Everything You Need to Know About Dividing Perennials

Stuck in an end of season gardening rut? Try a different way to keep your landscape in pristine condition      – by dividing your plants. 


Why?

Oftentimes gardeners only think about dividing plants when they want to increase their garden size. If you have a favorite perennial division is a quick and easy way to propagate that plant, yielding multiple buds where you once had one. But there are other benefits to dividing plants beyond it being a low-cost source for seeds.

As perennials age their plant clumps increase in both size and flowers. Robust growth is typically heralded in botany, but perennials can be aggressive growers and a significant size increase will have the plants jostling one another and beginning to overpower other flowers in the bed.  Division not only controls the spread of these ever-growing plants into areas where they aren’t wanted, but without it clumps can die in the middle leaving an unsightly bare hole.

Aging and overcrowding can take a real toll on the health of a plant, making it more susceptible to diseases. Dividing perennials rejuvenates the flower; giving it more space to grow, protection from fungal and insect infestations and encouraging good health.

When?

Changing seasons are the best time of year to start dividing plants. In the spring shoots are breaking ground and colder locations have passed the threat of winter, while autumn gives plants time to develop roots and leaves and recuperate after the summer heat.

The ideal situation for division is after the plant has flowered and when the soil temperature is warmer than the air temperature, which encourages root growth and discourages fast top growth. You should only divide when you know you can provide the necessary care for newly planted perennials, as immature transplants require ample watering until their root systems are established. And be sure any new plants divided in the fall are in the ground at least six weeks before continuous frost.  

Beyond weather the best time to divide perennials truly depends on what perennial you’re dividing. Spring and early summer blooming perennials should be tackled in the fall and fall bloomers in the spring. The majority of perennials will need dividing roughly every three to five years, while some like garden mums, blanket flower and asters require division more frequently, every one to two years, to maintain their vigor.

Which?

Many perennials will benefit from this spring/fall split but there are others that won’t take well to being divided. Bleeding heart, butterfly weed, monkshood, false indigo and baby’s breath are just some the plants that send down deep tap roots that keep them anchored and opposed to being moved.

Peonies and hostas rarely demand division but if they are a favorite and the need to make more of them arises they can adjust to new surroundings. Be advised that peonies may not bloom the following year after transplant as they are a sensitive plant that takes longer to adjust.

In addition to knowing when and how often perennials need to be divided, you can keep your garden in tip-top shape by learning the signs that a plant is giving you that it’s time for division.

If you have a perennial that has stopped flowering or is producing significantly smaller flowers than normal, division would be the solution. This situation can often be seen in coreopsis or bearded iris. As mentioned in the benefits above, if a center of the clump dies out and new growth occurs only on the outer edge of the clump then it is time to divide. Some plants, such as yarrow, lamb’s ear and black-eyed susan, can even develop this growth pattern in just a few years. Perennials can also reach this state after long periods of neglect.

You should also watch for growth in tall perennials, like garden phlox or bee balm, which only occurs at stem tips while bottom leaves become fewer and fewer. This can lead to stems becoming crowded within the clump. Other taller plants, such as sedum and campanula, will have stems that may become floppy when overcrowded.

Click here to have one of our experts come out for a free evaluation to determine which of your perennials are perfect candidates for division and how we can help rejuvenate your garden for fall.