Hardy garden ferns can ride out winters far below zero. Tropical ferns start to suffer in the 40s.
Native hardy ferns like wood ferns, Christmas ferns, and ostrich ferns survive temperatures of -20°F to -40°F once they go dormant, while tropical ferns like the Boston fern get damaged below about 40 to 45°F and lose their top growth to frost.
So how cold can ferns tolerate in your yard? It comes down to which group your fern belongs to, how long the cold lasts, and whether the roots sit in a pot or in the ground.
Once you know those three things, you can decide which ferns stay out, which come in, and which just need a little mulch.
The Short Answer Depends on the Fern
Hardy ferns handle hard freezes because they go dormant and protect their crowns underground, while tropical ferns have no dormancy to fall back on. A quick frost and a long freeze also hit ferns in very different ways. If you are stocking up, a soil moisture meter is the first thing I would pick up.
What Temperatures Can Hardy and Tropical Ferns Handle?
Hardy ferns fall into two camps. Deciduous types, like ostrich, lady, cinnamon, and sensitive ferns, let their fronds die back in fall and sleep through winter. Evergreen types, like Christmas fern and many wood ferns, keep green fronds that can sit under snow.
Most hardy natives grow in USDA zones 3 to 8. That means their roots survive winter lows of -40°F at the cold end.
Tropical ferns sit at the other extreme:
- Boston fern: best at 60 to 75°F, stressed below 50°F, damaged around 40 to 45°F
- Maidenhair (tropical types): wants steady warmth and no drafts
- Bird’s nest fern: no frost tolerance
- Staghorn fern: most types want to stay above 50°F
Why Does a Brief Frost Differ from a Long Freeze?
A light frost forms on fronds when the air dips near 32°F for a few hours. That burns leaf tissue, but the soil stays warmer, so the crown and roots usually survive.
A hard freeze is different. When temperatures stay below about 28°F for many hours, water inside the plant’s cells turns to ice and ruptures them.
For a tropical fern, that ice reaches the crown and kills the plant. A hardy fern is built for it, as long as it has hardened off with gradually cooling fall weather first.
How Cold Is Too Cold for Tropical Ferns?
Tropical ferns start slowing down at 50°F and take real damage in the low 40s. Frost kills their fronds, and a freeze kills the whole plant.
When Should You Bring a Boston Fern Indoors?
I’d bring a Boston fern (Nephrolepis exaltata) inside before night temperatures drop below 50°F. In the Northeast and Midwest, that’s often mid to late September. In the Southeast, it might be late October or November.
Waiting until frost is in the forecast is cutting it too close. Cool nights in the 40s cause yellowing and leaf drop even without frost. My guide on when to bring ferns inside covers timing by region.
Boston ferns live outdoors year-round only in USDA zones 9 to 11. Even in zone 9, a rare cold snap into the 20s will kill the top growth, so keep frost cloth handy.
Can Staghorn and Other Tropical Ferns Survive Frost?
No, frost is too much for them. A staghorn fern grows on tree bark in the wild, with its roots up in the air. That leaves it with no soil to buffer the cold.
Most common staghorns want to stay above 50°F, and a few tougher types handle brief dips into the high 30s.
I’d treat all staghorn ferns as frost-tender and move mounted plants into a garage or house once nights drop toward 45°F. Bird’s nest and tropical maidenhair ferns need the same care.
Which Hardy Ferns Can Survive a Cold Winter?
Wood ferns, Christmas ferns, sword ferns, and polypodies all shrug off freezing winters across most of the US. Picking one rated for your USDA zone is the single best way to avoid winter losses.
Wood Ferns and Other Cold-Hardy Garden Choices
Dryopteris species, often called the hardy wood fern group, are some of my favorite picks for cold gardens. Fancy fern (Dryopteris intermedia) is hardy to zone 3, and male fern grows in zones 4 to 8.
Autumn fern handles zones 4 to 9 and adds copper-colored new growth.
Other tough choices for zones 3 to 8:
- Ostrich fern: tall and spreading, zones 3 to 7
- Lady fern: lacy, zones 3 to 9
- Christmas fern: evergreen, a woodland staple in the East
- Cinnamon fern: handles wet soil, zones 3 to 9
- Japanese painted fern: silvery fronds, zones 4 to 9
Western Sword Ferns and Polypodies
Western sword fern (Polystichum munitum) is the big evergreen of Pacific Northwest woods. It grows in roughly zones 5 to 9 and keeps its leathery fronds through wet, chilly winters.
Polypodies are small ferns that creep over rocks and logs. Western polypody grows along the West Coast, while rock polypody in the East handles zone 3 cold. Both often curl up in dry or frozen weather and flatten out again when it warms.
How USDA Zones Help You Choose
Your USDA zone tells you the average coldest night of the year. Here’s how a few zones translate:
- Zone 3: -40 to -30°F
- Zone 5: -20 to -10°F
- Zone 7: 0 to 10°F
- Zone 9: 20 to 30°F
Pick a fern rated at least one zone colder than yours if you want a safety margin. That cushion helps in a winter that runs colder than average. For more on cold-season care, see my post on whether ferns survive winter.
Cold Limits for Common Ferns at a Glance
Use this table as a starting point when you’re deciding what stays outside. The numbers are approximate lows for established plants in the ground.
| Fern | Type | Approx. Low Temperature | USDA Zones |
|---|---|---|---|
| Boston fern | Tropical | Damage below 40 to 45°F | 9 to 11 |
| Staghorn fern | Tropical | Keep above 50°F | 10 to 11 |
| Bird’s nest fern | Tropical | Keep above 50°F | 10 to 11 |
| Western sword fern | Hardy evergreen | About -20°F | 5 to 9 |
| Christmas (eastern sword) fern | Hardy evergreen | About -30°F | 3 to 9 |
| Male fern | Hardy wood fern | About -30°F | 4 to 8 |
| Ostrich fern | Hardy deciduous | About -40°F | 3 to 7 |
| Japanese painted fern | Hardy deciduous | About -30°F | 4 to 9 |
How to Read Temperature Ranges Without Treating Them as Guarantees
These limits assume a healthy, established fern in good soil. A fern planted last month has a smaller root system and less stored energy, so it’s more vulnerable its first winter.
Ratings also assume normal conditions. A fern in soggy clay, a windy corner, or a pot will take damage well above the number in the chart. Treat each figure as a best-case line.
Why Can the Name Eastern Sword Fern Be Misleading?
“Sword fern” gets attached to very different plants. Christmas fern (Polystichum acrostichoides) is sometimes called eastern sword fern, and it’s a tough native that survives zone 3 winters.
The Boston fern is also a “sword fern,” but it belongs to a tropical group called Nephrolepis. If a plant tag just says “sword fern,” check the Latin name. Polystichum means hardy, and Nephrolepis means bring it inside.
What Makes One Cold Night More Damaging Than Another?
The same 25°F reading can barely touch a fern one night and wreck it the next. Wind, soggy soil, freeze length, and container growing all change the outcome.
Wind, Wet Soil, and the Length of a Freeze
Wind pulls moisture out of evergreen fronds when the ground is frozen and roots can’t replace it. That’s why sword and Christmas ferns in exposed spots look scorched by late winter.
Wet soil adds a second problem. Water that pools in the crown can freeze right against the growing point and kill it. Ferns in well-drained, leafy soil handle cold far better than ferns sitting in puddles.
Length matters too. A dip to 28°F before sunrise does little, but three days below 20°F lets cold reach deeper into the soil and the crown.
Why Potted Ferns Get Colder Than Ferns in the Ground
Soil in the ground holds heat, so roots a few inches down stay much warmer than the air. A pot has cold air on every side, so its roots chill almost as fast as the air does.
A good rule: a fern in a container is about two zones less hardy than the same fern planted in the ground. A zone 5 fern in a pot in a zone 5 garden is at real risk. Small pots and hanging baskets freeze fastest.
What Does Cold Damage Look Like on a Fern?
Cold-damaged fronds turn dark, limp, and mushy at first, then dry to brown or black within a few days. Damage often shows first on the outer fronds or the side facing the wind.
Frost-Burned Fronds Versus Normal Winter Dormancy
Frost burn shows up fast. Fronds look water-soaked and dark green or black the morning after a cold night, then collapse. On a tropical fern, the whole plant can go limp at once.
Dormancy is slow and even. Deciduous hardy ferns turn yellow, then tan or golden brown over a few weeks in fall. That’s normal, and the crown is resting safely below.
How to Tell Cold Injury from Dryness or Sun Scorch
Timing is your best clue. If the damage appears right after a cold night, it’s cold injury. Browning that creeps in over weeks points to something else.
- Dry air or underwatering: crispy brown tips and edges, spreading slowly
- Sun scorch: bleached or pale patches on the sunny side