Whether they’re charming, compact millifloras in pastel shades, spreading varieties that cascade over a hill, or grandifloras with three-inch blooms in bold purple, all Petunia species belong to the nightshade family and are native to South America, in regions with hot and humid weather.
While you’ll find more selections of the modern hybrid varieties than ever before, all petunias still share this “warm weather preferred” trait.
They will not tolerate frost – at least not more than a touch of it. And they’re not cold hardy.
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These tender perennials are able to survive outdoors in winter only in warm-weather USDA Hardiness Zones 9 to 11, coming back year after year with proper care.
Even if your flowers can’t endure once freezing weather sets in, you can prepare them to bloom well after chilly temps first arrive, and make preparations all summer to make sure they’ll stay alive as long as possible.
I’ll also share my tips for those in cooler areas who want to encourage petunias grown as annuals to flower for as long as possible into the fall.
Petunias thrive when nighttime temperatures are in the 55 to 65°F range, and daytime temps are between 61 and 80°F.
They will succumb to sub-freezing temperatures in a hurry, and anything below 40°F may kill them, though Wave® petunias can tolerate temperatures as low as 35°F.
Petunias may be able to survive a dusting of snow or a light frost, especially if they’ve been acclimating gradually as autumn temperatures set in.
Petunias are a beloved warm weather flower known for their colorful, abundant blooms. But when the thermometer dips, these tropical plants are put to the test. Cold snaps and frosts quickly damage their delicate flowers and foliage. So what low temperature can petunias handle before succumbing to chill damage?
In this article, we’ll explore the ideal conditions and cold tolerance threshold for petunias. You’ll learn how to spot signs of frost injury and techniques to protect plants from freezing. We’ll also look at new cold hardy petunia varieties to try.
The Ideal Temperature Range for Petunias
To thrive, petunias prefer consistent warm weather with daytime highs around 70-85°F (21-29°C) Nighttime temperatures shouldn’t drop below 55°F (13°C) These tropical annuals and tender perennials evolved in sub-tropical and tropical climates without freezes.
Cool weather below 50°F (10°C) causes petunia growth to slow and flowers to fade Hot temperatures above 90°F (32°C) can also reduce blooming and plant vigor. Providing an ideal temperature range allows petunias to bloom prolifically through the frost-free months.
Shelter petunias from chilling winds that shock them. Morning sun with some afternoon shade helps prevent overheating on hot days. Consistent warmth coupled with fertile, well-drained soil gives petunias their best shot at performance.
How Cold Can Petunias Tolerate?
Most petunia varieties lack innate cold hardiness when grown outside their native warmer ranges. Exact low temperature tolerance depends on the cultivar, exposure, and conditions. But in general, petunias decline rapidly when lows drop below 40°F (4°C). Brief light freezes down to 32°F (0°C) may only injure exposed plant parts. But few petunias can withstand a hard freeze below 30°F (-1°C) for longer than a few hours.
Temperatures that frequently fluctuate above and below freezing are also detrimental to petunias. The repeated thawing and refreezing of plant tissues causes ruptured cells, wilting, and collapse. For optimal health, keep petunias above 40°F (4°C) overnight whenever possible. Take protective measures as cooler weather sets in.
Spotting Cold Damage on Petunias
It’s easy to spot chilling injury on affected petunias. Signs of cold damage include:
- Wilted or water-soaked leaves
- Leaves, flowers, or stems turning black or brown
- Flowers damaged or browned
- Plant death after harsh freezes
Younger, smaller plants are most vulnerable to cold stresses. But no petunias escape unscathed once frigid winter hits. If your plants show cold damage, don’t give up on them immediately. Further protect them while you assess if the crown and roots survived. The plants may rebound once warmed again.
Protecting Petunias from Frost and Freezes
When cold snaps threaten, be ready to take action to shield petunias. Here are some tips:
Cover Above and Below
Drape fabric row cover or old sheets directly over plants, securing the edges with rocks or landscape staples. Remove daily when temperatures climb. For containers, cluster pots together and enclose in a temporary cold frame or greenhouse.
Mound mulch, leaves, or straw around the base of plants. This insulates roots and prevents soil from freezing. Ensure soils are moist (but not wet) before frost hits.
Provide Warmth
Water during daytime hours right before a freeze to hydrate plants. Avoid wetting foliage at night when melting and refreezing occurs. Place strings of holiday lights or low-wattage bulbs under covers to provide radiant heating.
Move Indoors
Dig up garden plants and pot or bag the roots to overwinter in a sunny indoor location. Transfer containers to an unheated garage or greenhouse that stays above 40°F (4°C).
With vigilance, you can keep petunias alive through light frosts and extend their season. But once hard freezes set in, they’re generally finished for the year. Time to bring out the frost blankets!
Seeking Out Cold Hardy Petunia Varieties
Through selective breeding, some newer, more cold tolerant petunias are emerging. While no varieties are fully winter hardy, some cultivars better handle chill:
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‘Supertunia Bordeaux’ – Reportedly hardy to zone 7 and handles cool weather well.
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‘Supertunia Snowdrift’ – Features above average resistance to cold, rain, and wet.
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‘Purple Pirouette’ – Tolerates light frost and overwinters in zone 9. Blooms earlier than most.
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‘Potunia Plus Papaya’ – Bounces back after frost and does OK in cool, wet conditions.
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‘Crazytunia Black Mamba’ – Touts a bit more tolerance to chilly, damp weather than most.
Seek out petunias described as “cold tolerant” or for zone 7 and warmer. Pick plants rated for your specific planting zone. While not frost-proof, tough varieties resist cold better and recover quicker.
Overwintering Petunias as Houseplants
In zones with freezing winters, overwinter a few special petunias inside to carry plants over year to year. Here’s a simple technique:
In fall before frost, take 4-6” tip cuttings from healthy plants. Remove any flowers. Dip the cut end in rooting hormone and plant in sterile media like perlite or vermiculite.
Provide high warmth around 70-75°F (21-24°C) and enclose cuttings in a plastic bag to boost humidity. In about 6-8 weeks, new roots should develop.
Transplant the rooted cuttings into small pots with sterile soil. Grow on in a sunny window until spring. Then harden off and transplant outside after danger of frost passes.
This allows you to keep prized petunia cultivars from season to season. Taking cuttings is also a great way to make more plants of your favorites.
Caring for Petunias in Cool Weather
To help transition petunias as temperatures cool:
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Hold off on fertilizing which stimulates cold-sensitive new growth.
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Leave spent blooms on plants instead of deadheading. Pruning stresses plants going into dormancy.
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Keep soils moderately moist but not wet heading into winter. Dry soil freezes more readily.
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Choose a protected spot sheltered from wind which desiccates and kills tender growth.
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Monitor weather forecasts daily and be ready to take protective action as needed before frost arrives.
With extra care, your petunias can continue thriving well into fall most years. A bit of coddling goes a long way!
Answers to Common Cold Weather Petunia Questions
Some quick tips to address common concerns about growing petunias as colder months approach:
What temperature will kill petunias?
- Temperatures below 30°F (-1°C) for longer than a few hours will kill most plants. Damage starts around 32°F (0°C).
Should you deadhead petunias in cold weather?
- Stop deadheading once cool weather arrives to limit plant stress. But continue pruning until just before frost hits.
Can you plant petunias too early outside?
- Yes, planting too early while night temps are cool delays growth and flowering. Wait until lows reach 50°F (10°C).
Do petunias go dormant in winter?
- No, they don’t go fully dormant. Petunias are tender annuals or short-lived perennials that die off with hard freezes.
Should you fertilize petunias in fall?
- Avoid fertilizing which spurs tender new growth prone to cold damage. Let plants slow down and harden off naturally.
Enjoy Vibrant Petunias While You Can!
Give your petunias the best shot at thriving through fall by providing ideal care and protection. A little TLC goes a long way when cold snaps blow in! With the right attention, these tropical beauties will grace your garden or containers far into the season with abundant cheery blooms.
What Is the Growing Season for Petunias?
These cottage garden and hanging basket favorites thrive in warm weather.
Whether you start them from seeds indoors eight to 10 weeks before your average last frost date, or purchase transplants from a nursery, outside of Zones 9 to 11, you must wait for reliably warm weather before planting petunias out.
Keep in mind that some of the tactics that will help you delay the ill effects of freezing at the end of the season don’t work early on, when the plants are immature and more likely to succumb to temperature shock.
Only petunias that thrived all summer and then gradually acclimated to increasingly chilly temperatures in the fall are able to benefit from late-season efforts to keep them alive in colder weather.
A full-sun location or fabric covers won’t work on newly-planted petunias facing early spring frosts, for example, even if they’re growing in containers.
After all risk of frost has passed, be sure to harden off your plants before transplanting into the garden or leaving the containers out full time.
To do this, set them outside in a sheltered location for one hour on the first day, then gradually increasing the amount of time they spend outdoors over the course of a week to 10 days.
If an unexpected cold snap is in the forecast, bring them back indoors and start the process again once temperatures start to rise again.
As for the rest of the growing season, grown as annuals, most cultivars will begin blooming in midsummer and continue all the way up until the first heavy frost hits.
Want to learn some ways to push that blooming period to the limit? Read on.
Tips to Extend Blooming in Cooler Zones
Outside of Zones 9 to 11, you can still maximize the length of time these annuals stay alive, and even how long they keep blooming. Use these strategies:
Be diligent about selecting which type of petunia to grow if you want to enjoy them right up to the first frost, and maybe a little beyond.
For example, if you opt for spreading petunias, it’s far more difficult to cover them if there is a surprise early frost.
Multiflora varieties tend to be more compact and manageable.
If you’re willing to grow yours in containers, you can choose a variety that you can transport indoors for a day or two to withstand the first couple of frosts, or consider moving them indoors throughout the winter, either dormant or while continuing to put on new growth.
You can learn more about how to overwinter petunias indoors in our guide.
For these portable pots, select a smaller, more lightweight type, like ‘Supertunia Mini Vista Pink Star.’ It’s available in four-packs from Home Depot.
Or plant just one trailing petunia per hanging basket, so each will be light enough to move as needed when temps get cooler.
This also means you can easily move the plants into the shade or AC when sweltering midsummer weather makes them flag.
Shock Wave® ‘Coconut’ is one variety that will fill up a basket with a single plant. It has bright white flowers with yellow centers. It’s available in six-packs of plants from Burpee.
This part of the plan involves some tradeoffs. Ideally, you’ll transplant or sow in a space where the plants receive at least six hours of full sun per day.
But you can grow these flowers without quite that much sun. Even though it causes reduced flowering, planting in part shade will make the plants live longer in areas with scorching hot summers.
Here’s the thing, though. As autumn closes in, it’s colder in shady areas, so the plants’ roots and stems might freeze and die more quickly than those planted in sunnier spots.
The best compromise? Plant in full sun to stave off daytime exposure to those premature chilly temps, but use shade netting to reduce the ill effects of strong sun in the warm months, if that’s an issue for you.
Avoid planting in low-lying areas of the garden as cold air tends to accumulate in valleys and frost is more likely.
Also grow annual petunias in a spot where your flowers will be protected from wind, which can break the plants and dry the soil.
Many newer Wave® series or spreading petunias have been bred to continuously bloom without deadheading.
But if you’re growing older varieties like the grandifloras, you can extend the blooming season well into the fall by diligently removing spent blooms from the plants.
The idea is to remove them before they rob the plant of energy. After flowers fade, the plants will turn energy to producing seeds that could go to producing more blooms instead.
Use sanitized scissors to avoid spreading disease pathogens, and snip the faded flowers where they meet the stem.
It may not seem kind when you do it, but cutting the plants back to half their height in midsummer will help them grow back stronger, and bloom longer.
Perform this chore when you notice the stems getting leggy and losing their leaves.
Not only will the severe cut promote new growth and another round of blooms, it will make the plants sturdier, and better able to withstand chilly weather in subsequent weeks.
When growing these annuals in the garden, start with a balanced granular fertilizer worked into the soil ahead of planting, about eight ounces per 25 square feet.
In midsummer, start using liquid fertilizer once every three weeks to give petunias an energy boost.
Opting for the liquid is the best option when the plants have already spread over the soil surface, which makes it difficult to scratch in the granules.
Dilute the liquid and either pour it onto the soil or spray it on the leaves, according to the manufacturer’s instructions.
To encourage hanging basket and container plants to flower as long as possible, add some granular, slow-acting fertilizer to the growing medium when you plant, and consider adding more midway through the growing season.
The granules release nutrients gradually, which keeps the fertilizer in the pot longer. Liquid fertilizer is far more susceptible to getting washed out with the frequent watering that’s necessary for container-grown annuals.
Some hanging basket aficionados do both, giving their petunias an occasional boost from liquid fertilizer sprayed on the foliage along with granules added to the soil.
If you’re watering once a day or more often, this might be a good idea for your petunias, too.
How Much Cold Can Petunias Take? (The answer may surprise you)
FAQ
At what temperature do I need to cover my petunias?
Many newer cultivars are able to withstand hotter temperatures, but no petunias thrive in chilly temperatures. When spring temperatures dip below 40 degrees F, either cover your outdoor petunias with frost cloth or bring them indoors. Temperatures below freezing will kill them in a jiffy.
Are petunias ok in 40 degree weather?
Petunia Cold Tolerance
and daytime temps between 61 and 75 degrees F. (16-18 C.). However, petunias usually tolerate temperatures as low as 39 degrees F.
What temperature is too cold for annuals?
Temperatures below 32 degrees can freeze the leaf tissue of cold-wimpy plants and turn them to mush after just a few hours. Some of the most tender annuals and veggies might not die but suffer cold-induced setbacks even when overnight lows dip below 40.
Do petunias grow in cold weather?
Petunias are a unique and beautiful flower that can bring a bright splash of color to any garden. With proper care, they can provide blooms all summer long. To ensure petunias thrive in colder climates, it is important to keep temperatures above 45F and to cover them during frost.
What temperature do petunias like?
Read on to learn more about the cold tolerance of petunias. Petunias prefer nighttime temperatures between 57 and 65 degrees F. (14-16 C.) and daytime temps between 61 and 75 degrees F. (16-18 C.).
Are petunias temperature sensitive?
That’s because petunias are exceptionally temperature-sensitive. Typically, these plants prefer to live somewhere between 50℉ and 80℉. This temperature range provides the optimum environment for petunias to grow. Anything lower than that, and the plants won’t be able to carry out normal life processes.
How do you keep petunias warm?
Frost is a petunias nemesis. To keep your plants warm, cover them with a frost cloth or bring them inside if they are in pots. Think of it as tucking them in for a chilly night. Mulch acts like a cozy blanket, keeping the soil temperature steady.
Can a cold snap kill a Petunia plant?
But cold snaps below 30°F (-1°C) for longer than a few hours will likely kill the entire plant. Temperatures fluctuating above and below freezing are also detrimental. The repeated thawing and refreezing ruptures plant cells. So in general, sustained temperatures below 40°F (4°C) spell trouble for your petunias.
How does cold stress affect petunias?
Each upward or downward shift introduces a ripple of change, altering the rhythm of growth and blooming. In the chill of cold temperatures, petunias face a daunting adversary – cold stress. Reduced growth becomes the anthem of their struggle, as metabolic processes slow their tempo, stifling the vibrancy of life.