When we travelled through Estonia a few years ago there were long stretches of the Canadian Golden Rod (Solidago canadensis) growing beside the road and looking bright against the dark forests.  Here it is growing beside the North Esk river but can also be found in many areas of disturbed land as it is spreads easily.  It was first introduced from North America in the 17th century and became popular in gardens for its showy golden flowers.   Now it is widely recognized as one of the most widespread invasive species in Europe and has caused problems by suppressing the native vegetation.   However it is very popular with insects and there were several bees and hoverflies on these flowers.