How and when to divide perennials

Perennials grace our gardens year after year with their variety of brilliant colors and unique foliage forms.

Spring is a great time to divide many of our favorite garden perennials. The plants are small, easy to handle, and they have the entire growing season to get re-established in your garden. But how do you know how often to divide perennials? Are there some perennials you shouldn’t divide?

The term “perennial” literally means to last or exist for a long time and to be enduring or continually recurring. A perennial plant re-grows year after year from the same root. Some perennial plants are very long-lived, like an oak tree, while others are short-lived, like some of the newer coneflower cultivars.

Plants that are not winter-hardy are considered annuals in Minnesota, but are perennial in their native growing environment. An example is Tithonia rotundifolia, which is grown as an annual flower here, but is a perennial shrub in its native Mexico and Central America.

Dividing perennials or “division” is a form of propagation: you are creating multiple plants from a single plant.

Some plants, like coral bells (Heuchera), are very easy to divide. The plant crowns are right at the surface of the soil, and small plants, called “offsets,” develop from the larger central plant. Other plants, such as hosta and ornamental grasses, have dense roots that require a sharp knife, garden fork, or even an axe to cut apart.

After a few years in the garden, perennials may start to produce smaller blooms, develop a bald spot at the center of their crown, or require staking to prevent their stems from falling over. All of these are signs that it is time to divide.

Why divide perennials?

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.

Division promotes plant health and can rejuvenate a plant. A perennial should be divided when it dies out in the middle (the oldest part) of the plant, produces smaller flowers or leaves, and blooms less.

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.

Dividing perennials is also a good idea if plants have become crowded, as it increases air and light around plants, which can increase blooming as well as reduce disease and insect pests. Plants like catmint (Nepeta), hyssop ( Agastache), and Japanese painted fern (Athyrium niponicum) can be divided every year if necessary.

To increase the number of plants

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

Guidelines for dividing perennials

• 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 of 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.
   • 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

• 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

• 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 plant’s roots to become established. This is particularly important in colder, northern climates.

Dividing specific perennials

Some plants, such as Baptisia (false indigo), do not respond well to division because they have long, deep roots that are difficult to dig out without breaking them off. In fact, Baptisia shouldn’t be divided or moved if possible.

Similarly, peonies have deep, fleshy roots that break easily. They can be tricky to dig up and must be replanted at the proper depth of 1-1/2 inches. It can take a few years for these divisions to become established enough to bloom.

Download the PDF or bookmark the Dividing perennials spreadsheet to find information specific to 125 common perennials.

The plants are listed in alphabetical order by common name. Their scientific names are given in italics.

You will find information on when to divide, how often to divide and other helpful tips.