I have new hydrangia plants should I have cut them down to the ground?

no....improper pruning could lead to no flowers next year. https://www.thespruce.com/when-to-prune-hydrangeas-1403320

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)

Hi Charlie, Never prune to the ground! How you prune depends on the type of Hydrangea. Some bloom on old wood so if you trim them now, you'll fewer flowers or maybe none at all. If they bloom on new wood the time to trim them is after they are finished blooming. There is one variety called Endless Summer that blooms on both old and new wood, but it's not recommended to trim it unless it really gets huge and even then, you're supposed to trim it a little at a time after it blooms. If it's blooming well, I personally wouldn't trim it at all. We had one put in when we did some landscaping 1 1/2 years ago and we didn't trim it. It still looked beautiful the second summer. Wishing you the best & Happy New Year!