, such as against a warm, south-facing wall or into an unheated garage or shed. Wrapping the container itself with bubble wrap or burlap will provide an additional layer of insulation for the root ball.
All temporary covers must be removed promptly at sunrise, especially if the day is sunny, to prevent the plants from overheating and to allow for proper air circulation. A light watering of the soil in the late afternoon before a predicted frost can slightly raise the ground temperature, offering minor but helpful protection.