Geranium Temperature Tolerance

What temperature do geraniums need?

Zonal geraniums (Pelargonium x hortorum) thrive between 45°F and 85°F. Below 32°F, they face frost damage. Above 95°F, they show heat stress. Both extremes reduce bloom output and can kill the plant if prolonged.

True perennial geraniums (Geranium spp., called cranesbill) are a completely different story. They go dormant in winter and tolerate temperatures as low as –30°F in USDA Zone 4. These two plants share a name but require very different cold-weather management.

Understanding which geranium you have — Pelargonium or Geranium — is the first step in protecting it from temperature extremes.

Cold tolerance: zonal geraniums (Pelargonium)

Zonal geraniums are frost-sensitive. They originate from the warm, dry climate of South Africa and have no built-in cold hardiness.

The key temperature thresholds:

• 32°F: Light frost damage possible. Leaf edges may turn mushy or translucent.
• 28–30°F: Significant tissue damage. Most plants will not recover without intervention.
• 25°F or below: Plants die if exposure lasts more than 2–3 hours.

In practice, a brief overnight dip to 30°F in early October — typical for many US climates — can kill a zonal geranium left uncovered on a porch. Experienced gardeners watch the forecast closely starting in late September.

USDA zone guide for zonal geraniums

In Zones 7–9, mild winters occasionally allow geraniums to survive outdoors if protected under frost cloth. But this is unreliable. The safest approach is to treat them as annuals or bring them inside each fall.

Signs of cold damage in geraniums

After an unexpected frost, zonal geraniums show damage within 24–48 hours:

• Leaves turn translucent or blackened
• Stems become soft and waterlogged-looking
• Flowers collapse and turn brown
• The base of the plant may feel mushy at soil level

Minor cold damage (light frost, brief duration) often spares the lower stems and roots. Cut the damaged growth back to healthy tissue and keep the plant at 55–65°F indoors. Recovery is possible if the root zone survived.

Cold tolerance: perennial geraniums (cranesbill)

True perennial geraniums are cold-hardy to Zone 4, surviving temperatures to –30°F with standard garden conditions. Some species, including Geranium maculatum (wild cranesbill, native to eastern North America), tolerate even colder climates with consistent snow cover.

The most popular perennial geranium in US gardens — ‘Rozanne’ — is rated for Zones 4–9. This means it overwinters in the ground in most US states without any special care.

Snow cover is actually beneficial for perennial geraniums. The insulation effect of snow protects the crown from freeze-thaw cycles, which can heave roots out of the ground in Zones 5–6. In snowless winters with frequent thaws, mulching with 3–4 inches of straw around the crown prevents heaving damage.

In Zone 3 and colder (average winter lows below –40°F), even perennial geraniums may need extra mulch protection to survive the coldest years.

Heat tolerance: both types

Both zonal and perennial geraniums show heat stress when daytime temperatures exceed 90–95°F for several consecutive days. The effects are most visible in flowers before they appear in foliage.

Heat stress signs

Managing summer heat

In Zones 8–9, afternoon shade between 1–4 PM noticeably reduces heat stress. An east-facing location (morning sun, afternoon shade) suits zonal geraniums much better than a south-facing exposure that gets direct sun through the hottest part of the day.

Consistent watering during heat waves matters. In extreme heat, container geraniums may need water twice daily — morning and late afternoon. However, avoid watering in the evening, which promotes fungal disease.

Mulching in-ground geraniums with 2–3 inches of wood chips keeps soil temperatures cooler and reduces water evaporation. Soil temperature at root depth can run 10–15°F cooler than air temperature when mulched.

Our full-sun perennials guide covers which perennials genuinely tolerate full sun and heat better than geraniums — useful when replacing geraniums in very hot exposures.

Hardening off: why it matters

Geraniums moved directly from a warm greenhouse or indoor space to outdoor temperatures perform worse in both cold and heat than gradually hardened plants.

A plant grown at 70°F indoors and suddenly placed outside at 50°F experiences transplant stress. Its cells aren’t prepared for the temperature shift. Hardening off — incrementally exposing the plant to outdoor conditions over 7–14 days — allows the plant to produce protective compounds and acclimate physically.

Hardening schedule

Properly hardened geraniums tolerate about 10–15°F lower temperatures than unhardened ones. This doesn’t extend their frost hardiness past 28°F, but it reduces the risk of cold shock from early May frosts in Zones 5–7.

Overwintering zonal geraniums

The goal is to keep them alive at 50–60°F until spring — not to grow them actively, just to maintain them.

Three methods work consistently in US climates:

Method 1: Houseplant overwintering

The most reliable approach. Before the first frost:

1. Cut the plant back by one-third.
2. Repot if it was in-ground or in a very large container.
3. Move to a south-facing window where it receives 4–6 hours of light daily.
4. Water when the top inch of soil dries out — every 7–10 days.
5. No fertilizer needed until March.

Plants kept this way often survive to become large, established specimens the following year. A one-year-old geranium that overwinters indoors will produce more flowers than a new transplant purchased each spring.

Method 2: Bare-root storage

Less reliable but uses almost no space. After the first light frost:

1. Dig up the plant, shake off excess soil.
2. Hang the roots upside down in a cool, dark space — 45–55°F, like a basement or garage.
3. Once a month, soak the roots in water for 30–60 minutes, then re-hang.
4. In late February, pot the plants and move to a bright window.

Success rate is lower than houseplant overwintering. Some plants will not revive. Reserve this method for situations where indoor window space is limited.

Method 3: Stem cuttings

Take 3–4-inch stem cuttings in August or September, before the plant is stressed by cold. Remove lower leaves, allow the cut end to callous for one hour, then insert into a moist potting mix. Root in a bright indoor location at 65–70°F.

Cuttings root in 3–4 weeks. These young plants overwinter better than large, root-bound specimens. This is also a free way to multiply your favorite geranium varieties.

Our annuals that bloom all summer guide places zonal geraniums among the longest-season bloomers — useful context when deciding whether overwintering an established plant is worth the effort versus buying new transplants each spring.

Spring planting timing by zone

Do not plant geraniums outside until nighttime temperatures reliably stay above 45°F. A brief cold snap to 32°F after planting is survivable if it lasts only a few hours. Sustained cold at or below 32°F will kill newly planted transplants.

Approximate safe outdoor planting dates for zonal geraniums:

Last frost dates vary by location within each zone. Check your local extension service’s last frost records or the USDA Plant Hardiness Zone Map at planthardiness.ars.usda.gov for precise first and last frost averages in your zip code.

Cold-climate success strategies

Gardeners in Zones 4–6 can extend geranium performance by choosing planting locations that moderate temperature extremes.

South-facing walls: A location against a south-facing brick or concrete wall absorbs daytime heat and radiates it at night, creating a microclimate 5–10°F warmer than open garden beds. Geraniums placed against such walls in Zone 6 sometimes survive a light frost that kills plants in open beds.

Container mobility: Keeping geraniums in containers rather than in-ground makes it practical to move them inside quickly when frost is forecast. A container garden on a porch or patio can be wheeled inside in minutes.

Row cover protection: Lightweight frost cloth (1–1.5 oz.) raises temperature inside by 4–8°F for brief periods. Draped over geraniums before a predicted frost, it can protect down to about 25°F.

For geraniums in containers year-round, our perennial geranium guide covers the cranesbill types that return in the ground every spring without any indoor overwintering — a lower-maintenance alternative for gardeners tired of annual storage.

Pairing with heat- and cold-tolerant companions

When temperatures stress geraniums, companion plants carry the display. Pairing geraniums with plants that tolerate a broader temperature range produces a more consistent garden bed.

Heat-tolerant companions:

• Lantana — thrives in 100°F heat, full sun; fills in when geraniums slow bloom. See our lantana guide.
• Salvia — many annual and perennial salvias tolerate 95–100°F without flower drop. See our salvia guide.

Cold-tolerant companions for shoulder-season gaps:

• Pansies — tolerate 20°F; plant in fall to fill spots after geraniums are pulled.
• Snapdragons — bloom in cool temperatures, bridging the gap between geranium removal and spring.

Pairing geraniums strategically means the garden holds color even during the weeks when geraniums are stressed or dormant. Our geranium care guide covers the full range of geranium care, from planting through fertilizing and deadheading, with specific guidance for both Pelargonium and true geranium types.

The Old Farmer’s Almanac covers US frost dates by location with a searchable database that goes back decades — worth bookmarking for fall overwintering decisions.