Creeping phlox (Phlox stolonifera) can survive winter lows in U.S. Department of Agriculture plant hardiness zones 5 to 9. Moss phlox (Phlox subulata) and sand phlox (Phlox bifida), also creeping phlox species, can survive even colder winters. Moss phlox is hardy in USDA zones 3 to 9, while sand phlox is hardy in USDA zones 4 to 8. Late spring frost and winter weather can do some damage, though. Provide the plants with protection from these weather extremes.
Creeping phlox plants are not as sensitive to frost as many other plants, but temperatures of 20 degrees Fahrenheit for a few nights after a period of warm spring weather will damage them and temperatures below 40 F should prompt protective measures. When damaged, the leaves may wilt or look scorched with dry, brown areas. Flower buds can be killed or damaged so that the petals appear irregular and distorted. Creeping phlox plants recover quickly, but flowering could be reduced.
Fall frost is not a problem for creeping phlox plants. They have finished blooming by then and the foliage is mature enough to handle the cold until true winter temperatures set in.
Tightly closed flower buds may bloom normally after a damaging frost.
Monitor weather reports and cover creeping phlox when temperatures below 40 F are predicted in the spring. Cover plants each evening, and uncover them when temperatures warm the next day.
To cover creeping phlox, spread several layers of newspapers, a garbage bag or a sheet over them. Anchor the covering with rocks so that it is flush with the soil and does not blow off. Overturned buckets and cardboard boxes can also be used to cover creeping phlox.
Winter Weather Damage
Frost heaving and winter drying are big problems for creeping phlox plants. When the soil thaws and freezes throughout the winter and spring, their shallow roots can be heaved, or pushed up, out of the soil and exposed to the elements. Creeping phlox plants are especially prone to heaving during the first winter or two after planting. Their shallow roots can take a year or two to get a firm hold in the soil.
Winter drying occurs when creeping phlox foliage is exposed to cold winter winds and direct sunlight. Even though they are evergreen, the plants go dormant in the winter and can’t absorb moisture to keep the leaves hydrated.
To protect creeping phlox against heaving and drying, pile 3 inches of shredded pine bark mulch, straw or pine needles over the tops of plants after two or three hard frosts in the fall. This helps moderate soil temperatures, reducing the freeze-thaw cycle severity, and protect the foliage from harsh, cold, drying wind and sun.
Check the creeping phlox on warm days throughout the winter and early spring to see whether the roots have been pushed up. If they have, gently press the plant back down and firm the soil or spread a few inches of fresh soil over the roots and firm it by hand. Remove the protective covering in the spring when there is no longer any risk of the soil freezing.
Shoveling snow or placing needle-leaved evergreen branches over creeping phlox plants also helps prevent winter wind and sun damage to the leaves.