How does Trifluralin kill newly germinating seeds, with almost no effects on established ones?

Question

Trifluralin (2,6-Dinitro-N,N-dipropyl-4-(trifluoromethyl)aniline) is a pre-emergent herbicide used in landscape beds before the application of mulch (my use for it, anyway). It kills the weeds as they germinate, while not harming most established plants.

From what I understand, the mode of action is as a mitotic inhibitor, which disrupts the microtubules that pull the cell apart when it divides (stops cell division).

How does this only affect germinating weeds, while not noticeably affecting weeds that have emerged from the soil (I have to hit those with another herbicide)?

Answer 1

Mitotic (cell division) inhibitors in plants are pretty much what a chemo therapeuticum means for cancer, i.e., rapidly dividing tissues will be affected most. Germinating seeds depend on mitosis for growth. An established plant will also be affected (it will stop growing), but it will stand its ground. To push the analogy - chemotherapy is often commenced after surgical removal or irradiation therapy to remove the tumor, and then chemo is applied to make sure any surviving tumor cell will not start to divide. Your herbicide will prevent germination (cf. cancer re-growth), but will not grossly affect established plants (cf. existing tumor).