Originally classified as a single species; Pteridium aquilinum, bracken has now been re-classified into about 10 species. It is a survivor – able to adapt to a wide range of climates and conditions, so it needs to be kept in check for the benefit of the environment, wildlife and those farmers whose livelihood depends on grazing livestock on clean pastures. In this short article I shall look at the plant; how it spreads, its positive and negative attributes and measures available to control it.
Bracken – Fronds
1/2 - Bracken Fronds
2/2 - The Crozier stage of Bracken growth
There are about 12,000 species of fern that belong to the plant kingdom group Pteridophyta. Ferns are one of the oldest groups of plants to have evolved on the earth, with fossil records dating them back to the Carboniferous period, 350 million years ago. They reproduce by spores rather than by flowers and seeds and have an alternating life cycle but, like flowering plants, they do have a vascular transport system (xylem and phloem), roots, stem and leaves (known as fronds). Once established bracken will spread and colonise an area with fronds sprouting directly from the rhizomatous root system.
Bracken thrives in most habitats except for deserts and poorly drained land, occurring in all parts of the world except Antarctica. In the UK it was originally an inhabitant of woodlands, moorlands and lowlands but today it can also be found in upland regions, where the removal of large areas of trees to make way for sheep pastures has allowed it to become more invasive. Bracken will tolerate soils with a pH ranging from 2.8 to 8.6 and is found growing in the saline mists of coastal areas, although a combination of high winds and salt can cause scorching of the young fronds. Cold inhospitable climates in the higher regions of the UK limit the spread of bracken to those areas below 600metres.
In cooler climates, bracken is a deciduous plant dying back completely in the autumn and sprouting again in late spring from the underground rhizomes. The first shoots are often referred to as the ‘crozier’ or ‘fiddlehead’ stage as they resemble a shepherd’s crook or the curved end of a violin.
The underground root system for bracken consists of thickened storage organs found deep in the soil that are attached to thinner rhizomes growing much nearer the surface (see diagram above), from which the bracken fronds sprout. These young fronds have a covering of bronze coloured hairs and are easily damaged by late frosts. The emerging shoots develop into large individual triangular fronds, each growing directly from the rhizomes and forming dense thickets. The fronds may grow up to 2.5m or more in height.
Bracken conquers new ground mainly by extension of the rooting system, however there is also a sexual stage involving the spores. These are microscopic and produced in structures known as ‘sori’, located in a linear fashion on the undersides of the fronds. Production of spores takes place only in bracken that has been established for 3 to 4 years. The spores ripen from July to August but are not released until the autumn – usually in October. A single frond can produce several hundred million spores but many do not survive to become new bracken plants, as successful development of the sexual stage of the life cycle is dependent on frost-free conditions with adequate moisture, and without fungal attack on the germinated spores. Those spores that do germinate will eventually form a small (~1cm tall) immature sporophyte stage to complete the life cycle but, due to the size, these are rarely seen.
Once established into a new area, bracken will dominate and squeeze out the existing vegetation by a combination of tactics. One of these involves the release of allelopathic chemicals into the soil. These are antagonistic molecules that discourage other plant species from taking root and may remain in the soil long after the bracken has been removed. Allelopathic chemicals, together with the dense shading canopy produced by the fronds and deep litter on the surface from several years of decaying bracken, will make it difficult for other vegetation to get established again even after total loss of cover by fire damage.
Looking at some of the positive characteristics of this plant, it has provided man with a source of food; the immature fronds have been eaten as a delicacy, after thorough cooking to remove toxins. The deeper rhizomes that contain stores of starch were used in baking by some cultures in remote parts of the world and both fronds and rhizomes have been used to brew beer. However, medical authorities and toxicologists advise against consuming any part of the bracken plant as it is known to contain carcinogenic and other substances toxic to humans and animals. Mature fronds were used for animal bedding and as a roofing material in primitive civilizations, whilst the ash from the burnt fronds provided a good source of potash for glassmaking during the mid to late Middle Ages. Today bracken is still harvested in parts of the UK to make commercial composts.
Bracken can offer the right conditions of shading and humidity to support several plants normally found in woodland areas; such as wood anemone, bluebell, chickweed-wintergreen and common dog violet. The presence of dog violets and bracken on south-facing hillsides provides a valuable habitat for the pearl-bordered fritillary – a rare butterfly listed in the UK Biodiversity Action Plan. This is also an important habitat for reptiles, such as adders.
Two British birds, the whinchat and nightjar, have adopted bracken as their preferred habitat as it provides good cover and a degree of protection for their young. Some other birds such as the skylark, lapwing and yellowhammer use thick bracken as a protected feeding site. However, where bracken has invaded moorland it then excludes many bird species that are common to this habitat.
Apart from the invasive characteristics of bracken the main negative aspects of this plant are its poisonous properties when consumed by man and other animals. Bracken may cause poisoning in some grazing animals although they will normally avoid it when alternative food is plentiful. It is in times of hardship that sheep may suffer blindness and cattle can get severe stomach ulceration from consuming significant quantities of the plant. Raw bracken contains the enzyme thiaminase, which can cause a vitamin deficiency in horses, leading to a condition known as ‘staggers’. The young emerging fronds contain the most toxins with levels decreasing as the plant matures through the season.
The discovery of carcinogenic substances in the spores and in the plant tissues of bracken is a major concern. The microscopic spores are readily spread by wind that might blow them towards nearby human habitations, whilst decaying bracken can release a carcinogen known as ‘ptaquiloside’ that can leach into water supplies. Factors that are now being linked to the high incidences of oesophageal and stomach cancer in many areas of the world where bracken is abundant.
Ecologically, a small amount of bracken can be a benefit to the biodiversity of an area. However, the other side of the argument suggests that the careful removal of bracken will encourage displaced species to return, which is ultimately more beneficial to man and to wildlife.
Mechanical methods of control include:
Damaging the fronds by partially cutting or bruising each stem several times. This can be repeated through the summer as the fronds grow back and if such treatments are maintained for several years, it can have the effect of reducing the reserves of the underground rhizome system causing a gradual weakening of the growth.
Use of livestock to trample down the fronds. The animals must be provided with sufficient food (hay/silage) to prevent them from grazing the bracken. This can open up the ground and allow frost to penetrate the soil and damage the rhizomes. This method is not usually applicable to amenity situations.