cincinnana

Hey Zb dave.

I think your issue is environmental. Compounded with lack of water that could offset the slow roast.
Like all the previous posts have said.

I think your landscape cloth is toasting your plants and roots.

I can say this because I am a lover of dyed black mulch in a few of my plantings......the stuff looks good, but add some sun to it and it cooks your plants.

For giggles....put a thermometer under the landscape cloth around your plants in question at mid day and take a reading, my guess is 138-+ degrees.

Do it again at dusk....my guess 131-+

And in the morning my guess is 92-+

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