Although day lilies are tough enough to be divided in the summer months, why disrupt your garden while they are blooming? Waiting until after the flowers are gone makes more sense. If your day lilies were under-performing this year, or seem very crowded, dividing these perennials will entice them to produce more flowers.
As their name implies, each individual flower lasts only one day, but the large number of < flowers on each stem provide a three week period of bloomfor most cultivars. There are nearly 60,000 different daylily cultivars that range in color from near white through yellow, orange, and red, to brown and violet. These tough perennials have a reputation for low maintenance and require little in the way of special care. Daylilies are very adaptable and can be grown in almost any soil in every corner of the country. They bloom best if given full sun, and they produce more flowers if they are divided periodically. Division is also a great way to expand your planting and share plants with gardening friends. Late summer is the best time to divide your daylilies. Their roots will have time to grow before winter comes if this task is completed in early September.
How to Do It
Step 1. After the blooms are gone from your day lilies, cut the plant back to 8 to 12 inches to make it easier to work with.
Step 2. Insert a pitch fork into the soil about a foot from the base of the plant. Apply pressure to the handle to raise the clump, and do the same thing all around the root ball until you have loosened the whole thing, and lift the plant out of the ground.
Inspection of the clump at this point will generally reveal weak or thin areas at the center of the clump. Plunge two garden forks, placed back to back, into the clump. After the forks are fully inserted, gently push the handles apart. This will cause the roots to separate, forming two smaller clumps.
Dig a wide shallow hole. The depth should be slightly less than the height of the rootball. The width of the hole should be 6 to 9 inches greater than the width of the rootball.
Place the rootball in the hole and backfill with soil. Lightly tamp the soil into place. Apply mulch to a depth of one inch to discourage weeds and protect the roots from drying out and from temperature extremes. Water thoroughly. The foliage can be cut back to a length of about 12″ which helps the plant retain moisture while it is getting established.
Your newly divided daylilies will reward you with extra bloom for years to come. Most daylily varieties may be left untouched for a period of four or five years before they need to be divided again.
* Thanks to the U.S. National Arboretum for the above information.