Now - at the start of the growing season and right before warm weather arrives - is NOT the time to move anything. It would be far better to wait until early fall or late winter, when the plants are entering or in dormancy. If you must do it now, be prepared for transplant shock - prepare the new planting location before digging the plants, attempt to move when temps are cool and the day overcast and pay very close attention to watering for the rest of the season.

but I'm not sure when is the best time?

As with most woody plants, when dormant.

Dan

if it can wait.. do it when the leave of the trees are changing color..

just about everything is dormant at that time...

if it cant wait... its still cool enough in z5/6 ... at night ...

i would do it ASAP with a forecast of rain or cloud cover for a day or two after ... o would not do it if temps are supposed to creep near the 80's ....

big ball of soil with them ... watering properly.. and mulch heavily

i grow my hydrangea in all shade.. i doubt that its the issue.. next to grass... it might be drought.... though it did take it a few years to bloom regardless ...

ken