I have Red Russian Kale and Collards that I want to grow for a late-season crop, but I keep procrastinating. Is late August/Early September too late to put them out in order to get enough greens to have a mess or two to eat? I live in USDA zone 6 (on the line between 6a and 6b) and we get our first frost usually sometime from the middle of October to the middle of November.
USDA planting zone has less to do with it than latitude (which determines day length) and the time you have between now and hard freezes. (Planting zone actually has nothing to do with it -- unless you're planning on harvesting in January / February when you'll likely experience the lows that go along with living in zone 6.)
Red russian kale (at least the packet I can see) matures in about 50 days, and kale is frost hardy. So even if you expect some frosts between now (Aug 23) and harvest time (Oct 12), you can expect to harvest full-size leaves. You could even get away with later planting if you harvest baby leaves (25 days).
Collards mature in 50-60 days depending on variety, and are also frost hardy. So you should be able to get a crop of these as well.
Planting now is probably ideal, since both are supposed to taste better after a couple of frosts.
Note that day length gets short quickly in higher latitudes, and shorter days means slower growth. So if you plant too late, even though these vegetables won't die from frost, they may grow too slowly for you to get a large crop. In other words: the later you plant, the longer you should expect those "days to harvest" numbers to get.
If growing from seed:
If growing from seedlings (transplants), you could most likely get away with planting through the month of September (IMHO).
Above dates come via:
The above document also contains a lot of helpful/useful information (at least I think it does).