Division is best done in the spring and early fall, at a time that will give the plants a few weeks to adjust before the weather becomes extremely hot or cold. For fall dividing, at least 6 weeks before the first frost date is a general rule. Those with severe winters (especially zones 3 & 4)are at greatest risk of losing plants over the winter, so spring is a better time to divide. In hot climates (for example zones 9 & 10) more plants are lost in the summer months due to heat stress, so dividing in the fall may be best.