The OSU guide recommends that turnip planting should be from Feb 15 to March 10 depending on where you are in the State. The earliest date is for the SE and the latest for the NW.

The Fall guide recommends turnip planting is Aug 1 - Sep 15, with the date direction going the opposite way. Mustard is Sept 10 - Oct 10.

Here is a link that might be useful: Fall Guide

In most years turnips and mustard do better in the fall in Oklahoma.

Yes, as the others have noted, it is far too late for a spring planting and too early for a fall planting. It isn't that they won't grow if planted now, but rather that their flavor and texture would be unpleasant because of the warm temperatures, rendering them inedible.

The last of my 2012 fall-planted mustard plants are bolting now and I am getting ready to rip them out of the ground so I can use their space to grow peppers. They have been bolting for about a month and I just haven't bothered to remove them since I didn't need that spot yet for spring planting. The fall-planted turnips bolted in March.

Most cool-season plants do as well in fall or better in fall than they do in spring here. We just get too hot too early some years and that can cause the cool-season stuff to bolt, to produce poorly or to develop an unpleasantly strong flavor. If I cannot get the cool-season plants in the ground by the end of March, I wait until fall to plant them. I generally plant broccoli the latest of all the cool-season crops but even it is in the ground no later than the first week in April. Even a 2-week or 3-week delay in getting cool/season crops planted can mean a poor harvest or no harvest.

Dawn