I would plant them the last week of August. Yes, it may still be hot, but if you wait until much later, they won't be ready before frost. You can pre sprout them by soaking overnight, then put them on damp paper towells in a dish and set on top of the refrigerator for 2 or 3 days. Plant them out when you see tails. You will need about 75 frost free days from
planting seed out till harvest. It takes a little more time for a fall crop than a spring crop.
Gardening at 5000 feet elevation, zone 4/5 Northern Utah, Frost free from May 25 to September 8 +/-