As soon as temperatures drop to freezing (32 degrees Fahrenheit), petunias die. They can grow perennially only in the warmest corners of the United States (USDA zones 10 and 11). As a result, many gardeners treat petunias (Pentunia [group]) like annuals and replace them every year. However, you can ensure your petunias grow back after the winter by bringing them indoors or propagating new plants.
Bringing Petunias Indoors
If your petunias are growing in pots, bring them inside once the low temperatures start to dip below 40 degrees. This is when a petunia's delicate foliage and flowers may experience damage or injury. Once the potted petunias are inside, you have to decide whether you want them to go dormant or continue flowering throughout the winter.
Dormancy is a natural period of rest that allow perennials to store nutrients and conserve energy in preparation for spring growth. Perennials will become dormant when nights get longer in the winter. It's possible to force perennials like petunias to avoid dormancy and continue to flower and grow, but they'll require extra care and a grow light.
Caring for Dormant Petunias
Once your petunia is inside, place it in front of your sunniest window. Petunias still need plenty of light until they become dormant. In fact, they might go into shock and die if they suddenly receive too little light. If you don't have a suitable window for your petunias, keep them under a grow light until sunset each day.
Do not prune or fertilize the plant again until spring. Gradually start watering it less each week. When the leaves have dropped from the plant, cease watering altogether and store your dormant petunias in a cool, dark place.
In about mid-February, you can place them in front of the sunny window again and give them a drink of water. To help them transition to being outdoors full time, harden them off as if they were seedlings.
Caring for Nondormant Petunias
Caring for nondormant petunias in winter is just like caring for petunias any other time of year. Preventing dormancy is the tricky part.
Sufficient sunlight is the critical factor that prevents petunias from becoming dormant. You'll need to invest in a grow light so that the petunias are exposed to light for eight to 12 hours each day. Continue to water and fertilize the petunias as if it's the middle of summer. Feel free to prune the petunias as needed, as this can stimulate new growth and prevent dormancy.
Plants sometimes need supplemental humidity when grown indoors, but petunias actually prefer low humidity. If you run a humidifier for other indoor plants, try to place your petunias in a separate room or away from the humidifier on the other side of the room.
Propagating Petunias for Fresh Growth
If you grow petunias in the ground rather than in pots, you can still enjoy fresh (and free) petunia plants each year. Before your petunias suffer frost damage, cut the stems about 3 to 4 inches down from the central flower bud. Then, remove all buds or flowers from your cuttings.
Strip the lower half of the stem of all leaves. Dip the bare stem in water and then in powdered rooting hormone. Then, plant the cutting in potting soil, water thoroughly and place it under a grow light or in front of a sunny window. New roots will develop throughout winter, giving you a new selection of petunias to plant in the spring.