Japanese Knotweed
Originally an ornamental plant Japanese Knotweed can grow to 7ft in one season. Although it is not unattractive to look at it is capable of spreading relentlessly, overwhelming other garden plants. It does not produce seeds but can sprout from very small sections of rhizomes. Much of its spread is probably via topsoil or construction traffic.

Giant Hogweed

Giant Hogweed is a tall plant with thick bristly stems. It can reach a height of 11.5ft when the flat-topped flowers appear, and has a spread of about 3.5ft. After flowering the plant usually dies, but not before spreading its seeds. Although it makes an impressive plant, it is invasive and potentially harmful. Always wear gloves, and cover your arms and legs when working on or near it. Contaminated clothing and tools are potentially hazardous too. Wash any skin that comes in contact with the plant immediately.

Mares Tail
Also known as Field Horsetail. This is one of the most dreaded weeds. Once established it has roots that are usually too deep to dig out. The plant has creeping rhizomes, which may reach 6.5ft below the soil surface.

Ragwort
Common ragwort can become a serious weed of waste or other uncultivated ground. Ragworts are poisonous weeds. Cattle and horses are particularly susceptible to poisoning but it is rarely a problem in gardens.

Himalayan Balsam
Himalayan balsam is a relative of the busy Lizzie. It is a tall, robust, annual producing clusters of purplish pink helmet-shaped flowers. These are followed by seed pods that open explosively when ripe, shooting their seeds up to 22ft away. Each plant can produce up to 800 seeds. It is now becoming a problematical weed. Himalayan balsam tolerates low light levels and tends to shade out other vegetation.