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Isoxaben (N-[3-(1-ethyl-1-methylpropyl)-1,2-oxazol-5-yl]-2,6-dimethoxybenzamide) 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.

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Here's the mode of action, as stated by Dow AgroSciences:

Isoxaben belongs to the Benzamide family of herbicides and inhibits cellulose biosynthesis in the cell walls of susceptible weeds (WSSA group 21). This means that cells cannot divide during the reproductive cycle; therefore, they cannot grow, causing death. While cell division does not occur, this mode of action should not be confused with mitotic inhibition that occurs with dinitroaniline herbicides.

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)?

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As with your other question on mitotic inhibitors ("How does Trifluralin kill newly germinating seeds, with almost no effects on established ones?"), inhibiting cellulose synthesis will inhibit growth. Hence, an established plant will stand its ground, but newly germinating plants cannot grow without cellulose synthesis and hence will fail to germinate.

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