I sprayed Roundup 2-3 days ago on some tall, tough weeds. If I mow those same weeds down, will the Roundup still kill them?

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To completely kill the plant, you need to wait until the Roundup has been transported to the roots via the plant's sap.

How long that takes depends on the size of the plant and how fast it is growing, but a good rule of thumb is "don't chop it down until it looks like it is nearly dead."

Personally I would wait 1 or 2 weeks, not 2 or 3 days. There is no benefit from rushing to finish the job and having to do it again in a few months time when the weeds grow back.

Really "tough" and large plants may not be completely killed by a single application anyway. If they still look fairly healthy after a week, spraying them again will usually finish them off.

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