Division is probably the simplest way to propagate herbaceous perennials. It is done because the parent plant has become unkempt, outgrown its space or to increase the stock.
When to divide
- 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
Healthy ones, obviously; younger plants will have more vigour.
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.
Rather than providing such a list, I shall follow the definition that an herbaceous perennial is a plant 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.
One way of grouping herbaceous perennials is by the type of root system. This is useful as it affects the manner of division. NB roots are sometimes referred to as crowns.
Different types of roots –
- Fibrous roots / crown
- Woody crown
- Fleshy crown
- Tap root
- Fleshy roots
- Tuberous
- Rhizomes
- Off sets / plantlets
How
The presumption here is that only some plants are being divided rather than the rejuvenation of a whole border. The technique is the same, but the logistics different.
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.
Replenish the soil by mixing in compost.
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.
NB – each new plant needs to have a growth tip / section of crown plus roots for it to re-establish.
A Few 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 and any guidelines that need to be followed.
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
Dividing the Lifted Parent Plant
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.
Washing the roots of all perennials before splitting used to be the preferred method. Now it’s done where it makes the job of division more efficient. For example, where the roots are congested or soil is hiding the tuber. 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!
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.
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 – are 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 this is where the knives and saws are used.
If required dust or wash the cut edges with a fungicide or organic equivalent.
Tubers, Rhizomes
These are usually visible above ground, giving you an idea of both the clump’s size and the size 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. If cut, they may be treated with fungicide.
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; 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.
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.
Marie Shallcross
Plews Garden Design and Consultancy