The general thought is you may move them safely any time the ground is not frozen, or they are not in active growth. The period of active growth is immediately after flowering when the new foliage buds are opening.

They have a shallow dense rooting habit with roots extending out at least as far as the widest branches. If weight is a consideration and you must take less of a rootball when you dig, try for a diameter equal to 2/3s the plants height. Then drag on a tarp or sheet of cardboard to it's new location rather than lift by the stem - the weight of soil on roots can pull them off, break them.

Be careful not to plant any deeper than it was originally growing, mulch after planting to keep roots cool and conserve moisture.

Thank you.