When should I divide my perennials?

A Question of the Week

iris

Perennial herbaceous plants are preferred by many gardeners because they come back year after year, eliminating the need to replant every spring. However, to maintain a lush and attractive perennial garden, it is necessary to devote time and energy into dividing perennials every so often. Not all perennial plants require division, but most do and will show signs that it is time to divide, such as fewer blooms or a dead spot in the center of the clump. Ornamental grasses, irises and daylilies are particularly notorious for the latter.

As perennial plants grow, they produce new shoots and roots around their periphery. Over time, roots and shoots begin to crowd one another and compete for water, light and nutrients, resulting in larger perennial clumps but less robust plants. Additionally, restricted airflow amid dense growth can make some plants more susceptible to diseases. Dividing plants into smaller segments helps alleviate competition between roots and shoots and often results in new growth and more flowers. From a practical standpoint, division is an excellent way to increase the number of plants in your garden without spending any money.

When to Divide Perennials

Some gardeners make a point of dividing their perennials every three to five years. Others wait to divide until their plants are producing fewer blooms or developing dead centers. How much time should elapse before division depends entirely on the plant being grown and the growing conditions where it has been planted. For example, full-sun perennials grown in sunny locations with consistently moist soils and plenty of nutrients may need to be divided every few years. The same plants grown under less than ideal conditions will grow more slowly and need to be divided much less often.

It is important to divide a perennial plant at the right time of year. Many gardening guides suggest dividing spring flowering perennials in the fall and fall flowering perennials in the spring. While some perennials do respond better to being divided at a certain time of year, in practice, they can be divided at either end of the season depending on when a gardener has the time.

Spring and fall are the best times to divide because the weather is typically cool and wet, reducing plant water loss and stress after being transplanted. Perennials can be divided in the summer months as well, but it will take longer for them to become established, and their bloom may be reduced for a longer period while they recover. If summer division cannot be avoided, wait to do so until after they have finished flowering, and wait for a cloudy, wind-less day.

Many New Hampshire gardeners prefer to divide all their perennials in the spring. A good argument for this is that plants that are divided in the fall may not have time for their roots to become established before winter. This makes perennials prone to heaving out of the soil with freeze and thaw cycles, damaging roots and crowns. If you decide to divide in the fall, aim to do so at least eight weeks before the ground freezes.

How to Divide Perennials

The amount of effort it takes to divide perennials will depend on the species and how long it has been since the clump was last divided. Ornamental grasses and iris can require a significant amount of effort to break into smaller pieces, while hosta and beebalm are usually quite simple. The task can be made even easier and be less stressful for plants if you water the soil in the area a day or so before digging. The actual process of division can be broken into a handful of steps:

  1. Dig up the entire perennial clump with a garden shovel, spade or fork. Try to keep the root ball intact by digging 8 to 12 inches from the crown of the plant, going all the way around it with your digging tool before trying to pry it out of the soil.
  2. Lift the perennial from the soil, and then gently remove enough loose soil that you can see the roots. Some gardeners prefer to place the root ball in water and wash soil from the roots.
  3. Break the plant into smaller sections by cutting the crown with a sharp spade or knife, gently pulling the roots apart with your hands, or using two garden forks to pull the clump apart. The type of root system a plant has will make it obvious which of these methods will cause the least damage to the plant and be most effective.
  4. All divisions should have a minimum of three shoots to be successful and bloom again quickly.
  5. Once the plant has been divided, the divisions should be planted as soon as possible. Dig new holes twice as wide as the root ball and about the same depth. Place the division in the hole and back fill with soil, making sure the crown sits at or just slightly above the soil line. Gently pack soils around the roots to eliminate air pockets.
  6. Water divisions thoroughly immediately after planting. Continue to water for the remainder of the growing season whenever the soil is dry.
  7. Wait to fertilize plants until the following season. Newly divided perennials will not be able to utilize nutrients until their roots systems have become established.

Perennials can be a beautiful addition to many landscapes if you keep their maintenance in mind.  Many perennials benefit greatly from regular division. Done properly, division will reward you with healthy new growth and an abundance of new blooms for years to come.


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Author(s)

Emma Erler
Landscape and Greenhouse Field Specialist
Instructor Field Specialist
Phone: 603-641-6060
Office: Cooperative Extension, Taylor Hall, Durham, NH 03824

Ask UNH Extension
Master Gardeners & Extension Specialists
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