In my case I had it sprayed in mid July. The sprayer operator was shocked by the height of the giant ragweed, and expressed doubts about the effectiveness of spraying that late. What it did accomplish was to kill a large percentage of both marestail and giant ragweed, and the sick plants that didn't completely die stopped flowering before seed heads formed. The sick plants also are letting a lot more sunshine down onto the switch. Where the grass got more sunshine, it's now about 2-3 feet tall, and putting out seed.
That said, you might have missed your opportunity for this season. Those switchgrass seedlings have a limited number of days to grow this year. I would probably save my money for a pre-emerge application of gly and atrazine next spring.
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