Impatiens capensis Meerb. (1775) has been naturalized in the British Isles since the early 19^th century. Though native to North America, the species is now established along waterways of lowland Britain and continues to increase its distribution across northern and central Europe. There have been two homonyms of I. capensis for a further two plant species with different sporophytic chromosome number. Impatiens capensis Thunb. (1794) is native to southern Africa, has a sporophytic chromosome number of 2n  =  14, and now carries the accepted scientific species name I. duthieae L.Bolus (1921). Impatiens capensis Boj. ex Baker (1883) is native to Madagascar, has a sporophytic chromosome number of 2n  =  14, and now carries the accepted scientific species name I. firmula Baker (1883). Karyological analyses of plants from three geographically separated I. capensis Meerb. populations from the River Thames Valley proved the sporophytic chromosome number of 2n  =  20 and estimated the mean sporophytic unreplicated nuclear DNA amount of 2C ≌ 1.55 pg, which corresponds to a genome size of approximately 1.51264 Gbp per nucleus.