What's the best time for dividing perennials? While spring is, generally speaking, the best time to divide, those who really wish to "get it right" will want to treat each perennial on a case by case basis, as some plants prefer to undergo the operation in the summer or autumn.
Before continuing with the subject of when to divide perennials, however, let's briefly look at why dividing perennials is sometimes advisable. There are three reasons to divide perennials:
- To propagate them
- To check their spread into areas where they aren't wanted
- To promote the health and longevity of the perennial
Of the reasons for dividing perennials cited above, #3 may be the most common reason for dividing perennials. 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. Dividing perennials in these cases is often just the "shot in the arm" they need for rejuvenation.
Below I group perennials according to when the best time is for dividing them. This list is not, of course, exhaustive, being just a sampling; use it as a springboard for further research on the best times for dividing perennials. Note also that, in cases where a perennial can be divided either in spring or at some other time of the year, I have listed it in the spring (as being the best time of year for dividing perennials, generally speaking).
I have intentionally left perennials difficult to divide off the list. Meanwhile, I have also intentionally left aggressive spreaders such as ajuga off the list, but for the opposite reason: They're easy to divide at any time during the growing season, being more vigorous than we'd like them to be, ideally. Finally, because the list contains so many more entries for spring than for the other seasons, I have made it more readable by breaking down the spring entries into sub-categories.
The Best Time for Dividing the Following Perennials is Spring:
- Perennial Flowers
- Autumn Joy and other stonecrops
- Bachelor Buttons
- Black-eyed Susan
- Bleeding heart
- Brunnera macrophylla 'Jack Frost'
- Creeping phlox
- False dragonhead
- Garden phlox
- Lenten rose
- Montauk daisy
- Red hot poker plant
- Shasta daisy
- Silver King artemisia
- Stella d'Oro and other daylilies
- Wood spurge
Ornamental grasses are generally divided in spring; examples include:
For the plants listed above, keep the following general guideline in mind: If the plant blooms in spring, divide the perennial after blooming; otherwise, divide the perennial as it emerges, in early spring.
The Best Time for Dividing the Following Perennial is July/August, After Blooming:
The Best Time for Dividing the Following Perennials is Late Summer to Early Fall:
Note that the above are only generally suggested times for dividing perennials, not the "Gospel truth." If you read up on the subject of the best time for dividing perennials, you will find, for example, that some gardeners prefer fall to spring as the time for perennial division. Their reasoning is that, by dividing perennials in fall, the plant has several months to recuperate before summer's heat returns. This argument may be compelling in warm climates; but in cold climates, one could point to the disadvantage that the newly divided perennial would then have to face the challenges posed by winter.
By all means, experiment. Just be aware that the consensus is that the best time for dividing perennials, generally speaking -- especially in cold climates-- is spring.