Tristan da Cunha acquired an estimated 137 non-native vascular plants that can be categorized into four species types; weeds (trees, shrubs, agricultural weeds), grassland species (grasses), garden escapes (vegetables), and other ruderal species. Vascular plants were accidentally introduced in a variety of ways including; impurities in flower or vegetable seeds, seeds or plant fragments from other imported plants and in soil, attached to containers, cars or people. The majority of invasive weed species that has been introduced to the island are spread by seed and cover 50% of arable land in widely distributed patches. These species include prickly sow-thistle ("Sonchus asper"), smooth sow-thistle ("Sonchus oleraceus"), smooth hawksbeard ("Crepis capillaris"), scrambling fumitory ("Fumaria muralis"), green field speedwell ("Veronica agrestis"), groundsel ("Senecio vulgaris"), and nutgrass ("Cyperus esculentus"). Other invasive weed species that have a more localized distribution in plots include prickly sow-thistle ("Sonchus asper"), smooth sow-thistle ("Sonchus oleraceus"), smooth hawksbeard ("Crepis capillaris"), and groundsel ("Senecio vulgaris"). Whether a species is distributed locally or widely depends on the seed's dispersal mechanisms; larger seeds that have not adapted to wind dispersal will be distributed locally, while smaller seeds have adapted to wind dispersal will be widely distributed.