If you want to encourage more blooms and avoid self-seeding, cut dead flowers off yarrow. ‘The spent or fading blooms should be cut during mid-to-late-summer to encourage another round of blooms,’ says Ashleigh Smith of True Leaf Market. It’s also a good idea in order to keep the plant where you want it. ‘Yarrow can be quite aggressive in its spreading. Deadheading can help control its spread by reducing the number of seeds it drops,’ says Gene Caballero of GreenPal.