Zoysia, Torpedograss, and Quinclorac

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Questions: is the yellowing of zoysia normal? How often can I selectively spray the new torpedograss that has since appeared?

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Probably. Need to know the following?
What did you apply?
How much did you apply?
Did you add a surfactant?

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once you have torpedo grass you always have torpedo grass

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For a long time I would have agreed. I put down emerald zoysia over batture dirt in 2005 and had two spots that torpedo came up. It took 18 years to finally be free of it. I had tried everything imaginable, including straight 41% glyphosate/roundup. What finally worked for me over the course of about three summers, was spraying properly mixed Quinclorac over it, and then sprinkling the unmixed micro-pellets on the wetted grass I had just sprayed, with the thought of providing extended retention time in the soil. I repeated that each time it returned. Last year was the first year it never came back up. It hasn't this year either.

If you want to have any chance of getting rid of it, the first thing you have to let go of is saving the good grass in the treatment area. That's never going to happen. But it will come back.

Edit: At its worst, the two areas I had only reached maybe 10sqft each. And it got to that extent in times when I got apathetic/lazy towards it. But if you stay on it, even if you're not successful, you can keep it from getting worse. But the method I described above worked for me. If you have a much larger infestation, that will require a lot of effort, even by my method.

Awful news

It will recover. I transitioned my front yard from St Aug to Bermuda with the same quinclorac and MSO. It does yellow it a bit.

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I think the ratio was 2TBSP per gallon water. And yes, I used a surfactant

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The label says 1 tbsp per gallon for spot spraying. Maybe you went a little heavy. The surfactant will also damage the turfgrass in high temps.

Next time, go lighter, even if it requires more applications, and mix iron with the quinclorac as per the label. This will help with the yellowing.

But still, I think you'll be ok.

A product called DRIVE should kill the torpedo grass without affecting zoysia. It will require more than one treatment and may take up to a year to get all torpedo grass.

Yes I know. Drive is just easier to use.