Hardiness: The salad radish is an annual that does best in cool weather. In northern areas plant salad radishes before the last expected frost in spring; in the south, plant in autumn or winter. They are not hurt by frost. Cold weather makes them sweeter.
This is what I suspected. I've always waited to early to mid May to plant
when I could start them in mid to late April through mid May. They would just get progressively hotter.
So, given they can tolerate a frost Hoop, you can plant
and havest the last of the sweet radishes and replace them in the garden with a summer veggie.