Splitting a plant is a task typically done on perennials who start to show signs of needing it to be done. These signs can include “balding in the crown” or dead center of the plant, producing smaller blooms, or requiring staking of steams to keep them from falling over. A reduction on plant production might not be the only reason to divide the plant. Remember that Perennials are plants that grow back each year. Dividing or splitting a single perennial into multiple plants helps the plant perform better. When perennials are divided, there is more space for roots to grow and absorb nutrients and water. Dividing perennials can help manage the size of the plant. You will have more plants of the same kind to add to your garden when you divide a perennial.

Reasons to Split plants:

To rejuvenate the plant and stimulate new growth

Overcrowded plants compete for nutrients and water. Restricted airflow can lead to diseases.

Dividing the plants into smaller sections reduces this competition and stimulates new growth as well as more vigorous blooming.

To control the size of the plant

Since plants grow at varying rates, division may be used to keep plants that spread rapidly under control.

To increase the number of plants

Division is an easy and inexpensive way to increase the number of plants in your garden.

Guideline For Splitting Plants

• Divide perennials on a cloudy, overcast day as dividing on a hot sunny day can cause the plants to dry out.
• Water the soil a day in advance if the area to be worked on is dry. Ideally, divide plants when there are a couple days of showers in the forecast to provide enough moisture for the new transplants.

How to divide perennials

1. Dig up the parent plant using a spade or fork.
2. Gently lift the plant out of the ground and remove any loose dirt around the roots.
3. Separate the plant into smaller divisions by any of these methods: 
   • Gently pull or tease the roots apart with your hands; 
   • Cut them with a sharp knife or spade; 
   • Or put two forks in the center of the clump, back to back, and pull the forks apart.
4. Each division should have three to five vigorous shoots and a healthy supply of roots.
5. Keep these divisions shaded and moist until they are replanted.

When to divide

Divide when the plant is not flowering so it can focus all of its energy on regenerating root and leaf tissue. 

Divide fall blooming perennials in the spring because

• New growth is emerging and it is easier to see what you are doing.
• Smaller leaves and shoots will not suffer as much damage as full-grown leaves and stems.
• Plants have stored up energy in their roots that will aid in their recovery.
• Rain showers that generally come along with the early season are helpful.
• Plants divided in spring have the entire growing season to recover before winter.

Divide spring and summer blooming perennials in the fall because

• There is less gardening work to do in the fall compared with spring.
• It is easy to locate the plants that need dividing.
• Perennials with fleshy roots such as peonies (Paeonia spp.), Oriental poppy (Papaver orientale) and Siberian iris (Iris siberica) are best divided in the fall.
• When dividing plants in the fall, time it for four to six weeks before the ground freezes for the plants roots to become established. This is particularly important in colder, northern climates.

Source: University of Michigan Extension