Gled. ex Scop. 1760 not Raf. 1814 (Pteridaceae)
Bracken (Pteridium) is a genus of large, coarse ferns in the family Dennstaedtiaceae. Ferns (Pteridophyta) are vascular plants that have alternating generations, large plants that produce spores and small plants that produce sex cells (eggs and sperm). Brackens are noted for their large, highly divided leaves. They are found on all continents except Antarctica and in all environments except deserts, though their typical habitat is moorland. The genus probably has the widest distribution of any fern in the world.
The word bracken is of Old Norse origin, related to Swedish bräken and Danish bregne, both meaning fern. In the past, the genus was commonly treated as having only one species, Pteridium aquilinum, but the recent trend is to subdivide it into about ten species.
Description and biology
Bracken, like heather, is typically found in moorland environments, and is commonly referred to by local populations in the north of England as 'Moorland Scrub'. It is also one of the oldest ferns, with fossil records over 55 million years old having been found. The plant sends up large, triangular fronds from a wide-creeping underground rootstock, and may form dense thickets. This rootstock may travel a metre or more underground between fronds. The fronds may grow up to 2.5 m (8.2 ft) long or longer with support, but typically are in the range of 0.6–2 m (2.0–6.6 ft) high. In cold environments, bracken is deciduous and, as it requires well-drained soil, is generally found growing on the sides of hills.
Fern spores are contained in structures found on the underside of the leaf called sori. The linear, leaf-edge pattern of these in bracken is different from that in most other ferns, where the sori are circular and occur towards the centre of the leaf.
Pteridium aquilinum (bracken or common bracken) is the most common species with a cosmopolitan distribution, occurring in temperate and subtropical regions throughout much of the world. It is a prolific and abundant plant in the moorlands of Great Britain, where it is limited to altitudes of below 600 metres. It does not like poorly drained marshes or fen. It has been observed growing in soils from pH 2.8 to 8.6. Exposure to cold or high pH inhibits its growth. It causes such a problem of invading pastureland that at one time the British government had an eradication programme. Special filters have even been used on some British water supplies to filter out the bracken spores.
Bracken is a characteristic moorland plant in the UK which over the last decades has increasingly out-competed characteristic ground-cover plants such as moor grasses, cowberry, bilberry and heathers and now covers a considerable part of upland moorland. Once valued and gathered for use in animal bedding, tanning, soap and glass making and as a fertiliser, bracken is now seen as a pernicious, invasive and opportunistic plant, taking over from the plants traditionally associated with open moorland and reducing easy access by humans. It is toxic to cattle, dogs, sheep, pigs and horses and is linked to cancers in humans. It can harbour high levels of sheep ticks, which can pass on Lyme disease. Grazing provided some control by stock trampling, but this has almost ceased since the 2007 foot-and-mouth disease outbreak reduced commercial livestock production. Global climatic changes have also suited bracken well and contributed to its rapid increase in land coverage.
Bracken is a well-adapted pioneer plant which can colonise land quickly, with the potential to extend its area by as much as 1–3% per year. This ability to expand rapidly at the expense of other plants and wildlife, can cause major problems for land users and managers. It colonises ground with an open vegetation structure but is slow to colonise healthy, well managed heather stands.
Bracken presents a threat to biodiversity. Many species occur only on upland moorland, tied to features unique to the habitat. The loss and degradation of such areas due to the dominance of bracken has caused many species to become rare and isolated.
- Pteridium aquilinum - nearly cosmopolitan
- Pteridium arachnoideum - Mexico, Central and South America, Galápagos
- Pteridium caudatum - Mexico, Central and South America, Florida, West Indies
- Pteridium centrali-africanum - Zaire, Zambia, Tanzania, Burundi
- Pteridium esculentum - China, Southeast Asia, Australia, New Zealand
- Pteridium falcatum - Guangxi
- Pteridium feei - Mexico, Central America
- Pteridium lineare - Yunnan
- Pteridium revolutum - China
- Pteridium tauricum - Caucasus
- Pteridium yunnanense - Yunnan
Other plant associations
Bracken is known to produce and release allelopathic chemicals, which is an important factor in its ability to dominate other vegetation, particularly in regrowth after fire. Its chemical emissions, shady canopy and thick litter inhibit other plant species from establishing themselves – with the occasional exception of plants which support rare butterflies. Herb and tree seedling growth may be inhibited even after bracken is removed, apparently because active plant toxins remain in the soil.
Bracken substitutes the characteristics of a woodland canopy, and is important for giving shade to European plants such as common bluebell and wood anemone where the woodland does not exist. These plants are intolerant to stock trampling. Dead bracken provides a warm microclimate for development of the immature stages. Climbing corydalis, wild gladiolus and chickweed wintergreen also seem to benefit from the conditions found under bracken stands.
In China, bracken is known as juecai, and are eaten like vegetables or preserved by drying.
Bracken rhizomes can be ground into flour to make bread. In the Canary Islands, the rhizome was historically used to make a porridge called gofio. Both fronds and rhizomes have been used to produce beer in Siberia, and among indigenous peoples of North America.
P. esculentum rhizomes were traditionally used by the Māori people of New Zealand as a staple food, and is known as aruhe. It was eaten while exploring or by hunting groups away from permanent settlements. The plant was widely distributed across New Zealand as a result of prehistoric deforestation, and planting on rich soils, which produced the best rhizomes. The rhizomes were dried, and could be heated and softened with a pounder (patu aruhe), after which the starch could be sucked from the fibers. Patu aruhe were important ritual items, and several distinct styles were developed.
Bracken has traditionally been used for animal bedding, which later breaks down into a rich mulch that could be used as fertilizer. It is still used this way in Wales. It is also used as a winter mulch, which has been shown to reduce the loss of potassium and nitrogen in the soil, and to lower soil pH. Green bracken ferns average 25 percent potash and can contain as much as 55 percent.
Bracken contains the carcinogenic compound ptaquiloside, which causes damage to DNA, thus leading to cancers of the digestive tract. High stomach cancer rates are found in Japan and North Wales, where bracken is often eaten, but it is unclear whether bracken plays a role. Consumption of ptaquiloside-contaminated milk is thought to contribute to human gastric cancer in the Andean states of Venezuela. The spores have also been implicated as carcinogens.
However, ptaquiloside is water-soluble and destroyed in heat (by cooking) and alkaline conditions (by soaking). Korean and Japanese cooks have traditionally soaked the shoots in water and ash to detoxify the plant before eating. Ptaquiloside also degenerates at room temperature, and denatures almost completely at boiling temperature. Despite this, moderation of consumption is still recommended to reduce chances of cancer formation. The British Royal Horticultural Society recommends against consumption of bracken altogether, either by humans or livestock.
Ptaquiloside has been shown to leach from wild bracken plants into the water supply, which has been implicated in high rates of stomach and esophogeal cancers in areas with high bracken growth, such as Wales and South America.
Uncooked bracken also contains the enzyme thiaminase, which breaks down thiamine (vitamin B1). Excessive consumption of bracken can lead to vitamin B1 deficiency (beriberi), especially in animals with simple stomachs. Ruminants are less vulnerable because they synthesize thiamine.
In cattle, bracken poisoning can occur in acute and chronic forms, acute poisoning being the most common. Milk from cows that have eaten bracken may also contain ptaquiloside, which is especially concentrated in buttermilk. In pigs and horses, bracken poisoning induces vitamin B1 deficiency.
Hydrogen cyanide is released by the young fronds of bracken when eaten by mammals or insects. Two major insect moulting hormones, alpha ecdysone and 20-hydroxyecdysone, are found in bracken. These cause uncontrollable, repeated moulting in insects ingesting the fronds, leading to rapid death. Bracken is currently under investigation as a possible source of new insecticides.
Many sites have archaeological remains dating from the Neolithic and Bronze Ages through to the Industrial Revolution. The root systems of established bracken stands degrade archaeological sites by disrupting the strata and other physical evidence. These rhizomes may travel a metre or more underground between fronds and form 90% of the plant, with only the remainder being visible.
Some small level of scattered cover can provide beneficial habitats for some wildlife, at least in the UK (as given above). However, on balance, removing bracken encourages primary habitats to re-establish, which are of greater importance for wildlife. Control is a complex question with complex answers, which need to form part of a wider approach. Management can be difficult and expensive; plans may need to be about cost-effective, practical limitation and control rather than give an expectation for eradication.
All methods need follow-up over time, starting with the advancing areas first. Given the decades elapsed to arrive at the current levels of coverage on many sites, slowing or reversing the process will be also of necessity long-term, with consistency and persistence from all parties being key.
- Cutting — once or twice a year, repeatedly cutting back the fronds for at least 3 years
- Crushing/rolling — using rollers, again for at least 3 years
- Livestock treading — during winter, encouraging livestock to bracken areas with food. They trample the developing plants and allow frost to penetrate the rhizomes. In May and June, temporary close grazing or mob stocking on small areas away from nests, particularly using cattle, horses, pigs or ponies may crush emerging bracken fronds resulting in reduced bracken cover. Sufficient fodder will be required to prevent livestock eating the bracken. This may suit steep areas where human access is difficult and herbicide undesirable.
- Herbicide — Asulam (also known as Asulox) is selective for ferns; glyphosate is not; but the latter has the advantage that the effects can be seen soon after application. They are applied when the fronds are fully unfurled to ensure that the chemical is fully absorbed. Rare ferns such as adder's tongue (Ophioglossum vulgatum), killarney (Trichomanes speciosum) and lemon-scented ferns can also be found in similar habitats and it is important that these are not destroyed in the process of bracken control.
Natural England recommends that only Asulam can be sprayed aerially, Glyphosate requires spot treatment, e.g. using a weedwiper or knapsack spray. The toxicity of Asulam is low and has been generally highly cost-effective but its use is now restricted by the EU after 2012, at least until specific registered uses can be defined.
Selective sprays like Starane, Access, Metsulfuron 600WG, etc. work well but only if sprayed in late autumn so the rhizomes store food for winter and hence absorb the poison.
On archaeological sites, chemical control is usually required as mechanical methods may cause damage.
- Allowing plants to grow in its place, e.g., the establishment of woodland, causes shade that inhibits bracken growth. In the UK, trees, notably rowan, have done well since grazing reduced greatly after the foot-and-mouth epidemic in 2000 but young saplings struggle in high bracken. In decades to come, tree shade cover may increase, if permitted, and so may reduce bracken growth, but this is both long-term and in some cases is contentious in the change it would bring to traditionally open heath or moorland, both aesthetically and as a valuable habitat.
- Burning — useful for removing the litter, but may be counter-productive as bracken is considered to be a fire-adapted species
- Ploughing — late in the season followed by sowing seed
Any bracken control programme must be completed, or bracken will re-establish.
A Bracken Control Group was established in 2012 to provide best-practice guidance for all bracken control techniques. The Group has also been responsible for submitting an application for an Emergency Authorisation to secure the continued availability of Asulam for bracken control, following the decision not to register the product under new regulations in the EU. Registration has been re-applied for but this will not be available until 2017 at the earliest. Until re-registration is approved the Group will aim to keep Asulam available under the emergency provisions. Bracken Control Group website
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|Wikimedia Commons has media related to Pteridium aquilinum.|
- World fern species list — Pteridium
- Independent page devoted to bracken (uses older classification scheme)
- Alonso-Amelot ME, Avendaño M (March 2002). "Human carcinogenesis and bracken fern: a review of the evidence". Curr. Med. Chem. 9 (6): 675–86. doi:10.2174/0929867023370743. PMID 11945131. Archived from the original on 2011-10-06.
- Natural England — Bracken control, vegetation restoration and land management[permanent dead link]
- RSPB: Bracken management in the uplands
- Bracken for Butterflies by Butterfly Conservation
- Edibility of Bracken: Identification and edible parts of bracken