Under Section 14 of the Wildlife and Countryside Act 1981, as amended by the Wildlife and Natural Environment (Scotland) Act 2011, it is an offence to plant, or otherwise cause to grow, in the wild any plant outwith its native range. ‘In the wild’ is defined by exception – gardens, parks, arable land and improved pasture are not considered to be in the wild. Just about anywhere else can be considered to be in the wild, including woodlands, scrub, rough grasslands and waste ground in urban situations.
There is no legal requirement to control non-native species, such as Japanese knotweed, that are already established in the wild. The aim of the legislation is to prevent the spread of non-native plants from non-wild areas, such as gardens, into the wild. For example, gardeners are expected to take reasonable steps to prevent plants like Japanese knotweed, which readily grows from fragments of roots and shoots, escaping into the wild by not fly-tipping garden waste, or allowing fragments of plants to get washed into ditches.
Responsibility for control lies in the first instance with the owner of the land concerned. Although it may be an offence to cause or allow a non-native species, such as giant hogweed, to escape into the wild, the presence of the plant in itself is not an offence and in such circumstances, where the plant is already widespread, there may be no enforcement action that Scottish Natural Heritage (SNH) can take. SNH do have powers to enforce action where a landowner’s inactivity may prevent the success of a co-ordinated eradication program. However, voluntary action by landowners is the preferred course of action.
SNH has published web pages to advise and inform on the legislation relating to non-native species in Scotland: http://www.snh.gov.uk/protecting-scotlands-nature/protected-species/non-native-species/
For a quick summary of the non-native species issues, the SNH “Frequently asked questions” webpage is a very useful starting point: http://www.snh.gov.uk/docs/B1113471.pdf
The Scottish Government has published a Code of Practice to help people and organisations understand their responsibilities: http://www.gov.scot/Resource/0039/00398608.pdf