Broom, also known as Scot’s or Scotch Broom, is a shrub with yellow flowers. It can be found in many parts of the British Isles and across Europe, but it has also been introduced to many other parts of the world. It is naturally found in rocky places woodland clearings and heathland.
Currently invasive in Australia, New Zealand, and North and South America, broom has spread to many different habitats, including grassland, woodland and riverbanks. It can take over large areas of land, and prevent native species growing. This invader forms dense clumps up to five metres in height. Broom also invades farmland and in North America even causes problems for motorists because it grows so large that its presence on roadside verges blocks the view of the road.
Broom produces seeds in huge numbers, up to 60 000 seeds per square metre and the seeds are tough, surviving many years in the soil. This means broom is very difficult to control as it can regrow after clearance from the bank of seeds remaining in the soil.
Studies of broom in both its native habitat, and the places it has invaded have shown that it grows larger and lives longer when it is invasive. In its native range in Europe, broom it has a large group of insects that feed upon. These broom grazers are often highly specialised and many of them eat nothing but broom. They may attack specific parts of the plant such as the roots, seeds or leaves. In addition, fungi, bacteria and viruses can also cause diseases of broom. These insect grazers such as leafminers, and pathogens reduce the survival of broom plants and are important in controlling its growth.
Most of the species that naturally attack broom have not been transported to other parts of the world. The native insects of invaded countries such as New Zealand, have not had time to adapt to this new invader and only a few of them have added broom to their diet so far. Native insects that specialise on a single plant species are very unlikely to adapt to a new plant so it is mostly generalists, species that feed on a wide range of plants, that will eat the invader. Because these species eat so many different plants they are unlikely to be useful in controlling broom. The specialist broom grazers from Europe may be important as biological control agents as many are so specialised for eating broom that they are very unlikely to attack other species.
Managing the problem
Controlled experiments have helped us to understand the life cycle of broom and the factors that can help or prevent it from growing. This information is vital in working out how to control broom in the places it has invaded. Studies of broom in Europe have shown that the hard coated seeds of broom are encouraged to grow by disturbances such as fires. This means that if you control by burning it, you are actually likely to increase the broom problem as more plants grow the following year. By understanding the ecology of the invader it is possible to work out control plans that will be effective in the long term.