Phlox is a popular garden flower, cherished for its abundant, colorful blooms that span from spring through fall. This genus includes nearly 70 different species, ranging from low-growing groundcovers to tall, upright border plants. A common concern for gardeners is whether Phlox can survive a sudden drop in temperature. The plant’s ability to withstand freezing weather depends entirely on the specific variety being grown and the intensity of the cold event. Understanding the difference between plant types is the first step in determining how well a Phlox will endure an overnight frost.
The Critical Distinction: Annual Versus Perennial Phlox
The survival rate of Phlox during a freeze is fundamentally tied to its life cycle, separating the genus into annual and perennial categories. Perennial varieties, such as Garden Phlox (P. paniculata) and Creeping Phlox (P. subulata), are generally cold-hardy. Depending on the species, these plants are adapted to survive winter in USDA Hardiness Zones 3 to 9.
These hardy plants prepare for winter by entering dormancy, where the above-ground stems and foliage die back after the first hard frost. Although the aerial parts may appear dead, the root crown and underground structures remain alive, waiting for spring to regenerate new growth. This adaptation allows the plant to endure prolonged periods of freezing temperatures.
Annual Phlox, primarily Phlox drummondii, behave differently in cold weather. Since these varieties complete their entire life cycle within one growing season, they are not equipped to enter true dormancy. Even a light frost can kill the entire plant, meaning they must be replanted each year after the last expected freeze has passed.
The Science of Cold Damage in Phlox
Frost damage is caused by a physical process that occurs at the cellular level, not just the cold temperature itself. Healthy plant cells are full of water, giving the tissue firmness. When the temperature drops below freezing, ice crystals begin to form in the extracellular space (outside the cells).
This ice formation makes the surrounding solution more concentrated, causing an osmotic imbalance. Water is then drawn out of the plant cells into the extracellular space, effectively dehydrating the cells. This severe cellular dehydration damages the cell membranes and causes the cell walls to collapse, resulting in the characteristic water-soaked or wilted appearance of frostbite.
If the temperature drop is rapid and severe, ice crystals can form directly inside the cell structure, a process known as intracellular freezing. Intracellular ice formation is usually fatal because the expanding crystals physically rupture the cell walls and membranes. Perennial Phlox species possess mechanisms that increase their resistance to this internal freezing, making them far more cold-tolerant than annuals.
Protecting Established Phlox from Cold Snaps
When a sudden cold snap is forecast, gardeners can take several steps to mitigate potential damage to established perennials and newly planted stock.
Watering
One effective preventative action is to ensure the soil around the plants is thoroughly watered a day or two before the freezing event. Moist soil holds significantly more heat than dry soil. This stored thermal energy radiates upward during the night, providing a slight warming effect to the air immediately surrounding the plants.
Mulching
Applying a layer of insulating material is beneficial, especially for protecting the roots of recently planted perennial Phlox. A two to four-inch layer of organic mulch, such as shredded bark or straw, should be placed over the root zone and around the crown. This mulch helps stabilize the soil temperature and minimizes the damaging cycle of freezing and thawing, which can cause plants to heave out of the ground.
Covering
For tender new growth or during a deep freeze, covering the plants offers direct protection from cold air. Breathable fabrics, including frost blankets, old bed sheets, or burlap, are the preferred materials for temporary insulation. The cover should be secured to the ground to trap residual warmth and removed promptly the following morning once temperatures rise above freezing. Leaving covers on too long can cause the plant to overheat or risk damaging the foliage if plastic materials touch the tissue.
Recovery and Care After Frost Damage
Once the threat of freezing temperatures has passed, the recovery phase begins with assessing the extent of the damage. Tissues injured by frost typically display a blackened, scorched, or soft, water-soaked appearance. For established perennial Phlox, it is important to be patient and avoid hasty decisions about the plant’s survival.
The damaged foliage should not be pruned immediately, as this dead material provides insulation against any unexpected late frosts. Waiting allows the plant to begin pushing out new growth from the undamaged root crown, which signals recovery. Once healthy new growth is clearly visible, the dead stems and leaves can be cleanly cut back. Removing the dead tissue promotes better air circulation and reduces the possibility of fungal diseases.