Western Brackenfern

Bracken Fern, courtesy of Chris Evans, River to River CWMA, Bugwood.org
Photo courtesy of Chris Evans, River to River CWMA, bugwood.org
Common Name(s):
Brackenfern
Western Brackenfern
Bracken Fern
Bracken
Brake Fern
Brake
Hog-brake
Eagle Fern
Scientific Name:
Pteridium aquilinum (L.) Kuhn
Scientific Name Synonyms:
Pteris aquiline
Asplenium aquilinum
Allosorus aquilinus
Ornithopteris aquiline
Filix aquiline
Filix-foemina aquiline
Pteris latiuscula
Symbol:
PTAQ
Description:
Life Span: Perennial

Origin: Native

Growth Characteristics:  Bracken Fern occurs throughout the world.  In North America, fronds usually begin to emerge between March and early May. Frost-killed fronds are replaced through mid-July. Spore maturation and dispersal begins at the base of the frond and proceeds up to the tip resulting in an extended period of seed dispersal. It has extensive rhizomes.

Flowers: Young bracken fern plants produce spores by the end of 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. 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. 

When spores germinate, they produce bisexual, gamete-bearing plants about 0.25 inch (0.6 cm) in diameter and one cell thick. These tiny plants (gametopytes or prothalli) have no vascular system and require very moist conditions to survive. The young spore-bearing plant (sometimes called a sporling) 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.

Fruits/Seeds: not applicable

Leaves: The leaves or fronds of bracken fern are normally from 1 to 10 feet (3-30 dm) long including a stipe (leaf-stalk) that may be as long as 39 to 59 inches (10-15 dm) but is usually shorter than the leaf blade. The blades of the fronds are divided into pinnae, the bottom pair of which are sometimes large enough to give the impression of a three-part leaf. Each pinna is in turn divided into pinnules. Above the first division of the stipe into a frond, it is called a rachis. On fertile fronds the spores are borne in sori beneath the outer margins of the pinnules. The sori are protected by the inrolled pinnule margins on one side and a thin membrane called an indusium on the other.
Nectaries are found at the base of the pinnae during spring and early summer. The largest nectaries are found near the base of the frond and the nectaries get progressively smaller going up the rachis. Ants are attracted by and feed on sugars produced by these extra-floral nectaries. 

The fronds are killed by frost. 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.

Stems: not applicable

Roots: Rhizomes are the main carbohydrate storage organs.  Rhizomes also store water and are consistently around 87 percent water. Rhizomes can be up to 1 inch (2.5 cm) in 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. Thin, black, brittle roots extend from the rhizome and may extend over 20 inches (50 cm) deeper into the soil. Endotrophic mycorrhizae have been found on the roots of bracken fern.
In the first year rhizomes may grow to 86 inches (217 cm) long. By the end of a second year the rhizome system may exceed 6 feet (18 dm) in diameter. Bracken fern's aggressive rhizome system gives it the ability to reproduce vegetatively and reduces the plant's dependence on water for reproduction. The rhizomatous clones can be hundreds of years old, and 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. 
Ecological Adaptations:
In the west, bracken fern is generally found in open forests, pastures, and on open slopes. It is common following fires. 

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. In Utah, bracken fern is found at elevations of 5,500 to 8,000 feet (1676-2438 meters).

Soils: Bracken fern grows on a variety of soils with the exception of heavily waterlogged soils. Its 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. Bracken fern grows best on deep well-drained soils with good water-holding capacity, and it may dominate other vegetation on such sites. Bracken fern is characteristically found on soils with medium to very rich nutrients. It is absent from soils contaminated with zinc.

Associated Species: Aspen, subalpine fir, Englemann spruce.
Uses and Management:

Bracken fern is known to be poisonous to livestock throughout the United States, Canada, and Europe. Goats are the only livestock that normally eat bracken fern. Losses are greatest when livestock are fed hay mixed with bracken fern. Simple-stomached animals like horses, pigs, and rats develop a thiamine deficiency within a month. Vitamin B1 is effective in curing the animal if it is administered early. Acute bracken poisoning affects the bone-marrow of both cattle and sheep and causes anemia and hemorrhaging which is often fatal. Bright blindness and tumors of the jaws, rumen, intestine, and liver are also found in sheep feeding on bracken fern. Sheep and cattle are most often poisoned by bracken fern when young animals are moved from an area without bracken fern to a field containing the fern. Cumulative poisoning may occur in older sheep that have ingested small amounts of bracken fern over a period of years.

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.

Despite bracken fern's 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.

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

Bracken fern is 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. 

Bracken fern is competitive plant that invades cultivated fields and disturbed areas. It effectively competes for soil moisture and nutrients. Its 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. Control may be needed until tree seedlings are taller than the bracken fern and sturdy enough to withstand the weight of dead fronds. 

Bracken fern's production and release of allelopathic chemicals is an important factor in its ability to dominate other vegetation. The release of these toxic chemicals varies by environment or perhaps by variety of bracken fern. In the intermountain west, no allelopathic chemicals are released from the green fronds but are readily leached from standing dead fronds. Other vegetation may be inhibited for a full growing season after bracken fern is removed, apparently because active phytotoxins remain in the soil.

Timing is important in any treatment of bracken fern. The most effective time for treatment is summer just after the new fronds have fully expanded and starch reserves in the rhizome are at their lowest level. Two or more annual treatments and combinations of cutting and herbicide are more effective than single treatments or even single annual treatments.

Mechanical Treatment: Cutting early in the summer, allowing the rhizomes to regenerate a second crop of fronds, then recutting will deplete the resources of the rhizome much faster than a single cutting. However, single, annual cuttings or deep plowing can be effective during midsummer. 

Chemical control: Asulam is a relatively specific and environmentally safe herbicide that is very effective for bracken fern control. Asulam is more effective if the bracken fern is cut first. Dead fronds may need to be cut away from growing trees after spraying with asulam. Glyphosate (Roundup) is also effective and reduces carbohydrate reserves of the rhizome. Other effective chemical controls include amitrole-T, dicamba, karbutilate, picloram, 4-CPA, sodium chlorate/borate, chlorthiamid, and dichlobenil. Spraying vegetation with other herbicides may reduce competition and allow b