TO DIVIDE OR NOT TO DIVIDE, THAT IS THE QUESTION

In the garden, perennials provide many things that annuals can't--and the biggest plus for a perennial is it's ability to survive the cold winter and emerge new and fresh in the spring.  Dividing is necessary for most perennials in order to keep them vibrant, vigorous, and healthy.  Divisions also aid in keeping your beds neat, with less chance of foliar disease. Another big reason for division is that overcrowded gardens can struggle as plants compete for nutrients or water, not to mention air circulation.  You'll have to take each plant and decide what is best for it--some do better if left alone for years, like peonies and hosta; others need to be divided regularly to maintain their size and vigor, like tall bearded iris. 

I just divided one of my Asiatic Lily clumps.  To do this, I carefully dug up the area.  The bulbs were all different sizes.  I separated the bulbs by size; I wanted to replant only the biggest  ones in the same place.  The remainder I will share with family or friends.  The smaller, yet still fairly large ones, I can plant in another area that allows the bulbs to reach a bigger size. I took the biggest 11 and replanted them in well-prepared soil, amended with compost and bone meal, about 5 inches apart, and about 6-8 inches deep.  After, I watered in well and marked them.  Marking them helps me know the bulbs are there and there will be less chance of disturbing or harming them by digging into them.  In early spring, another application of a balanced fertilizer, like 10-10-10, or an incomplete fertilizer, like super triple phosphate, 0-45-0, will be applied for maximum growth and bloom. 

Some perennials do better when divided in early spring when plants are still dormant.  In spring the weather is cool, and the soil is moist.  The root systems are also storing energy, which helps the divisions recover from transplanting. To divide, I like to use a pitchfork and gently work my way around the plant, until I can gently lift it out, avoiding the breakage of roots.  If I can pull the roots apart, as in my day lilies, I will.  Others are tough to divide, like a peony.  These need to be cut with a sharp blade or shovel.  Some plants get woody in the center, like lamb's ear, coral bells, or some daisies.  Discard the old woody centers and plant the outside edges.  I like to form a round clump when doing this, to keep the plant uniform.  

Here is a partial list of plants that need division and the frequency of division:

Divide in EARLY SPRING, every 1-3 years

Aster

Beardtongue

Beebalm

Coral Bells

Centaurea

Delphinium

Garden Mums

Obedient Plant

Painted Daisy

Spiderwort

Divide in EARLY SPRING, every 3-5 years

Astilbe

Bellflower

Coneflower

Daylily

Gaillardia

Jacob's Ladder

Veronica

Sea Thrift

Divide in EARLY SPRING, infrequently 5-10 years

Goatsbeard

Hosta

Lady's Mantle

Lungwort

Meadowsweet

Meadow Rue

Siberian Iris

Japanese Anemone

Divide in LATE SUMMER or EARLY FALL

Asiatic Lily

Bearded Iris

Peony (infrequently)

Siberian Iris

Tall Phlox

Daylily

BEST NOT DIVIDED

Baby's Breath

Clematis

Flax

Lupine

Monkshood

Russian Sage