Ornamental · Osmundaceae
Interrupted Fern
Claytosmunda claytoniana
Give it damp shade and rich, acidic soil, and this tough native fern unfurls fresh fronds every spring.
Partial shade
Every 3–7 days
USDA 3–8
2–5 ft · 0.6–1.5 m
Overview
The interrupted fern is one of the easiest ferns to keep happy in a shady, damp corner. It earns its name from a quirk of the frond: a few pairs of brown, spore-bearing leaflets sit right in the middle, breaking up the green so the leaf looks interrupted. Those fertile leaflets show up in mid to late spring and wither away by summer, leaving a tidy green arch behind.
This is a true native of cool, moist woodlands, and it is built to last. It handles winters down to -40°F (-40°C) and shrugs off the deer and rabbits that flatten softer plants. Given steady moisture, it slowly forms a wide, vase-shaped clump that comes back bigger each year.
Plant it for the texture and the toughness rather than for flowers, because it has none. Its closest lookalikes are the cinnamon fern and the royal fern, but only this one wears that band of brown leaflets across the middle of its fronds.
Care
The short version: part to full shade, soil kept evenly moist and on the acidic side, and a yearly top-dressing of compost. Once it's settled it asks for almost nothing beyond water during dry spells.
Light
Part shade is the sweet spot, though this fern is unusually flexible and takes anything from full shade to full sun. The catch with sun is water: it only tolerates bright, open sites where the soil stays constantly moist.
Water
Keep the soil consistently moist and never let it dry out, watering more often through dry spells. This is a wetland fern at heart, so it even shrugs off boggy ground and short stretches of standing water that would rot most plants.
Soil
It wants moist, humus-rich, acidic ground, happiest at a pH of 4.5 to 6.5. Beyond that it isn't fussy and adapts to clay, loam, or sand, so long as the moisture is there.
Temperature
Few garden plants are this hardy. It handles a range from -40°F to 86°F (-40°C to 30°C), riding out brutal northern winters and coming back every spring.
Humidity
Like most woodland ferns it prefers the air on the damp side, ideally around 50% to 80% humidity. In the garden, steady soil moisture and a shaded, sheltered spot do most of that work for you.
Feeding
This fern barely needs feeding. A single spring top-dressing of compost or leaf mold is plenty, slowly enriching the soil the way leaf litter would on a forest floor.
Pruning
There's no routine pruning to do. Just cut away dead or tattered fronds in late winter or early spring, before the new growth pushes up, to keep the clump looking fresh.
Repotting
It isn't really a container plant, so think dividing rather than repotting. Lift and split established garden clumps every 3 to 5 years, which also revives a crown that has started to die out in the center.
Size & spacing
A mature clump reaches 2 to 5 feet (61 to 152 cm) tall and spreads 2 to 4 feet (61 to 122 cm) wide. Give each plant room for that full spread so the fronds can arch out without crowding.
When to plant
This fern goes in the ground in spring, after the soil thaws and starts to warm. New fronds unfurl as the season opens, the brown fertile leaflets appear in the middle of each one by mid to late spring, and they wither off by midsummer while the green stays on through fall.
Northern hemisphere
Southern hemisphere
Typical months for a temperate climate: start earlier in warm zones, later where spring frosts linger.
- Plant: spring
- Harvest: Non-flowering; fertile pinnae appear in the middle of the frond in mid-to-late spring and typically wither by midsummer
Hardiness
USDA hardiness zones: 3–8
Temperature: down to -40°C
Troubleshooting
An interrupted fern has very few problems, and nearly all of them trace back to dry soil or too much sun. Read the fronds and the crown for the early signs below, then fix the cause rather than the symptom.
Fronds browning, crisping, or scorching at the edges
Dry soil is almost always the culprit, and too much sun without enough water makes it worse fast. This fern wants its roots damp at all times, so water deeply and lay down a mulch of leaf mold or compost to hold that moisture. If a plant is sitting in full midday sun, move it to a shadier spot.
Fix:
- Water deeply to keep the soil consistently moist
- Mulch with leaf mold or compost to retain moisture
- Relocate to a shadier site if planted in full sun
A bald center with healthy growth only around the edges
This is just the rhizome crown aging, not a disease, and it happens to most established clumps in time. Lift and divide the plant in early spring, then replant the vigorous outer sections and discard the dead middle. Dividing every few years keeps the whole clump growing strong.
Fix:
- Divide the clump in early spring and replant the healthy outer sections
Toxicity
Good news for pet owners: this is a true fern with no known toxins, so it's safe to plant where cats, dogs, and other animals roam. It isn't a food plant, but a curious nibble won't hurt anyone, and the species is actually known for standing up to rabbit and deer browsing.
Safe for
People · Cats · Dogs · Horses · Rabbits · Livestock · Birds
Cats None
Safe to live alongside. A curious cat that nibbles a frond is in no danger, though a big mouthful of any plant can still bring on a little stomach upset.
Dogs None
No toxins to worry about here. A dog can brush past or chew a frond without harm, and the worst you might see from a real binge is some mild, passing tummy trouble.
Horses None
This true fern carries nothing that would poison a horse, and grazing animals leave it alone in any case.
Rabbits None
Rabbits can share a garden with it freely. The plant is known for shrugging off rabbit browsing rather than causing any harm when nibbled.
Livestock None
Cattle, goats, and sheep face no danger from it. Stock generally pass it by, and a stray mouthful does no damage.
Birds None
Nothing in the fronds threatens birds, so backyard poultry and wild visitors can pick around it without concern.
People None
Handling it is perfectly safe. There's no irritant to speak of, though it isn't a food plant, so wash your hands after working in the soil out of plain garden habit.
No documented toxicity. Handle normally and wash hands after working with soil.
Varieties
There isn't a long list of named selections for this fern. The one form worth knowing is the wild type itself.
Osmunda claytoniana var. vera
This is the typical wild form of the interrupted fern, an infraspecific variety named by Wherry and recognized in Kew's Plants of the World Online.
How to propagate
You multiply an interrupted fern by splitting an established clump rather than fussing with seed, since it carries spores instead.
DivisionModerate
Early spring · 1-2 years
Division in early spring is the reliable way to make more. Lift the rhizome crown, split it, and replant, knowing the species creeps along slowly by horizontal rhizomes and a new division takes 1 to 2 years to settle in and bulk up. The fern also seeds itself from spores off the fertile pinnae, but raising it that way is slow, fussy work.
Myths
The interrupted fern produces flowers or seeds.
It never flowers and sets no seed. Like all ferns it spreads by spores, carried here on a band of brown fertile pinnae set into the middle of an otherwise green, sterile frond.
Interrupted fern, cinnamon fern, and royal fern are all the same plant.
They are three separate species in the family Osmundaceae. You tell the interrupted fern apart by the brown fertile pinnae that break up the middle of the frond, leaving green leaflets above and below them.