Ornamental · Osmundaceae
Cinnamon Fern
Osmundastrum cinnamomeum
Give it wet, acidic shade and the cinnamon fern throws up tall green fronds and rust-colored spikes year after year.
Partial shade
Every 2–4 days
USDA 3–9
2–5 ft · 0.6–1.5 m
Overview
Cinnamon fern is a big, clump-forming native of North American bogs and wet woodlands, and it grows the same way across much of eastern Asia. In spring the crown sends up a ring of upright green fronds that can reach 2 to 5 feet (60 to 150 cm) tall, vase-shaped and elegant. From the center rise separate fertile fronds covered in rust-colored spores, the cinnamon-brown spikes that give the plant its name.
It earns its keep in the damp, shady corners where most plants sulk. Pond edges, rain gardens, stream banks, and low spots that stay wet all suit it, and it shrugs off cold down to USDA zone 3. The growth is steady rather than fast, but a happy clump widens to 2 to 4 feet (60 to 120 cm) across and comes back larger each year.
What it asks for is simple: shade, acidic ground, and water that never quits. Let the soil dry in summer and the fern protects itself by going dormant early, dropping its fronds well before fall. Keep it wet and it stays lush from spring fiddlehead through the first hard frost.
Care
The short version: part to full shade, rich and acidic soil that stays consistently moist to wet, and steady water all season so it never dries out and quits early.
Light
Cinnamon fern is happiest in partial to full shade, the dappled light of a woodland floor. It will take full sun, but only where the soil stays wet enough to keep the fronds from scorching at the edges.
Water
This is the one thing the fern won't forgive. Keep the soil consistently moist to wet, watering every 2 to 4 days in dry weather, and never let it dry out. It even handles standing water and boggy ground, but a summer drought sends it into early dormancy.
Soil
Plant it in rich, humusy, acidic ground that holds moisture, ideally a pH of 4.5 to 6.0. Boggy, low-lying spots that stay wet are perfect, and a thick layer of leaf mold keeps the roots cool and damp.
Temperature
It's tough as nails in the cold, hardy from -40°F to 86°F (-40°C to 30°C), which covers USDA zones 3 through 9. Hard winters don't faze it, since the crown sits dormant underground until spring.
Humidity
Coming from wet woods and bog edges, it likes the air on the damp side, around 50% to 80%. In a naturally moist, shaded site the humidity takes care of itself, so there's rarely anything to manage.
Feeding
Rich soil means this fern barely needs feeding. A light spring top-dressing of compost or leaf mold is plenty, and in good ground you can skip even that.
Pruning
Leave the healthy green fronds alone through the season. The only cut worth making is clearing the old, dead, or frost-damaged fronds in late winter, before the new fiddleheads push up.
Repotting
Established clumps rarely need disturbing. When the crowns grow crowded and overlap and the fronds start coming in smaller, that's the sign to lift and divide rather than simply move the plant.
Size & spacing
Expect a vase of fronds 2 to 5 feet (60 to 150 cm) tall, spreading 2 to 4 feet (60 to 120 cm) wide over time. Give each crown that much room so the mature clump has space to fill out.
When to plant
There's no harvest to time with this fern, just one planting window that matters. Set new plants out in spring as the soil warms and the fiddleheads start to rise, which gives the crown a full season to root in before winter.
Northern hemisphere
Southern hemisphere
Typical months for a temperate climate: start earlier in warm zones, later where spring frosts linger.
- Plant: spring
Hardiness
USDA hardiness zones: 3–9
Temperature: down to -40°C
Troubleshooting
Cinnamon fern has no serious pests or diseases, so nearly every problem traces back to one thing: dry soil. Read the fronds and they'll tell you when the plant is thirsty. Catch it early and a quick deep soaking usually turns things around.
Brown, crisp frond edges that look scorched
Dry soil is almost always the cause. Cinnamon fern comes from bogs and wet woods and burns at the margins the moment its roots run short of water, and the damage shows up worst on whichever side catches the most sun. Water deeply to bring the soil back to evenly moist, mulch with leaf mold to hold it there, and move the plant into more shade if it sits in full sun.
Fix:
- Water deeply to keep soil consistently moist
- Mulch with leaf mold to retain moisture
- Provide more shade if sited in full sun
Fronds yellow and collapse early in summer
When the soil dries in midsummer the fern protects itself by going dormant, so the fronds yellow and fold down ahead of schedule. This looks alarming but rarely kills the plant. It almost always comes back the next spring if you resume steady deep watering and work more organic matter into the soil to hold moisture.
Fix:
- Resume consistent deep watering
- Improve soil organic matter and moisture retention
Toxicity
Good news for households with pets and children. Cinnamon fern is a true fern with no known toxin, so a curious nibble won't poison a cat, dog, or anyone else. The only thing to mind is the wiry, cinnamon-colored hairs at the base of the fronds, which can scratch or irritate sensitive skin, so wash your hands after handling them.
Safe for
People · Cats · Dogs · Horses · Rabbits · Livestock · Birds
Cats None
Cinnamon fern is a true fern and carries no known toxin, so a curious cat that nibbles a frond should be fine. A pet that gorges on any plant may bring up a little, but there's nothing here that poisons.
Dogs None
Dogs can be around this fern without worry. There's no toxic compound in the fronds, though the stiff fibers at the frond bases are better left unchewed.
Horses None
The fronds hold nothing that harms horses. It's a safe planting around paddocks and pasture edges.
Rabbits None
Rabbits can graze this fern safely. The young fiddleheads and mature fronds carry no toxin.
Livestock None
Cattle, goats, and sheep won't be poisoned by it. The plant is safe in fields and along wet ground where stock roam.
Birds None
Birds face no danger from the fronds or spores. Backyard flocks can scratch and forage around the crowns without trouble.
People None
The plant isn't poisonous, and the fiddleheads of related ferns are eaten in spring. The only thing to mind is the wiry hairs at the frond bases, which can scratch sensitive skin, so wash your hands after handling them.
The cinnamon-colored hairs and fibers at the frond bases may cause mild mechanical skin irritation in sensitive individuals.
Varieties
Cinnamon fern is grown mostly as the wild species, but botanists recognize a couple of regional forms worth knowing.
var. glandulosum
This form carries glandular hairs along the frond axes and turns up in the southeastern reaches of North America.
var. asiaticum
The Asian variant ranges from the Russian Far East south into Indochina.
How to propagate
You won't grow this fern from seed, because ferns don't make any. The dependable route is dividing an established clump, with growing from spores as the patient gardener's alternative.
DivisionModerate
Early spring before the fronds unfurl · 1-2 years
The reliable way to make more plants is to lift and split the crown of an established clump in early spring, before the fiddleheads open. Each piece takes a year or two to settle in and fill out. You can also raise it from spores sown fresh on a moist, sterile mix, but that route is slow and fussy.
Myths
Cinnamon fern is where culinary cinnamon comes from.
The name points only to the cinnamon color of the fibers and the fertile fronds, not to the spice. Real cinnamon is the bark of unrelated Cinnamomum trees, so this fern has nothing to do with what ends up in your kitchen.
Cinnamon fern spreads and reproduces by seed.
Ferns don't make seeds or flowers at all. This one reproduces by spores carried on separate upright fertile fronds, the rust-colored spikes that give the plant its name.