Dividing day lilies is best done in the spring and early fall so that the plant has a few weeks to adjust before the weather becomes extremely hot or cold.

If you are dividing day lilies in the fall, at least 6 weeks before the first frost date is a good general rule to follow. If you are dividing day lilies in the spring, do so after the last frost date.

If you have severe winters (zones 3 & 4) you have a greatest risk of losing plants over the winter, so spring is a better time to divide. In hot climates (zones 9 & 10) more plants are lost in the summer months due to stress from heat so dividing in the fall may be a better choice.