The internal biologial make up of weeds and grasses is different. The weeds, like trees and shrubs have Cambium Cells. These cells provide support in the main trunk and branches of the tree or weed. The selective weed killer is a growth hormone that mimics the actual growth hormone 'Gibberelline' - the plant hormone responsible for secondary thickening. The selective turf herbicide is absorbed by the weeds and after a while, the weeds go into super growth, with their stems twisting and distorting. The weeds actually outgrow themselves and when their overall structure has literally had it, they die. Grasses do not have Cambium Cells so although they might absorb some of the chemical, it has no affect - if applied correctly in the first place!