Digging up and dividing most summer-flowering herbaceous perennial plants is an excellent way to perk them up when they’re looking tired, giving you free plants in the bargain.
Some perennials are either not suitable for division, or else seed so readily that there is no point, so research each plant first.
You will need a fork, spade, maybe a serrated garden knife, and water.
Why Divide Perennials?
Division reinvigorates perennials, giving them more space, air flow, access to nutrients, and makes them more attractive.
There are three main reasons to split up your clumps of perennials:
- Revitalize ageing plants & encourage flowering: Over time, most perennials become crowded and less attractive. Some produce fewer and smaller flowers, others “go bald” in the middle of the clump
- Control their size: When a perennial is happy, it’s likely to spread too much and crowd out its neighbours
- Propagate new plants: Even if none of the above apply, getting more plants for free is always a good deal
If none of the above reasons apply, then there is no need to divide.
Division is not necessary to maintain the health of a healthy plant that’s looking pretty and filling the space you want it to.
Helenium and Phlox are the most demanding of division: every two-three years tends to suit them best. Older heucheras and primulas might creep into that range as well to keep them really fresh looking.
When to Divide Perennials
The best times to divide summer-flowering perennials are:
- Spring, March-May, ideally when they show signs of growth, but before they fully leaf out
- Autumn, September to late October / early November, depending on your location: by the time the weather gets wet and cold, it’s better to postpone division until the following Spring
Spring-flowering perennials are best divided in Summer (June-August) as soon as they finish flowering.
How to Divide Perennials
- Water the Plants in Advance:
- If the weather has been dry-ish, water thoroughly a day before dividing to reduce stress and loosen the soil.
- Lift the Plant:
- Use a garden fork or spade to dig around the base of the plant. You can lift the whole thing, or slice across it to lift part and leave the rest in place.
- Inspect it:
- Shake off excess soil and examine the root system for dead, diseased, or insect infested sections to remove.
- Divide the Plant:
- Divide the plant into sections, each with a few healthy shoots and roots. Some plants are easy to separate by hand, some are tough and have to be forcibly sliced using a sharp spade or serrated garden knife.
- Replant Right Away:
- Re-plant the divisions at the same depth as they were before.
- Water well:
- Water the plants in after dividing, and in dry weather.
Perennial Plants That Don’t Like Being Divided
These are your plants: you are in control! But some perennials do not respond well to division, especially plants with long tap roots. You can try, but the chance of failure is high.
- Dictamnus albus, Burning Bush
- Lupins
- Hollyhocks
- Hellebores
- Dieramas
Plants with a deep taproot, like lupins, hollyhocks, acanthus, most verbascums, poppies and eryngiums can’t be divided easily, but they either spread by seed, or you can propagate them via root cuttings in late Autumn or early Spring.
Dividing
As perennials mature past a few years, they tend to form clumps that bulge or spread. In many cases, you can dig these up, split them, and replant the pieces – remember to water them as if they are new plants in their first summer.
September-October is a great time to divide early flowering perennials, so they can put down some roots before winter, and Spring is also fine for late flowering plants, right as the new foliage shows signs of emerging. Remember that if your soil is sticky clay, early Spring is always the best time for moving any perennials.
If you water them well, you should often get away with splitting them during the growing season if you have no choice.