"In a dense stand of turf under a medium maintenance program it was found that soil temperatures greater than 73° F. were required for significant crabgrass seed emergence. It has also been reported that minimum temperatures of 55° to 58° F. at daybreak in the upper inch of soil for 4-5 days will encourage the initiation of crabgrass germination. Other research has shown that mean soil temperatures of 62° to 65° F. are required for germination." http://viette.indigofiles.com/CrabgrassControl.pdf Also this comment was made: "This research suggests that under average turf density the germination and emergence of crabgrass can continue for up to 12 weeks during a typical growing season in the Northeast. An application of a preemergence herbicide too early in the season, before soil temperatures warm adequately, may result in the need for additional crabgrass control measures later in the season."