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I aerated, overseeded and applied starter fertilizer last Sept 15th. Would I be safe to apply a pre-emergent such as Scotts fertilizer with Halts pre-emergent this spring, or would some of the grass seeds from last fall still be germinating and/or too weak to survive the chemicals?
I'm in Minnesota, so I expect ground temps to be around 50° around mid-late April.

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The pre-emergent used in that product is Pendimethalin (3,4-Dimethyl-2,6-dinitro-N-pentan-3-yl-aniline), and its mode of action is microtubule assembly inhibition in very immature plant shoots (In other words, it inhibits microtubules, the structures which pull cells apart in cell division (mitosis), thus shutting down growth). This is the same mode of action that trifluralin based pre-ems use.

For this pre-em, I'd wait until the 3 leaf stage (where most of the seedlings have 3 blades each). This will ensure that they are finished taking nutrients from the old seed, and are mature enough to live independently. You could probably do it before that point, but when working with herbicides, safe is always better than sorry.

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