What Temperature is Too Cold for Petunias? How to Protect Them From Frost

With their profuse blooms in shades of pink, purple, red, white and more, petunias are a summertime garden favorite These fast-growing annuals or tender perennials instantly fill beds, borders, containers and hanging baskets with vivid color

But petunias are tropical plants that evolved in warmer climates. They lack natural cold hardiness when grown in most regions. Even a light frost can damage or kill them. So what temperature is too cold for petunias?

Read on to learn the cold tolerance threshold for petunias and how to protect them when chilly weather strikes

Ideal Temperature Range for Petunias

Petunias thrive best with daytime temperatures between 70-85°F (21-29°C) and nighttime lows above 55°F (13°C). Temperatures below 50°F (10°C) or above 90°F (32°C) can cause flowers to fade and lower overall plant vigor.

Consistent warmth is key throughout the growing season. Petunias grow poorly and often refuse to bloom in cooler conditions. Shelter plants from cold winds which can shock them. Morning sun with afternoon shade helps keep plants from overheating in summer.

Providing an ideal temperature range allows petunias to perform at their best through the season until the first fall frost arrives.

Minimum Cold Tolerance for Petunias

The exact minimum temperature petunias can tolerate depends on the specific variety grown, as well as conditions like wind and exposure. But most cannot withstand consistent temperatures below 40°F (4°C).

Brief light freezes down to 32°F (0°C) may only damage exposed flowers and leaves. 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. Take action to protect them before you reach this frigid threshold.

Signs of Cold Damage in Petunias

It’s easy to spot cold injury on petunias. Here are some telltale signs:

• Wilted, water-soaked leaves
• Leaves, flowers or stems turning black or brown
• Damaged, browned flowers
• Plant collapse or death

Young plants are more susceptible than established, mature specimens. But no petunias can survive untouched through harsh winters in northern climates.

If you see cold damage on your petunias, don’t assume they are goners yet. Protect them from further chilling while assessing if the main crown and roots survived. There’s a chance they may rebound once warmed.

Protecting Petunias from the Cold

When chilly overnight lows or frosts threaten, be prepared to take protective action:

Cover Plants Outdoors

• Drape fabric row cover or old sheets directly over plants, anchoring the edges with stones or landscape pins.

• Remove covers daily when temperatures warm.

• For container plants, move pots together and enclose in a temporary cold frame.

Provide Insulation

• Mound mulch, chopped leaves or straw around the base to insulate roots from freezing.

• Prior to frost, ensure soils are moist but not soggy. Dry soil freezes more readily.

Maintain Warmth

• Water plants during the day to hydrate them. Avoid wetting foliage at night which can freeze.

• Place incandescent lights or strings of holiday lights under covers to provide radiant heat.

Transfer Indoors or to Greenhouses

• Dig up plants and pot or bag roots to overwinter in a sunny window.

• Move containers to an unheated garage or greenhouse with temperatures above 40°F (4°C).

With a little TLC, you can extend the life of petunias into fall and protect them from lighter frosts. But once hard freezes arrive, they are generally done for the season.

Cold Hardy Petunia Varieties

Most petunias are rated hardy in zones 10-11 only and grown as annuals elsewhere. But breeders are introducing more cold tolerant options suitable for zones 7-9 with protection. These emerging varieties better withstand light freezes. Some to look for include:

• Supertunia® ‘Bordeaux’ – Reportedly hardy to zone 7 with good chill tolerance.

• Supertunia® ‘Snowdrift’ – Features higher resistance to cool weather and rain than most whites.

• ‘Purple Pirouette’ – Survives light frosts and can overwinter in zone 9. Reblooms in spring before most petunias.

• ‘Potunia® Plus Papaya’ – Tolerates cooler and wetter conditions. Bounces back after frost.

• ‘Crazytunia® Black Mamba’ – Features some increased tolerance to wet conditions and cooler weather.

Check tags or plant descriptions to see if new varieties you are selecting highlight good cold hardiness. Resistant plants still need protection from hard freezes but can better handle chill.

Overwintering Petunias Indoors

If you live where winters get harsh, you can overwinter a few favorite petunias inside to carry stock plants over for next year. Here’s how:

Take Cuttings in Fall

• Clip 4-6″ cuttings from branch tips before frost damage. Remove flowers.

• Dip cut end in rooting hormone then plant in sterile media like perlite or vermiculite.

Provide Warmth and Humidity

• Place in bright window or under grow lights. Maintain 70-75°F (21-24°C).

• Enclose in a plastic bag or cloche to boost humidity around cuttings.

Transplant Rooted Cuttings

• In about 6-8 weeks, new roots should form. Transplant into pots with sterile soil.

• Grow on in a sunny window until spring then harden off and plant outdoors after danger of frost.

With a little diligence, you can keep petunias around from one year to the next. Taking cuttings is also a great way to propagate your favorite varieties.

FAQ About Petunias in Cold Temperatures

If you’re new to growing petunias, here are quick answers to some common cold weather questions:

What temperature will kill petunias?

Petunias often can’t survive below 30°F (-1°C) for very long. Chilling injury occurs around 32°F (0°C) and they succumb quickly to a hard freeze. Be ready to cover or move plants before frost hits.

Should you deadhead petunias in cold weather?

Yes, continue removing spent blooms into fall. Deadheading redirects energy into forming new flowers instead of seeds. But stop pruning once cold temperatures arrive to limit plant stress.

Can you plant petunias too early outside?

Yes, planting outdoors too early while nights are still chilly can delay growth and flowering. Wait until nighttime lows are above 50°F (10°C) to plant.

Do petunias go dormant in winter?

No, petunias are tender annuals or perennials, not plants that go dormant. They will die with freezing weather and regrow from seed or new transplants next spring.

Should you fertilize petunias in fall?

Avoid fertilizing once cooler weather arrives. Fertilizing stimulates new growth that is vulnerable to frost damage. Let plants slow down naturally as days shorten.

Enjoy Vibrant Petunias While You Can

There’s no denying petunias add vibrant color and abundant blooms to gardens and landscape pots during the warm season. Just be attentive to incoming cold snaps and weather forecasts so you can take action before it’s too late.

A little TLC to shelter plants from chill can prolong their season well into fall most years. Or overwinter a few favorites indoors as cuttings to replace next spring. With a little planning, you can ensure a solid start of petunias each new growing season.