How Siduron Controls Crabgrass Without Harming Fescue

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

University of Georgia weed expert Tim Murphy provides the following insight:

Siduron belongs to the urea herbicide family. While most urea herbicides inhibit photosynthesis, siduron is not a potent photosynthesis inhibitor. Instead, it has been reported to inhibit root growth of susceptible species such as crabgrass. This is an older product, and no recent research has clarified its exact mode of action.

There is undoubtedly a physiological reason why tall fescue, creeping bentgrass, and Kentucky bluegrass tolerate siduron while crabgrass does not. The precise mechanism remains unknown, but these cool-season species are clearly tolerant. Importantly, siduron does not affect germination; it controls crabgrass by inhibiting the growth of roots that emerge from the seed. It does not impede root growth from germinating cool-season turf seeds.

Note that siduron does not control annual bluegrass, so it has no utility in a fall seeding of tall fescue.