Given that excessive transpiration rates will cause more stress on full-grown plants than is necessary, some gardeners favor cutting back the foliage of such plants as irises and lilies to 4" to 6" above the ground. The remaining 4" to 6" of foliage will provide enough nourishment to the roots through photosynthesis; however, feeding the new plants with a water-soluble fertilizer high in phosphorus will also provide nutrients essential for establishing healthy roots.
Choose an overcast and cool morning to dig up and separate your perennials, rather than a hot and sunny day. Dividing on a cloudy day will keep the roots moist and the foliage from wilting excessively.
Aim to divide plants three to four weeks before the first hard frost or major snow fall. This will allow enough time for parent plants, and new plants, to get established before the ground freezes. To ensure winter survival of your new plants, make sure that they are well watered, and protect the developing crown from the cold and desiccating winds with a layer of mulch.