The Royal Horticultural Society defines ‘invasive weeds’ as those that occur outside their natural range due to direct or indirect introduction by humans.
There are many non-native plants in the UK which are considered to be invasive. The most widespread invasive species include:
- Japanese knotweed
- Giant hogweed
- Himalayan balsam
- New Zealand Pigmyweed
It is not an offence to have these plants growing on your land or in your garden, and there is no legal requirement to control them. However, it is an offence to let them to grow in the wild.
Invasive weeds in the UK are becoming an increasing issue for people, businesses, and the environment that they live in. Non-native plants can:
- Change ecosystems and habitats and have non-biotic effects, such as reducing or impeding water flow leading to flooding, or changing the PH or chemical composition of the soil, or lock up nutrients.
- Outcompete native plants either by habitat change or by spreading so rapidly as to crowd out slower growing species, threatening the long-term survival of species.
- Take a long time to become invasive. Many of the plants now considered invasive have been growing in the UK for over 100 years and for much of that time showed no sign of becoming a problem.
- Be expensive to eradicate. It is also very costly to restore degraded habitat, if it can be done at all.
Japanese Knotweed is one of the most common invasive weeds in the UK. It is a resilient plant that grows commonly along riverbanks, canals, motorways and rail embankments. Its red shoots and hollow, bamboo-like stems make it easily recognisable, and it appears in springtime with flat-based leaves before flowering with a small, white flower in late summer.
However, it can cause serious problems for people and businesses. It is strong enough to grow through walls and tarmac, and can damage infrastructure.
While it is not illegal to have Japanese Knotweed growing on your land, it is an offence to cause it to spread. This is very difficult to prevent as it spreads easily and quickly establishes in new areas.
Because of the plant’s expansive root system and ability to regenerate from small fragments, it is also very difficult to eradicate fully.
Japanese Knotweed grows rapidly in dense clumps and dominates an area with deep roots. It is resilient enough to grow in any soil type and as such it prevents the growth of native plants.
Giant Hogweed is widespread throughout the UK. Its green stem grows in early spring before developing into a dark red colour with purple spots during summer. The leaves are dark green with deeply cut lobes and ragged edges that can be 1 metre across and form in a rosette.
Its large, umbrella-like flowers only appear after four years, and, having flowered, it dies. It thrives in any habitat but particularly where the soil has been disturbed, such as riverbanks, derelict land or railway embankments.
The plant is a danger to both animals and people as it contains a large amount of poisonous sap. If the sap makes contact with skin (and in the presence of sunlight), it causes severe irritation, swelling and painful blisters.
Burns can last several months, and even once they have died down the skin can remain sensitive to light for many years.
As its flowers, it can disperse up to 50,000 seeds that remain viable in the ground for 15 years. As it spreads it endangers the survival of native plants.
The loss of other vegetation may lead to excessive erosion of soil as the Giant Hogweed dies back in winter.
Introduced to the UK in 1839, Himalayan balsam is now a naturalised plant, found especially on riverbanks and in waste places where it has become a problem weed.
Himalayan balsam tolerates low light levels and also shades out other vegetation, so gradually impoverishes habitats by killing off other plants. It is sometimes seen in gardens, either uninvited or grown deliberately, but care must be taken to ensure that it does not escape into the wild.
It is a tall growing annual, and produces clusters of purplish pink helmet-shaped flowers between June and October.
The flowers are followed by seed pods that open explosively when ripe.
Each plant can produce up to 800 seeds. These are dispersed widely as the ripe seedpods shoot their seeds up to 7m away.
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