Check out the two pictures below. The one one the left shows a strip of perennial ryegrass infested with dandelions, but the surrounding tall fescue is dandelion free. The p. rye and t. fescue were planted at the same time and have been managed identically for the past five years. The picture on the right is of a stand of Kentucky bluegrass (left side of picture) next to a stand of tall fescue (on the right). While the effect is less dramatic, it’s clear that the left side has dandelions while the right side does not. I’ve also noticed this effect in my tall fescue lawn – my neighbors have dandelions and I don’t. But I do have plenty of clover, which also has infested the tall fescue in the pictures below.