Is it all right to cut your hydrangeas down to the ground?

yes,here in zone eight.

No

Pruning hydrangea can be confusing.

It’s because it depends on the variety of hydrangea. Knowing your hydrangea type is an important factor. Looking at your leaves and blossom will help ID the variety, please look at the references below for best practice recommendations.

https://www.almanac.com/plant/hydrangeas

The timing of the pruning seem determines by the hydrangea variety according to this source.

Prune during the summer, just after the flowers fade:

Bigleaf or Florist Hydrangea (Hydrangea macrophylla)

Prune in late winter or early spring:

Hills-of-Snow or Sevenbark Hydrangea (H. arborescens ‘Grandiflora’),

Oakleaf Hydrangea (H. quercifolia)

Peegee Hydrangea (H. paniculata ‘Grandiflora’),

Tea of Heaven (H. serrata)

Prune as needed to control growth -

Climbing Hydrangea (H. anomala petiolaris)

Heres reference to consider

https://www.thespruce.com/when-to-prune-hydrangeas-1403320

It all depends on the type of hydrangeas you have. I would consult with a local greenhouse to find out what kind you have (take a picture of the whole bush, one of a close up of the leaves and one closeup of the flowers), and how to trim them back and when.

It depends on the type your have - whether they bloom on old or new wood. You don’t want to cut off any potential growth for next year. This article might help you! https://decorhint.com/how-to-care-for-hydrangeas-5-things-you-might-not-know/

Thank you