Dividing Herbaceous Perennials

Dividing herbaceous perennials is a quicker way to propagate the plants than growing them by seed; in fact, some will not come ‘true’ from seed.
As well as increasing the number of your herbaceous perennials, division also allows you to rejuvenate older plants, which may have become unkempt and tidy up a border where the plants are all “hugger mugger”.

When to Divide Herbaceous Perennials

General rule – every two to three years to maintain health (advisable); every five years max
summer-flowering plants in spring or autumn
spring-flowering plants in summer after flowering or autumn

Which plants

Herbaceous perennials are a wide category of plant species and any list of them will contain some that gardeners will disagree with and some that they reckon have been excluded. For the purposes of dividing herbaceous perennials, I like to follow this definition:

“Herbaceous perennials are plants with a root system that lasts more than 2 years; the top growth (foliage) remains green not woody (ie does not lignify) and may die back over winter.”

This definition does include some plants which would otherwise come under ‘bulbs’, but makes sense of the fact that they can be propagated this way.
A useful way of grouping herbaceous perennials is by the type of root system as this influences the manner of division.

Different types of herbaceous perennial roots and crowns –

Fibrous roots / crown
Woody crown
Fleshy crown
Tap root
Fleshy roots
Tuberous
Rhizomes
Off sets / plantlets

How to Divide Herbaceous Perennials

The presumption here is that only some plants are being divided rather than the rejuvenation of a whole border. (That’s a whole other blog or gardening lesson!)
NB – wearing gloves is advisable as some plants have a skin irritant sap.

If carrying out division on a particularly hot or windy day, minimise the time the roots are exposed.
Tip – place them on wet newspaper in a trug to reduce stress.
To minimise damage to the roots dig around the parent plant at the ‘drip line’. This is roughly the same as the diameter of the top growth. Those with sword-like leaves will have root spread wider than their foliage.
Dig down, cleanly severing any roots extending beyond the drip line. Work the spade under the clump from various points around the outer edge until the plant can be levered out of the hole. Use two spades / spade and fork if required.
With very large clumps, you may need to divide in situ, halving or quartering the clump before undercutting and lifting the pieces out. Take care not to damage tubers and rhizomes.
NB – each new plant needs to have a growth tip plus roots for it to re-establish.
Replenish the soil by mixing in compost: your own or bought in.
After division, reduce the amount of top growth to reduce stress on the new plants. Remove old leaves, then either cut stems in half or remove third – half of the stems from the base, depending on the habit of the plant. For example, Iris – remove whole stems; Geranium -either method is fine.

A Few Garden Tools

Established gardeners have their favourite tools for dividing herbaceous perennials, often including a really sharp penknife. Choose from the following, depending on the type and size of the parent plant, personal preference.

Border spade – smaller than a digging spade, useful if the plants are close together
Border fork – two are better than one
Hand fork
Perennial spade – a small spade with a point to the blade for dividing and cutting
Soil knife / sharp knife – a serrated edge works better on soil encrusted roots; a straight edge on cleaner and tuberous roots
Hand saw – for larger clumps
Lawn edger as an alternative for cutting; the rocking motion gives extra leverage
NB – make sure tools have sharp blades – see here for tips on sharpening

Dividing the Lifted Parent Plant

When dividing herbaceous perennials, the size and health of the parent will determine the number of new plants: 3 – 4 is usual, but 2 – 8 are possible. Go for the optimal size for speedy recovery and new growth; too small needs tlc before planting into the border.

The most vigorous growth is found in the outer areas of the clump, as growth is from the centre out. Take new plants from here for best success rate.
You may find washing the roots before splitting helpful where the roots are congested or soil is hiding the tuber. It can be a chore, so the lesser method is to place parent plants in a bucket of water and shake gently for most of the soil to drop off.
However, removing soil has the advantage of allowing the health of the roots to be easily seen. There is no point in spending time dividing and replanting withered or diseased roots and rhizomes!
Although not generally needed, you can dust the cut edges with an organic fungicide.

Fibrous and woody perennials

It can be helpful to lay the parent on its side as this enables the root ball to be seen, the likely number of new plants gauged and a point of entry established.
Smaller plants can often be split by hand or hand fork, gently but firmly teasing out clump into different sections. You may need to use a knife.
Larger parents are divided by inserting two forks or spades back to back into the approximate centre of the clump. Whichever goes in more easily is the one to start with. Really congested clumps may need a mixture of forks, then spades and possibly a saw before you start to get more manageable sized pieces which split easily.
Irishman’s cuttings – is a phrase used to describe those plants that separate as if by magic into your hand.

Fleshy perennials

Once dug up, have a good look at the clump as some pieces may be easily prised off by hand. If not, then use knives and/or saws.

Tubers, Rhizomes

These are usually visible above ground, giving you an idea of both the clump’s size and that of the existing rhizomes. The clump may naturally fall apart when dug, presenting obvious divisions.
Tubers and rhizomes can either be split off from the parent or cut into pieces.

Plantlets / Offsets

Only remove from the parent plant those offsets which have a root system. Remove the ‘umbilical cord’.
Some fibrous perennials such as heuchera split easily in a similar manner to the offsets of ajuga and strawberries; thus giving two methods of propagation.

Replanting and Aftercare

Whether planted back into the ground or in pots the following applies –

Basic rule for fibrous and woody perennials, offsets – plant at the same height as before
Rhizomes should be planted no deeper than half their diameter
Fleshy rooted perennials and tubers should be planted in the soil with the growth point or dormant bud just peeking out of the ground
Water well after replanting
Cut stems can be a vector for disease and a magnet for pests such as slugs. Check on the new plants regularly and take appropriate action, preventative if it is likely to be a problem.

Dividing Herbaceous Perennials by Season

Where division occurs in spring, plants will rapidly put on new growth. However, the formation of a good root system is more critical at this point than foliage and flowers, so be prepared to cut back. The strength of roots can be checked by giving a gentle tug – there should be some resistance.

Dividing plants in autumn generally allows for roots to form before dormancy occurs. Some frost protection may be needed.

And finally…

Apologies as there should have been a video to accompany this and it went too pear shaped to use! I will try again, add it here and post on social media. The problem with filming this on my own was the unstable tripod – no harm done except a surprised cat!

If you’d like personal help in sorting out your herbaceous perennials, you could try a Plews Gardening Lesson. And if you email a question we can answer it within the monthly Plews eNewsletter, as there may be a few of you asking about dividing herbaceous perennials.

As ever, there are suggested blogs below which you might enjoy. Happy gardening!