How do I care for ornamental grass?

Hi Mark, I can't help with the grasses but I find cutting back my own hydrangea every 2-3 years keeps it healthy and looking its best. I generally cut it back in really later fall.

Yes ornamental grasses need to be pruned. It also depends on what you have and a general rule of thumb is to prune late winter to early spring before new growth appears.

https://www.thespruce.com/cut-back-ornamental-grasses-1402420

https://m.wikihow.com/Prune-Ornamental-Grasses

As to hydrangeas, you need to know if your hydrangeas bloom on new wood or old wood. If you prune hydrangeas that bloom on old wood, you will not get any blooms next year. These sites will help.

https://www.gardeners.com/how-to/pruning-hydrangeas/8852.html

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

Absolutely do not prune your grasses. This is news to me, but I recently found out from a horticulturist that you DO NOT prune grasses. Them them alone.

https://www.provenwinners.com/learn/caring_for_ornamental_grasses

I have an app called iNaturalist. Take a photo of the grass, try to identify what it is, then look it up online for maintenance. Or bring photos to your local garden center, they'll be happy to help.