Fall is a great time to divide perennials. Dividing perennials promotes plant growth while allowing you to easily create more plants. Vigorously growing perennials like chrysanthemums and asters may need to be divided every season or two, while slower growing plants may never need division.
Let the plant’s growth habit dictate whether you need to divide it. Perennials need to be divided when:
- flowering is reduced or flowers get smaller
- the center growth dies out but there is growth around the edges
- bottom foliage is sparse
- the plant loses vigor
- the plant flops over or requires staking
- it has simply gotten too large for its space in the garden
- you want more plants for your garden or to share with friends.
Perennials should not be divided while they are in flower. Flowering demands a lot of energy from the plant and you do not want to stress a plant by dividing it at the same time. The general rule of thumb is to divide spring-and summer-flowering plants in the fall and fall-flowering plants in the spring.
Prepare ahead of time. Water your plants thoroughly a day or two before you plan to divide them and prepare the area where you plan to put your new divisions. Minimize the amount of time that the plants’ roots are out and exposed to the air. If appropriate, prune the stems and foliage down to about 6 inches from the crown to ease division and cut down on moisture loss.
Use a sharp pointed shovel or a spading fork to dig down and lift the plant or section of plant. Dig down all around the plant about 4 to 6 inches away from the base. Slide your tool underneath the root mass and lift the clump out. Shake loose soil off the root ball and remove any dead leaves or stems. Division of the clump will depend on the growth habit of the plant you are dividing. For any plant, remember that you need to maintain a portion of the growing point (crown) and a portion of the root system intact.
Some plants can easily by divided by teasing the root system apart and separating the crowns by hand. If the clump is more substantial, you may need to use two digging forks placed back to back to pull the crowns apart. In some cases, you may need to divide the clump by cutting with a sharp heavy knife or handsaw. Work your knife between the crowns and cut down through the clump and root ball making sure that each crown has a healthy portion of root attached. Likewise, a sharp pointed shovel can be worked between the crowns and used to cut down through the root clump. Whatever technique or tool you use, make sure each division contains at least 3 to 5 shoots.
Never allow your divisions to dry out. Keep a bucket of water close by to keep them moistened until planting. Prune off any broken or damaged roots prior to planting. Immediately plant your new divisions in the prepared garden bed or in containers filled with a clean, coarse potting mix. Plant divisions at the same depth that they were growing originally. Firm the soil around the roots and water well after planting. New divisions should be watched carefully and protected from drying out until the root system is well established.
Not all plants benefit from division. Butterflyweed, euphorbias, oriental poppies, Japanese anemones, and false indigo are all examples of plants that should not be divided. Likewise, most woody plants don’t divide well. Instead, look for a branch that has naturally layered near the base of the plant. Once rooted, the branch can be severed from the parent, dug up and replanted.
For detailed instructions on dividing many perennials, check out the following publication: