Large clumps of perennials may be divided in spring or fall. It is most important that division takes place when the plant is not in flower. That would be as perennials begin to show new buds at the soil surface in the spring, and after flowering in late summer or fall. For example we’ll start with daylilies in late summer.
When a clump is getting too big for the space, or we want more to plant in another spot, prepare by watering the plant liberally a day or two before digging. This helps with ease of digging and removing extra soil.
Dig the clump with spade or digging fork while retaining as many roots as possible. Shake soil from the roots or wash the soil away. Gently pull apart the roots so each division has 2 or 3 growing tips. Then clip foliage back to about 3 or 4 inches
Replant. Do not let roots dry out before replanting. (Daylilies, however, are very forgiving) This system can be used for other perennials with fibrous root systems.
Bulbs, and rhizomes are a little different because the roots so different, not fibrous like daylilies. For bulbs we’ll use the example of daffodils.
If you have a thick stand of daffodils which has not been blooming well, it is time to divide them. Large clumps can be dug. Dig deeply to get the whole bulbs. Remove the soil and separate bulbs. Large bulbs may have off-sets (smaller bulbs attached at the base of a larger bulb). Replant the largest bulbs; the smaller bulbs may be grown in a nursery plot for another year until they get large enough to bloom. Remember, plant deeply with point up and base down. You may also want to stir some fertilizer into the soil.
Perennials such as german iris, have thick horizontal roots at the surface of the soil called rhizomes. These can be divided by cutting apart. When clumps grow large and get crowded , lift the clump, shake soil from the roots. Cut foliage back to about 3 inches.
Use a sharp knife to cut the rhizomes apart. Each rhizome should have one or two growing buds. Leave the roots growing down from the rhizome. You can let the newly cut surfaces cure (dry for a few days) before replanting. You may also treat the cut surface with a fungicide. If you find borers in the clump or soft rhizomes, discard these. Replant only firm healthy rhizomes. Plant rhizomes at the soil surface as you found them.
Always prepare a planting bed deeply with organic matter to improve drainage. Most popular bulbs will not grow well in a wet soil. Dividing perennials every 3–5 years helps to keep them growing vigorously and your garden looking fresh.