How to Overwinter Petunias

As soon as temperatures drop to freezing (32 degrees Fahrenheit), garden petunias die. They can grow perennially only in the warmest corners of the United States (USDA plant hardiness zones 10 and 11). As a result, most gardeners treat petunias (‌Petunia‌ x ‌hybrida‌) like annuals and replace them every year. However, you can ensure your petunias grow back after the winter by bringing them indoors, propagating new plants, or giving them special care outdoors if you are located in the warmer end of their climate zones.

The process differs depending on whether you want to allow your petunias to go dormant or try to force them to continue growing and maybe flowering through the colder seasons. Here's how to overwinter petunias so they'll come back in late spring with a bang.

If your petunias are growing in pots, bring them inside once the temperatures start to dip below 40 degrees. This is when these flowering plants may experience damage or injury. Once the potted plants are inside, decide whether you want them to go dormant or continue growing throughout the winter.

Dormancy is a natural period of rest that allows perennials to store nutrients and conserve energy in preparation for spring growth. Perennials become dormant when nights get longer in the winter. It's possible to force perennials like petunias to avoid dormancy and continue to grow, but they'll require extra care and a grow light.

Consider the difference between allowing your petunias to go dormant and forcing a nondormant state:

Petunias are technically perennials, so yes, they could come back every year, but they usually don't since most gardeners grow them as annuals (you'll typically see them gracing hanging baskets or window boxes). Even in the warm growing areas within USDA plant hardiness zones 10 to 11, they are a challenge to maintain indefinitely, as they become leggy and cease their heavy flowering.

Be aware, however, that petunias are short-lived perennials, meaning it's unlikely that your petunias will continue to flourish past three years or so. Long-lived perennials are defined as those that live three seasons or more, although experts differ on this definition since some perennials are so extremely long-lived — for example, peonies that can last for decades. Short-lived perennials, however, decline after just a few years, although they may self-seed.

You can prolong their longevity and health by ensuring the best environmental conditions both in and out of their growing season so they can come back the next year.

Petunias thrive in temperatures from 40 to 90 degrees, making them hardy and colorful additions to a landscape or hardscape during the warm months of spring through summer. However, the color of the petunia flowers is more intense when temperatures are at the lower to middle range of their tolerance, and petunias that experience prolonged hot summers with temperatures in the 90s or above may fail to thrive.

Those growing their petunias in containers in areas with cold winters should bring the containers into a protected location before temperatures get too cold. If a frost is on the horizon, bring the container indoors and situate it in a warm location that gets bright light.

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. Here are the basics to successfully overwinter petunias, allowing them to become dormant:

1. Bring your petunias inside before the first frost and place them in front of your sunniest window. If you don't have a suitable window for your petunias, keep them under a grow light until sunset each day.
2. Gradually start watering them less each week. When the leaves have dropped from the plants, cease watering altogether and store your dormant petunias in a cool, dark place.
3. Do not prune or fertilize the plant again until spring.
4. In about mid-February, place them in front of the sunny window again and give them a drink of water.
5. Wait until spring to take them outside, at which time you should harden them off or gradually transition them to outdoor conditions as if they were seedlings.

Caring for nondormant petunias in the winter months is just like caring for petunias any other time of year. Preventing dormancy is the tricky part, and even if you can keep them from going dormant, they may not flower, and they may become leggy and lose their leaves. However, several factors come into play, and it's possible that you could be enjoying blooms in January.

Full sun and warmth are the critical factors that prevent petunias from becoming dormant, so here's what you'll need to do if you want to overwinter nondormant petunias:

1. Find a nice, warm place to situate your petunias inside your house — the sunnier, the better.
2. Invest in a grow light so that the petunias are exposed to light for about 10 hours each day.
3. Continue to water and fertilize the petunias as if it's the middle of summer.
4. Prune the petunias as needed, as this can stimulate new growth and prevent dormancy.

Be aware that because petunias' internal clock tells them that they should shut down for a few months, they may slip into dormancy anyway. If this occurs, it's best to just allow dormancy to maintain the health of the plant. In this case, follow the procedure for overwintering dormant petunias.

If you grow petunias in the ground rather than in pots, you can still enjoy fresh (and free) petunia plants each year. Here are some tips on propagating petunias:

1. Observe the weather and act before a frost comes.
2. Cut the stems about 3 to 4 inches down from the central flower bud and then deadhead the flowers from your cuttings.
3. Strip the lower half of the stem of all leaves.
4. Dip the bare stem in water and then in powdered rooting hormone.
5. Plant the cutting in potting mix, 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.

Lucky you if you live in a climate warm enough that you can grow petunias year-round. Be aware, however, that even in warm climates that don't experience killing frosts, petunias will still naturally experience a semidormant period when the days get shorter, in which they cease actively growing and flowering. Take steps to ensure that they come back from this period ready to grow and bloom in your flower garden.

1. Give your petunias a rest after their intense summer growth by cutting them back to about a third of their size in the fall.
2. Water only if your area experiences dry spells and the top inches of the soil are dry.
3. If the weather threatens to dip below 40 degrees, cover them with a light plant cover or a lightweight sheet.
4. When your petunias begin to put on growth again in the spring, prune off any dry or old stems to encourage new, leafy growth. Then, get ready for another blooming season!

In warm climates that don't experience frost or freezing temperatures, you can also plant new petunias in the fall for a spring garden.

When spring arrives, it's time to bring your petunias back from their slumber into the light and warmth. If they have been living indoors as houseplants, wait until there is no chance of frost. If you plan to transplant them back into the garden soil, the soil should be at least 60 degrees.

Since the plants have been in a cozy environment for several months, they need time to acclimate to outdoor conditions. This is called "hardening off" and means to slowly introduce a plant to outdoor conditions after a period of growing in a greenhouse or having been in a dormant state. This usually takes a week or two depending on the location and starts with placing them in a shaded, protected spot on warm days and then bringing them back in at night. Each day, expose them to a little more direct sunlight so they can relearn how to tolerate outside conditions. Protect them from heavy rains or winds as well.

While they are in this transition, don't let them dry out but don't water them too heavily, as you don't want to encourage immediate leafy growth until they are acclimated. After a couple of weeks, you can leave them outside and resume a normal watering schedule, either in their pots or replanted in the garden soil directly.

As they begin to regrow, pinch off the top inch or so to encourage a bushier structure. Fertilize at least once a month with a balanced fertilizer designed for flowers to support all the gorgeous blooms you are about to enjoy.