Q: Can you explain how a fertilizer/pre-emergent combination using the chemical siduron can control crabgrass seed germination and not fescue?

A: University of Georgia weed expert Tim Murphy says:

Siduron belongs to the urea herbicide family. Most members of this control weeds by inhibiting photosynthesis. Siduron is not reported to be a potent inhibitor of photosynthesis, but it has been reported to inhibit root growth of susceptible species such as crabgrass. This herbicide is an old product, and I am not aware of any new work on its mode of action.

There surely is some physiological reason why tall fescue, creeping bentgrass and KY bluegrass are tolerant. What is that exact mechanism? Don’t know. But, obviously these cool-season species are tolerant, while crabgrass is not. It does not affect germination, but controls crabgrass by inhibiting growth of roots that emerge from seed. It must not affect root growth from germinating cool-season turf seed.

This herbicide though does not control annual bluegrass, so there would be no use for it in a fall-seeding of tall fescue.