A species which was introduced for ornamentation but has become a troublesome weed in India :

Plants of water hyacinth (Eichhornia crassipes), the world’s most problematic aquatic weed, also called ‘Terror of Bengal’. They grow abundantly in eutrophic water bodies, and lead to an imbalance in the ecosystem dynamics of the water body. These plants which were introduced into India for their lovely flowers have caused havoc by their excessive growth by causing blocks in our waterways. They grow faster than our ability to remove them.