Bracken (Pteridium aquilinum) is one of the more successful ferns of the family Dennstaedtiaceae, having a cosmopolitan distribution Bracken can be found in most countries around the world. Bracken love soils with high levels of acidity and will thrive in deep loams and sands.
Rhizomes (creeping stems lying, usually horizontally, at or under the surface of the soil) are the key to the success of bracken; they spread underground allowing the stand of bracken to increase in size. In deep soils the Rhizomes can be up to a metre down. These Rhizomes have active and dormant buds, the active buds are normally produced from the Rhizomes lying above or just below the surface of the soil, these produce the fronds. The dormant buds produce the fronds in subsequent years.
The fronds normally appear around May and will grow from rigid, straw-colored, smooth stalks. Bracken fronds are large and triangular which are divided into three main parts with each part bipinnately subdivided. These fronds will grow up to 1.5 meters long and 1 meter wide.
Bracken can also spread by seed but this is a rare event and of less interest to those wishing to control bracken.
Bracken contains carcinogens and the fronds contain a number of toxins poisonous to animals such as cattle, sheep, pigs and horses when ingested, although they will usually avoid it unless nothing else is available. In cattle, bracken poisoning can occur in both an acute and chronic form, acute poisoning being the most common. Acute bracken poisoning causes ahemorrhagic syndrome or, in some cases, sudden death. Bright blindness in sheep is a progressive retinal atrophy that derives its name from the hyperreflectivity of the tapetum. In pigs and horses bracken poisoning induces vitamin B deficiency. It damages blood cells and destroys thiamine (Vitamin B1). This in turn causes beriberi, a disease linked to nutritional deficiency. Poisoning usually occurs when there is a shortage of available grasses such as in drought or snowfalls.
As bracken has such an effective security mechanism against attack by herbicides: application of a herbicide to the fronds may kill the active buds but it will have little effect on the dormant buds. Asulox attempts to get round this by being translocated and is therefore absorbed into the rhizomes and reduces the number of dormant buds. Asulox is different to other herbicides in that it is highly selective for bracken and has no impact or very little impact on other species. The exceptions are other fern species and measures must be taken to avoid applying Asulox to ferns if these are considered to be important.