My reference for "highly invasive and spreads easily" is https://en.wikipedia.org/wiki/Impatiens_glandulifera#Invasive_species https://en.wikipedia.org/wiki/Imp

My reference for "highly invasive and spreads easily" is https://en.wikipedia.org/wiki/Impatiens_glandulifera#Invasive_species https://en.wikipedia.org/wiki/Impatiens_glandulifera#Invasiv..., which has by now completely taken over most clear and shady areas in and near forests where I live. Summer lilac is definitely far from being that bad. (I live in the South of Germany, but the UK, where it was originally introduced in the 19th century, seems to also have a huge problem with it: https://www.cabi.org/invasivespecies/species/himalayan-balsam/ https://www.cabi.org/invasivespecies/species/himalayan-balsa...)