If a plant screams divide me it is because it is becoming less productive and blooming less.
If it is getting crowded out by new starts, or has a hole in the middle of the mother plant it wants to be split in two or more. Even soil gets depleted around perennials and that is a warning sign. You get those annoying weeds that you can’t remove without lifting and dividing the perennial, so dividing is inevitable. Or we just get greedy and want more of our favorite plants.
If you divide in the spring, it allows enough time for roots to settle in before hot summer weather. Spring division is best in early spring just when the growing tips of the plant have emerged. Perennials often bloom a little later than usual if spring divided.
There are lists that help you know how often to divide perennials and when is the best time. You might notice that a rule of thumb is if the perennial blooms after mid June it gets divided in early spring or simpler to remember, the season opposite of when it flowers. In the coldest regions, many do all the dividing in early spring so that plants fully recover before winter sets in and the ground freezes.
In our area…
Here is some guidance for early spring when the soil is workable (usually in late April or May in our area) every 1-3 years.
- Dendranthema x grandiflora
- Oenothera fruticosa
- Phlox paniculata
Early Spring every 3-5 years
- Lysamachia punctata
Early Spring every 5-8 years
- Iris sibirica
And there are plants that are especially unhappy about being divided and best left be like Baptisia, Dicentra, Aconitum, Lupine, poppy, and Peony. Peony is best moved or divided in late September in our area. Those with long tap roots do not tolerate division well. All can be divided, but just not as reliably.
Coming up, So You Want to Change Bloom Time on Your Perennials?, and Lush gardens of June Leave Gaps in the Garden.