When to divide daylilies and iris?
A good rule of thumb for flowering plants is to perform any routine maintenance after they’ve finished flowering, usually in the opposite season. Daylilies and irises are spring bloomers, so fall is a great time to divide them.
Since they grow from underground stems, daylilies and irises are prone to overcrowding and need to be dug up and divided at least every few years. If not divided, the plants will continue to grow, but will produce fewer flowers, or stop flowering altogether.
Daylilies are bulbs and can be pulled apart into separate plants, but irises are rhizomes that will need to be cut into separate pieces.
Both can be rather prolific in the right conditions and will easily grow into a very large colony in just a year or two. Dividing them (especially iris) promotes more flowers.
With both, you’ll want to first loosen the soil with a spading fork, taking care not to damage the colony if at all possible. Once you have them out of the ground, wash away the soil from around the roots, so you can see where to pull apart or cut them, and look for the healthiest pieces to replant. Any shrunken or soft pieces should be discarded, as should any pieces which have holes in them, which might indicate borers.
Daylilies prefer a little richer soil than irises, so if you need to amend your beds with a little organic matter, now would be a good time to do so. Trim back the leaves on both, leaving about six inches of top growth.
Prune away any roots that appear to be dried out or dead. If possible, dip the cut ends of your rhizomes lightly in a bit of sulfur, which will inhibit rotting until the cut surface heals. When replanting, don’t go too deep. With irises, cover the roots completely, but the base of the plant, where the rhizome and leaves are joined, should be just at ground level, and even exposed a bit. Daylilies should also be planted rather shallowly, with the bulbs laid on top of the planting area, then covered with a thin layer of soil and gently firmed in place.