When should I cut down my hydrengeas, spring or fall?

Depending on your plant species In late winter or early spring, these shrubs can be cut all the way back to the ground. Smooth hydrangeas will produce much larger blooms if pruned hard like this each year, but many gardeners opt for smaller blooms on sturdier stems. To reduce flopping, leave a framework of old growth or If it blooms on old wood (stems from the summer before the current one), its buds are being formed, and if you wait too late you may cut them off, meaning no flowers next spring. So these shrubs should be pruned immediately after their flowers fade.

Don’t prune these hydrangeas to the ground in late fall. Doing so removes all of next year’s flower buds.

Knowing if your shrub blooms on old or new wood will help you make timely cuts

Just something I found for your Question?

Do your hydrangeas bloom on new wood? Or old wood?

https://www.finegardening.com/article/pruning-hydrangeas

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

https://m.wikihow.com/Prune-Hydrangeas

If they bloom on old wood, do not go Edward Scissorhands on them, it could kill the plant. My girlfriends hubby did that. Killed a bunch of beautiful specimens.

Don't NEED to prune them at all---Then it really depends what kind they are, some bloom only on old wood/stalks/growth. People are confused about how to prune hydrangeas all the time. The three most common reasons for their confusion are the plant’s dead-looking appearance in winter, its failure to bloom in summer, and the reasoning that because it’s a shrub it needs to be pruned. But these popular woody plants can live long, floriferous lives without ever feeling the cold blade of a pair of Felcos. Hydrangeas, though, can handle pruning (which, if done at the wrong time, may be the cause for the lack of flowers), and sometimes you might want or need to cut them back a bit. For example, you may not like the look of the fading blooms or your shrub may be a bit too tall. Pruning hydrangeas can also improve a shrub’s vigor and increase the size of its flowers.

Not all of these shrubs should be pruned at the same time. Those that bloom on old growth should only be pruned after flowering. Others bloom on new growth and should be pruned before they wake up in spring or as they are going dormant in fall. Watch a video on pruning hydrangeas.