The Best Time for Dividing Perennials

Mango Popsicle red hot poker plants in flower.

David Beaulieu / The Spruce

While spring is, generally speaking, the best time for dividing perennialsornamental grasses, and grass-like plants such as sedges, those who really wish to "get it right" will want to treat each plant on a case by case basis. This is because some plants prefer to undergo the operation in the late summer or early autumn.

You can consider the rule of thumb: if the plant blooms in spring, it is best to divide the perennial after blooming; otherwise, divide the perennial as it emerges, in early spring. However, it would be best to research each species you might wish to divide, as there are some exceptions to this rule. That's where this article comes in handy.

Reasons for Dividing

Not all perennial plants need to be divided there are some that you should not attempt to divide because you will only harm the plant in attempting the operation. For those that can or should be divided, you can profit from their division in the following ways:

  • To get more of them to transplant to other portions of the landscape
  • To check their spread into areas where they are not wanted
  • To improve the appearance of a clumping perennial with a "bald spot" in the middle
  • To improve the health and longevity of the perennial plant

This last reason for dividing perennials is the most common one. We all know the scenario: A perennial that we have planted thrives, at first, and even spreads, only to peter out as the years go by. This happens due to overcrowding. Over time, there are too many roots competing for resources in the same small area.

Division in these cases is often just the "shot in the arm" the plants need for rejuvenation. Open up a new garden bed for your transplants, and amend the soil so that they will have plenty of nutrients.

Plants to Divide in Spring

Divide in Late Summer to Early Fall

These are only generally suggested times for dividing perennials, not the "Gospel truth." Some gardeners prefer to divide some of the perennials listed above for spring division in fall, instead. They reason that, by dividing perennials in fall, the plant has several months to recuperate before summer's heat returns. This argument is compelling in warm climates. But, in cold climates, there is the disadvantage that the newly divided perennial would then have to face the challenges posed by a harsh winter.

Do Not Divide These Plants

It is best to avoid trying to divide some plants. For example, some perennials have long taproots, meaning it is better to just leave them alone rather than trying to break them apart and move them. In other cases, the plants reseed so readily that it just doesn't make sense to risk damaging the plant by breaking it apart. Perennials (and sub-shrubs treated as perennials) to leave undisturbed include:

Plants to Divide at Almost Any Time

In the case of some perennial ground covers you do not have to worry about the best time to divide them because they are very vigorous growers—often, more vigorous than we would like them to be. The following are examples of perennials you can divide at any point in the growing season, except for during the very hottest periods of the summer: