Disturbing the root system of any plant interrupts its ability to feed and hydrate itself. Insuring that the roots are well saturated before disturbing them reduces the trauma.
If you find you must divide a plant with a great deal of top growth, cutting back the leaves by about 1/3 will lessen the amount of work the roots will need to do to maintain the foliage. This is often the case when dividing bearded iris, after they have bloomed.
