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
Evolutionarily, bracken may be considered one of the most successful ferns. 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. NBN distribution map for the United Kingdom
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 fern 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 fern is removed, apparently because active plant toxins remain in the soil.
Brackens substitute the characteristics of a woodland canopy, and are 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.
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Bracken fiddleheads (the immature, tightly curled emerging fronds) have been eaten by many cultures throughout history, and are still commonly used today as a foodstuff. The mature fronds may be toxic. Bracken fiddleheads are either consumed fresh (and cooked) or preserved by salting, pickling, or sun drying. In Korea, where they are called gosari (고사리), they are a typical ingredient in the mixed rice dish called bibimbap.
The British Royal Horticultural Society recommends against consumption of bracken either by humans or livestock, since it contains carcinogens linked with oesophageal and stomach cancer. However, the poison found in bracken, ptaquiloside, is both water-soluble and is destroyed in heat and alkaline conditions. Despite this, moderation of consumption is still recommended to mitigate chance of throat and stomach cancers.
The Māori of New Zealand used the rhizomes of P. esculentum (aruhe) as a staple food, especially for exploring or hunting groups away from permanent settlements; much of the widespread distribution of this species in present-day New Zealand is in fact a consequence of prehistoric deforestation and subsequent tending of aruhe stands on rich soils (which produced the best rhizomes). The rhizomes were air-dried so that they could be stored and became lighter; for consumption, they were briefly heated and then softened with a patu aruhe (rhizome pounder); the starch could then be sucked from the fibers by each diner, or collected if it were to be prepared for a larger feast. Patu aruhe were significant items and several distinct styles were developed.
In East Asia, Pteridium aquilinum (bracken fiddleheads) is eaten as a vegetable, called warabi (蕨 / わらび) in Japan, gosari (고사리) in Korea, and juécài (蕨菜) in China and Taiwan. In Korea, a typical banchan (small side dish) is gosari-namul (고사리나물) that consists of prepared bracken fiddleheads that has been sauteed. It is a component of the popular dish bibimbap.
Bracken has been shown to be carcinogenic in some animals and some have suggested it could have some part in causing the high incidence of stomach cancer in Japan. It is currently under investigation as a possible source of new insecticides.
Uncooked bracken contains the enzyme thiaminase, which breaks down thiamine. Eating excessive quantities of bracken can cause beriberi, especially in creatures with simple stomachs. Ruminants are less vulnerable because they synthesize thiamine.
It was traditionally used (and still is in certain areas like mid-Wales) for animal bedding, which later broke down to a rich mulch which could be used as fertilizer.
The plant is carcinogenic to animals such as mice, rats, horses and cattle when ingested, although they will usually avoid it unless nothing else is available. Young stems are quite commonly used as a vegetable in China, Japan and Korea. However, some researchers suspect a link between consumption and higher stomach cancer rates. The spores have also been implicated as a carcinogen. Danish scientist Lars Holm Rasmussen released a study in 2004 showing that the carcinogenic compound in bracken, ptaquiloside or PTA, can leach from the plant into the water supply, which may explain an increase in the incidence of gastric and esophageal cancers in bracken-rich areas.
In cattle, bracken poisoning can occur in both an acute and chronic form, acute poisoning being the most common. In pigs and horses bracken poisoning induces vitamin B deficiency. Poisoning usually occurs when there is a shortage of available grasses such as in drought or snowfalls.
Along with the DNA damage caused by ptaquiloside it is shown that chemicals in the fern can damage blood cells and can destroy Vitamin B1. This in turn causes beriberi, a disease normally linked to nutritional 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.
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