Remove the tarp and water more.

How much sun are they getting and is it afternoon sun? I don't grow them but leaves look like either not enough water or too much sun.

Good point about the sun. They prefer partial shade.

No tarp.

I’m keeping them watered ( I hope) and think they’ll die back in fall of course but bounce back next spring.

It looks like your astilbe is too dry. Mine are in almost full shade, with just a bit of sun in the morning. They need lots of water

Oh, and if you want longer blooms, you can plant multiple varieties with different blooming times. In this photo, the light pink ones are in full bloom,
the fat purple ones just started, and the skinny purple ones are barely starting. Each variety only blooms about 2 weeks, but if you layer them, you can get more than a month in that section of the garden. Mine have never re-bloomed, so now I just leave the dried flowers on until the next spring.

wow!!!! thank you everyone!

If you want to have a garden, you have to weed occasionally. My astilbe flowers turn brown like yours when they go to seed. You can cut them off or leave them.

I was surprised to learn astilbe are one of the few plants that like afternoon sun. Mine that get it are the healthiest in the yard.