Pteridium aquilinum

Bracken Fern

 Bracken Fern, Photo © by Earl J.S. Rook
Bracken Fern
Photo © by Earl J.S. Rook

Flora, fauna, earth, and sky...
The natural history of the northwoods

Name:
  • Pteridium, from the Greek pteris (pteris), "fern"
  • aquilinum, from the Latin, "eagle like"
  • Bracken, an old English word for all large ferns, eventually applied to this species in particular.
  • Other common names include: Brake, Brake Fern, Eagle Fern, Female Fern, Fiddlehead, Hog Brake, Pasture Brake, Western Brackenfern, Grande fougere, Fougere d'aigle, Warabi (Qué), Örnbräken, Bräken, Slokörnbräken, Taigaörnbräken, Vanlig Örnbräken (Swe), Einstape (Nor), Ørnebregne (Dan), Sananjalka (Fin), Adlerfarn (Ger), Kilpjalg, Kotkajalg, Põldsõnajalg, Seatinarohi, Sõnajalg (Estonia)

Taxonomy:
  • Kingdom Plantae, the Plants
    • Division Polypodiophyta, the True Ferns
      • Class Filicopsida
        • Order Polypodiales
          • Family Dennstaedtiaceae
            • Genus Pteridium
  • Taxonomic Serial Number: 17224
  • Also known as Pteris aquilina, Asplenium aquilinum, Allosorus aquilinus, Ornithopteris aquilina, Filix aquilina, Filix-foemina aquilina, Pteris latiuscula
  • Considered a single, worldwide species, although some disagree

Description:
  • A large, deciduous, rhizomatous fern
  • Fronds 1'-3' w/leaf stalk up to 3'' but usually shorter than leaf blade. Blades of frond divided into pinnae, the bottom pair sometimes large enough to suggest a three part leaf. Pinna divided into pinnules. On fertile fronds the spores are borne in sori beneath the outer margins of the pinnules. Fronds are killed by frost each winter and new fronds grow in spring. Dead fronds form a mat of highly flammable litter that insulates the below-ground rhizomes from frost when there is no snow cover. This litter also delays the rise in soil temperature and emergence of frost-sensitive fronds in the spring.
  • Rhizomes are the main carbohydrate and water storage organs (87% water). Rhizomes can be up to 1" diameter and branching is alternate. The rhizome system has two components. The long shoots form the main axis or stem of the plant. They elongate rapidly, have few lateral buds, do not produce fronds, and store carbohydrates. Short shoots, or leaf-bearing lateral branches, may be closer to the soil surface. They arise from the long shoots, are slow growing, and produce annual fronds and many dormant frond buds. Transition shoots start from both short and long shoots and may develop into either.
  • Roots thin, black, brittle extending from the rhizome to over 20" inches into the soil.
  • Brackenfern is a large, coarse, perennial fern that has almost horizontal leaves and can grow 1½ to 6½ feet tall (sometimes up to 10 feet). Unlike our more typical broadleaf perennials, this primitive perennial lacks true stems. Each leaf arises directly from a rhizome (horizontal underground stem), and is supported on a rigid leaf stalk. In addition, brackenfern does not produce flowers or seeds. Instead, it reproduces by spores and creeping rhizomes. This species often forms large colonies.
  • Root system - The black, scaly, creeping rhizomes (horizontal underground stems) are ½ inch thick, and can grow as much as 20 feet long and 10 feet deep. Stout, black, wide-spreading roots grow sparsely along the rhizomes.
  • Seedlings & Shoots - The curled leaves (fiddleheads) emerging from rhizomes in the spring are covered with silvery gray hair.
  • Stems - The leaf stalk (not a true stem) is tall (about the same length as the leaf), smooth, rigid and grooved in front. It is green when young, but turns dark brown later in the season.
  • Leaves - The leaf stalk supports a broad (3 feet long, 3 feet wide), triangular, dark green, leathery and coarse-textured leaf that often bends nearly horizontal. The leaf is divided into 3 parts, 1 terminal and 2 opposite. Each of the leaf parts is triangular and composed of numerous oblong, pointed leaflets, which are in turn composed of narrow, blunt-tipped subleaflets.
  • Fruits & Seeds - A continuous line of spore cases (spore-producing structures) is formed along the underside edge of leaflets, but the spore cases are partially or completely covered by inrolled leaf margins and are difficult to see. Spore cases produce minute, brown spores.
  • Biology: Spores of brackenfern are produced August through September. Brackenfern is one of the earliest ferns to appear in spring or after a fire. It sometimes forms large colonies of nearly solid stands. In the fall, it is one of the first plants to be killed by frost, resulting in large patches of crisp, brown foliage.
  • Brackenfern is resistant to many herbicides and is tolerant of various forms of mechanical control. However, effective control has been obtained by repeated removal of aboveground growth, which eventually exhausts the food reserves in the rhizomes.

Identification:
  • Distinguished from other large North Country ferns by the large three part leaf atop a tall stalk.
  • Field Marks
    • broad triangular leaf held almost parallel to the ground
    • smooth, grooved, rigid stalk about as long as the leaf
    • narrowed tip to leaflets

Distribution:
  • Global; throughout the world with the exception of hot and cold deserts

Habitat:
  • Fossil evidence suggests that bracken fern has had at least 55 million years to evolve and perfect antidisease and antiherbivore chemicals. It produces bitter tasting sesquiterpenes and tannins, phytosterols that are closely related to the insect moulting-hormone, and cyanogenic glycosides that yield hydrogen cyanide (HCN) when crushed. It generates simple phenolic acids that reduce grazing, may act as fungicides, and are implicated in bracken fern's allelopathic activity. Severe disease outbreaks are very rare in bracken fern.
  • Grows on a variety of soils with the exception of heavily waterlogged sites. Efficient stomatal control allows it to succeed on sites that would be too dry for most ferns, and its distribution does not normally seem limited by moisture. Grows best on deep, well-drained soils with good water-holding capacity, and may dominate other vegetation on such sites.
  • Rhizomes are particularly effective at mobilizing phosphorus from inorganic sources into an available form for plant use. Bracken fern contributes to potassium cycling on sites and is associated with high levels of potassium.
  • In northern climates bracken fern is frequently found on uplands and side slopes, since it is susceptible to spring frost damage. Fronds growing in the open or without litter cover are often killed as crosiers by spring frost damage, since the soil warms earlier and growth begins sooner. The result is that fronds appear earlier in shaded habitats.
  • A shade intolerant pioneer and succession species that is sufficiently shade tolerant to survive in light spots in old growth forests.
  • Light, windborne spores allow colonization of newly vacant areas.
  • Despite production of bitter-tasting compounds, chemicals that interfere with insect growth, and toxic chemicals, bracken fern hosts a relatively large number and variety of herbivorous insects.
  • Competition: Invades cultivated fields and disturbed areas, effectively competing for soil moisture and nutrients. Rhizomes grow under the roots of herbs and tree or shrub seedlings, and when the fronds emerge, they shade the smaller plants. In the winter dead fronds may bury other plants and press them to the ground. On some sites shading may protect tree seedlings and increase survival.
  • Allelopathy: Bracken fern's production and release of allelopathic chemicals is an important factor in its ability to dominate other vegetation. Farther north no allelopathic chemicals are released from the green fronds but are readily leached from standing dead fronds. Herbs may be inhibited for a full growing season after bracken fern is removed, apparently because active plant toxins remain in the soil.

Fire:
  • A fire-adapted species throughout the world. Not merely well adapted to fire, it promotes fire by producing a highly flammable layer of dried fronds every fall. Repeated fires favor Bracken.
  • Primary fire adaptation is deeply buried rhizomes which sprout vigorously following fires before most competing vegetation is established. Windborne spores may disperse over long distances.
  • Fire removes competition and creates the alkaline soil conditions suitable for its establishment from spores
  • Fuel loading in areas dominated by bracken fern can be quite high.

Associates:

History:
  • Considered so valuable during the Middle Ages it was used to pay rents.
  • Used as roofing thatch and as fuel when a quick hot fire was desired.
  • The ash was used as a source of potash in the soap and glass industry until 1860 and for making soap and bleach. The rhizomes were used in tanning leathers and to dye wool yellow.
  • Bracken still used for winter livestock bedding in parts of Wales since it is more absorbent, warmer, and easier to handle than straw.
  • Also used as a green mulch and compost

Uses:
  • Most commonly used today as a food for humans. The newly emerging croziers or fiddleheads are picked in spring and may be consumed fresh or preserved by salting, pickling, or sun drying. Both fronds and rhizomes have been used in brewing beer, and rhizome starch has been used as a substitute for arrowroot. Bread can be made out of dried and powered rhizomes alone or with other flour. American Indians cooked the rhizomes, then peeled and ate them or pounded the starchy fiber into flour. In Japan starch from the rhizomes is used to make confections. Bracken fern is grown commercially for use as a food and herbal remedy in Canada, the United States, Siberia, China, Japan, and Brazil and is often listed as an edible wild plant. Powdered rhizome has been considered particularly effective against parasitic worms. American Indians ate raw rhizomes as a remedy for bronchitis
  • Bracken fern has been found to be mutagenic and carcinogenic in rats and mice, usually causing stomach or intestinal cancer. It is implicated in some leukemias, bladder cancer, and cancer of the esophagus and stomach in humans. All parts of the plant, including the spores, are carcinogenic, and face masks are recommended for people working in dense bracken. The toxins in bracken fern pass into cow's milk. The growing tips of the fronds are more carcinogenic than the stalks. If young fronds are boiled under alkaline conditions, they will be safer to eat and less bitter.
  • Bracken fern is a potential source of insecticides and it has potential as a biofuel. Bracken fern increases soil fertility by bringing larger amounts of phosphate, nitrogen, and potassium into circulation through litter leaching and stem flow; its rhizomes also mobilize mineral phosphate. Bracken fern fronds are particularly sensitive to acid rain which also reduces gamete fertilization. Both effects signal the amount of pollutants in rain water making bracken fern a useful indicator.
  • Fronds may release hydrogen cyanide (HCN) when they are damaged (cyanogenesis), particularly the younger fronds. Herbivores, including sheep, selectively graze young fronds that are acyanogenic (without HCN) Lignin, tannin, and silicate levels tend to increase through the growing season making the plants less palatable. Cyanide (HCN) levels fall during the season as do the levels of a thiaminase which prevents utilization of B vitamins.
  • Toxicity: Known to be poisonous to livestock throughout the US, Canada, and Europe. Simple stomach animals like horses, pigs, and rats develop a thiamine deficiency within a month. Acute bracken poisoning affects the bone marrow of both cattle and sheep, causing anemia and hemorrhaging which is often fatal. Blindness and tumors of the jaws, rumen, intestine, and liver are found in sheep feeding on bracken fern.
  • Toxicity: All parts of brackenfern, including rootstocks, fresh or dry leaves, fiddleheads and spores, contain toxic compounds, and are poisonous to livestock and humans. Consumption of brackenfern causes vitamin B1 deficiency in horses, and toxins can pass into the milk of cattle. Young leaves of brackenfern have been used as a human food source, especially in Japan, and may be linked to increased incidence of stomach cancer. Humans working outdoors near abundant stands of the plant may be at risk from cancer-causing compounds in the spores.
  • Facts and Folklore:
    It was once thought that, if the spores of the brackenfern were gathered on St. John's Eve, it would make the possessor invisible.
    In the 17th century, live brackenfern was set on fire in hopes of producing rain.
    Brackenfern fiddleheads have been used as a food source; however, their consumption has been linked to various types of cancer in humans.

Reproduction:
  • Reproduces by spores and vegetatively by rhizomes
  • Most regeneration is vegetative. Many have searched for young plants growing from spores, but few have found them. However, spores do germinate and grow readily in culture.
  • Young plants produce spores by the end of the second growing season in cultivation but normally do not produce spores until the third or fourth growing season. A single, fertile frond can produce 300,000,000 spores annually. Spore production varies from year to year depending on plant age, frond development, weather, and light exposure. Production decreases with increasing shade. The wind-borne spores are extremely small. Dry spores are very resistant to extreme physical conditions, although the germination of bracken fern spores declines from 95-96% to around 30-35% after 3 years storage. The spores germinate without any dormancy requirement. Under favorable conditions, young plants could be found 6 to 7 weeks after the spores are shed. Under normal conditions the spores may not germinate until the spring after they are shed.
  • Sufficient moisture and shelter from wind are important factors in fern spore germination. Bracken fern spore germination appears to require soil sterilized by fire. On unsterilized soils spores may germinate, but the new plants are quickly overwhelmed by other growth. Temperatures between 59º and 86º F are generally best for germination, although bracken fern is capable of germination at 33º-36ºF.
  • A pH range of 5.5 to 7.5 is optimal for germination. Germination is indifferent to light quality; it is one of the few ferns that can germinate in the dark. Despite limitations on spore germination, genotype analysis in the Northeast indicates that many stands of bracken fern represent multiple establishment of individuals from spores.
  • When spores germinate, they produce bisexual, gamete-bearing plants about ¼" in diameter and one cell thick. These tiny plants have no vascular system and require very moist conditions to survive. The young spore-bearing plant which develops from the fertilized egg is initially dependent on the gametopyte until it develops its first leaf and roots. The first fronds are simple and lobed. They develop into thin, delicate fronds divided into lobed pinnae. They do not look like adult plants and are frequently not recognized as bracken fern. Cultivated plants begin to resemble adult fern after 18 weeks. The rhizomes begin to develop after there are a number (up to 10) of fronds and a well-developed root system or in the fifteenth week of growth under optimal conditions.In the first year rhizomes may grow to 86 inches long. By the end of a second year the rhizome system may exceed 6' in diameter.
  • Aggressive rhizome system gives it the ability to reproduce vegetatively and reduces dependence on water for reproduction. The rhizomatous clones can be up to 400' in diameter and hundreds of years old; some clones alive today may be over 1,000 years old.
  • Rhizomes have a high proportion of dormant buds. When disturbed or broken off, all portions of the rhizome may sprout, and plants growing from small rhizome fragments revert temporarily to a juvenile morphology.
  • Shaded plants produce fewerspores than plants in full sun
  • Bracken fern is a survivor. The fronds are generally killed by fire, but some rhizomes survive. The rhizomes are sensitive to elevated temperatures. During fires the rhizome system is insulated by mineral soil. Depth of the main rhizome system is normally between 3½" and 12" short rhizomes may be within 1½" of the surface and some rhizomes may be as deep as 40".
  • Well known postfire colonizer in eastern pine and oak forests. Fire benefits bracken by removing competition while it sprouts profusely from surviving rhizomes. New sprouts are more vigorous following fire, and bracken fern becomes more fertile, producing far more spores than it does in the shade
  • Spores germinate well on alkaline soils, allowing them to establish in the basic conditions created by fire.

Propagation:
  • Division most successful method

Cultivation:
  • Hardy to USDA Zone 3 (average minimum annual temperature -40ºF)
  • Characteristically found on soils with medium to very rich nutrients.
  • Cultivated and shaded plants produce fewer, thinner but larger fronds than open-grown plants.

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Last Updated on 26 February, 2004