nce temperatures approach 32 degrees F, scattered frosts occur, especially in low-lying parts of the garden. Frost on flowering plants leads to tissue damage, depending on the plant species and extent or duration of the freezing temperatures. Petunias, pansies and geraniums all demonstrate different frost tolerances. Of the three, petunias are least able to survive frost. Pansies readily survive and continue blooming when frosts occur. Geraniums lose leaves during frost, and suffer varying degrees of damage.
Frost is frozen dew on plant leaves, stems and flowers.
Petunias are tropical plants with relatively thin leaves, petals and stems. They lack tolerance for freezing temperatures. Look for wilted, collapsed or fully brown tissues on petunias when frost occurs. Geraniums, native to subtropical southern Africa, survive light frosts, but may lose flowers and uppermost leaves. The thick stems survive light frost. If temperatures rebound afterward, new buds and leaves sprout from the unharmed stems. Frost damage may not develop for 24 to 48 hours after the event. Frost doesn’t faze pansies.
Petunias sustain extensive damage once 32 F is reached.
When temperatures threaten to reach 32 F or below, focus attention foremost on petunias and then geraniums. Cover petunias with an old bed sheet or other cloth. The covering insulates the tender plants by trapping heat radiated from the soil, avoiding frost damage on plant tissues. Avoid using plastic sheeting, as it insulates poorly compared to cloth. If it’s windy during the night, use blocks or other items to weigh down the cloth so it’s not blown off the tender plants. Consider relocating plants in containers temporarily indoors overnight when frost threatens. Cover large geranium specimens with cloth or a large cardboard box to protect it from cold.
The geranium also suffers leaf loss or stem dieback in subfreezing temperatures.
Plants with moist roots and soil recover from cold temperatures. The day before a frost is forecast, water the roots and soil around the petunias, geraniums and pansies but keep the leaves dry. The soil moisture helps hold heat overnight to be released under the fabric covering to keep plants warm. Plants stressed by dry roots heading into a cold night sustain tissue damage more quickly and often more severely. Once the morning arrives, remove all protective coverings so plants again have sunlight and warm up.
Since pansies tolerate and readily survive frosts and nighttime temperatures hovering around 32 F, plant them in the fall, late winter or early spring. Delay planting petunias and geraniums until no further threat of frost occurs in your region. The local cooperative extension office, botanical garden or garden center staff can tell you when frosts occur. Wait until after the last expected frost date in spring to place petunias and geraniums outdoors. When the first frost threatens in fall, any petunias or geraniums need to be covered to avoid frost damage.