October spraying is fine if all goes well. Esshup gave great advice, and I'm not disagreeing with him, but a late September spraying would allow plenty of time to do a followup spray if needed.
I try to make most decisions based on best case/worst case scenarios. If you spray in late September or late October, and the application takes, then all's well. If the September treatment doesn't take, then there's time to reaccess, and reapply if needed. If you spray in late October, and it doesn't take for whatever reason, then another treatment may not be as effective. Systemic herbicides take longer to work, so let's say it takes 7-14 days before a proper evaluation can be determined. If a late October application doesn't take, then you're looking at 7-14 days for discovery + a day or two for evaluation and application prep + another 7-14 days to evaluate the second treatment. That could potently get you to mid to late November. As I said, esshup gave great advice, but I like a little wiggle room after a herbicide treatment.
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