Lots of good advice here so far. "Standard" brassica mix for us would consist of 3lbs PPT, 3lbs DER, 4 lbs daikon radish(like Groundhog or Tillage). Our average date of first frost is Sept 17th, so I would think you are in the same timeframe as what we used. Plant between July 1st and July 15th to ensure they reach maximum bulb size before first frost. There are many other things you can do to help with the possibility of a failure, some guys use split planting dates and plant the same mix in each area. Another way to achieve this is to get some longer season brassicas like kale and rutabagas and plant them in early/mid June and then follow up with the shorter season PPT/DER/Radish mix in July.
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