Any plant that is growing in 'the wrong place' can be described as a weed. So this broad definition can include not only non-crop plants growing in agricultural, horticultural or forestry crops, but also undesirable species in semi-natural areas or any plant that becomes established along roads, pavements or in other 'weed free' hard surfaces.

Following on from this, a plant that is a perfectly acceptable component of the flora in one situation may be considered a weed if it occurs on another site. Even native species that form a key part of certain habitats may be 'weeds' if they establish and spread across another habitat. For example, woody species like ash, birch, blackthorn and hawthorn frequently need to be controlled to protect semi-natural grassland and lowland heaths.

Japanese knotweed	Rhododendron ponticum	Ragwort

Other plants that are valuable to wildlife need to be controlled in some situations. Ragwort, for example, is toxic to livestock - and a particular problem where horses graze. Landowners are required by law (Weeds Act, 1959) to control ragwort and four other native species (broad-leaved dock, curled dock, creeping thistle and spear thistle) to prevent their spread onto agricultural land. 

Non-native species are a significant and increasing problem across the UK and Europe - particularly in aquatic, riparian (river-side) and woodland habitats, where 'escapes' from gardens, ponds and aquariums are displacing native British plants and seriously damaging important habitats. For more information see the 'Invasive Weeds' page.

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