I just started to see black tips on both the black eyed susan's and hillside black beauty. It seems to have covered all the leaves in an equal amount and what appears to be overnight. Any help would be great.

Do I maintain and let it fix it self?

I can't prune bad leaves because it is on 90% of them.

Classic overwatering or underwatering?

Tough to tell since I have clay that does maintain moisture, but new plants in drought conditions??????

Possibly a fungus?

I have no idea and don't know if the appearance on two different plants on the same day is relevant.

Background-

I am struggling on first perennial garden. Of course I planted it before the hottest week in 20 years. I planted about 2 weeks ago. No rain in two weeks and consistent 100 degree days. I started watering everyday for three or four days. Then I realized that the soil is more clay than i realized and it was still moist. Since then I have watered once every three days by using something similar to a spray wand and holding it by the root for about 8-15 solid seconds.

Two lavender munstead never made it a week. I had a black spot on tips of helleborus, but only a couple leaves. I trimmed these off and have seen no other occurrences. The toad lilies that were getting any afternoon sun have scorches brown leaves.

I have applied liquid fence rabbit deterrent, Eight by earl May last watering, and a slug deterrent a week ago. I am a little reluctant to just buy random stuff and spray unless I have strong conviction in the cause.

Thanks