Ice age coming?

That really sucks. I'll keep my fingers crossed for you that we have a nice long fall before the first freeze.

Sounds like a frost pocket. Sometimes, something as simple as a fan can offer some protection or an all night sprinkler.
Down here our latest frost that I remember was June 11, back in the 80s sometime. Only certain low lying frost pocket were affected, but not much.

I think I'd be moving that garden next year.
Sorry it's a trouble spot!

Around that same time I had one night of 38 degrees. I covered that night. There are low spots and frost pockets in Northern Wis. that freeze in any month of the year. This is almost one here (river bottom) and the OP definitely seems like one.

This week is cool too. Gotta watch it every night.

The low here IN La Crosse was 60F that night. It has been cool lately. Sounds like you garden in one of this states coldest spots! I've always noticed areas over by the I90-I94 interchange are always cold (Sparta/Tomah/Black River Falls). That would be are "frost pocket" in this area. I have the Mississippi River nearby, and that has a water temp of 80F right now, so i'm sure that moderates it a little. Better put in a big pond.