Die Geel Gevaar

10 years ago this was a happy suburban vlei with birds, fishes and butterflies. It has subsequently become completely clogged with a large variety of alien invaders. This yellow one seems to be new. >100 plants, up to 220 cm tall. Seems confined to c. 100 square meters. Growing strongly, flowering profusely at present. I must have overlooked them in recent years.
 
I am worried. Identification and advice, please. I can make a herbarium sample if necessary.