In 2003 I made the (with hindsight) error of applying fertiliser/weedkiller/mosskiller combo to my lawns in the spring. This was especially a mistake on my front lawn which at the time had relatively little grass among the weeds. Being 2003 the grass did not grow for several months, and what moved into the spaces left by the dead moss and weeds was yarrow, which had been largely unaffected by the weedkiller.

Now I don't mind having a bit of yarrow mixed in with a balanced mix of grass, clover, daisies, and other small wild flowers. But a lawn mottled with a few large yarrow patches is not so nice. Although it was a big job, I just pulled it out, taking especial care to remove as much rhizome as possible. I find pulling yarrow out much easier to do in summer when the soil is dry than in winter when it is sticky, though that depends on your soil type I suppose, and the degree of lawn thatch. Also the rhizomes are more stringy in the summer, and so long stretches come out more easily. The rhizomes are mostly on the surface and with some care come out rather easily. And when it resprouted from the bits of rhizome left over, I pulled that out. It hasn't completely got rid of it, but it is no longer locally dominant, and there are some areas which are now completely free of it. But most important the grass has had chance to recover, and I now have a more balanced mix of plants in the lawn.

Now if any bits of lawn look like the yarrow is becoming dominant, I just pull out any bits that look especially healthy, and associated rhizome. I keep the couch grass under control in much the same way.

I found an autumn lawn feed especially helpful for giving the desirable grasses a boost. Spring feeding now seems to be a mistake, since most years recently my lawn has stops growing in mid-summer.