8 clump-forming perennials to divide in September for healthier plants and better blooms next year

Keep your plants happy and healthy, and fill borders with new plants for free

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With summer coming to an end, there are plenty of jobs to be getting on with in the garden right now. And one task worth tackling before it gets too cold is overcrowded beds and borders.

Separating congested plants will help to rejuvenate older plants, free up growing space and give you new plants that can be replanted elsewhere, so knowing which clump-forming perennials will benefit from dividing in September and how best to do it is key.

‘It’s always best to separate plants in early September to give them time to establish before any hard frost hits,’ says Craig Wilson, co-founder and gardening expert at Gardeners Dream. ‘There are a few signs to watch out for that mean your perennials will need to be separated. For instance, if the plant is growing fewer or smaller blooms, beginning to look overcrowded, or the plant is looking bare from the middle, then you need to think about dividing perennials like these now.’

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Find out which perennials will benefit from being divided below.

1. Daylilies

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‘Daylilies are prone to becoming congested, which results in reduced flowering, so they will benefit from dividing in September,’ says David Denyer, flower and garden expert at eflorist. ‘To divide, lift the clump carefully and separate the individual fans by hand or with a sharp knife. Make sure that each division has a good section of roots attached. Replant as soon as possible and keep the soil moist while the plants establish.’

‘Rather than dividing everything on a set schedule, I’d first assess how the plant is performing,’ says Matt J Adams, founder and lead gardener at The Relentless Gardener. ‘If it is healthy and flowering well, there is usually no need to disturb it and sacrifice late-season colour when the plant can be divided once flowering has finished.

‘But for clumps that are becoming overcrowded or losing vigour, September can provide a good opportunity to lift, divide and re-establish them before winter.’

Where to buy daylilies:

2. Hostas

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‘Hostas form dense root clumps that can be divided every four to five years when overcrowded,’ says Chris Bonnet, founder of Gardening Express. ‘Once separated, replant them in partial shade.’

‘Hostas are easy to divide once their foliage starts to die back,’ advises David. ‘Don’t be afraid to dig up the entire clump and use a sharp spade or knife to separate it into smaller sections. Try to make sure each division has a healthy portion of roots and several shoots, then replant at the same depth as before and water thoroughly.’

‘When dividing perennials, it’s best to water plants thoroughly before lifting them, ideally the day before,’ suggests Matt. ‘This makes the soil easier to work and reduces the amount of stress placed on the plant. And have everything ready before you start – a garden fork, sharp spade or clean knife, gloves and somewhere prepared to replant the divisions.’

‘Dig around the outside of the clump with a fork, keeping slightly further away from the crown than you think you need to. Work around the plant and gently lever it from the ground rather than pulling hard on the stems.’

‘Once it is lifted, remove some of the loose soil so you can see how the roots and shoots are arranged. This is important because you want every new section to have a healthy piece of crown and a good amount of roots.’

Where to buy hostas:

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3. Cone flowers

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‘Coneflowers (or echinacea) benefit from being divided when mature clumps become crowded or lose vigour,' advises Chris. 'Lift and split the roots gently, then replant promptly in a sunny garden border and well-drained soil.'

‘How you split a clump depends on the plant,’ says Matt. ‘Smaller, fibrous-rooted clumps can often be teased apart by hand, whereas a large, established clump may need two garden forks placed back-to-back and used as levers. For particularly tough rootballs, a sharp spade or knife will make a much cleaner cut.’

‘Don’t be afraid of making a decisive cut – repeatedly hacking at a rootball can cause more damage than one clean division. I also wouldn’t try to make dozens of tiny plants. Larger, healthy divisions with several shoots and a strong root system will generally establish much more quickly than very small pieces,' adds Matt. 'If the centre of an old clump has become woody, weak or dead, discard that section and concentrate on the vigorous outer portions.’

Where to buy coneflowers:

4. Shasta daisies

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‘Shasta daisies can become congested after a few years, so divide them every two to three years to encourage growth, better flowering and gain extra perennial plants for free,' says Chris.

‘Replanting is just as important as dividing,’ advises Matt. ‘Try to replant the divisions as quickly as possible so that the roots don’t have an opportunity to dry out. Prepare the new planting hole first, making it wide enough to accommodate the roots naturally rather than forcing them into a small space. To help the roots re-establish, it’s always best to add some bonemeal to the hole as well to strengthen roots for better chances of survival during spring and summer.'

'Plant each division at roughly the same depth as it was growing originally,’ adds Matt. 'The crown should not be buried deeply, as this can lead to rotting and poor growth. Firm the soil gently around the roots and then give the plant a thorough watering.’

Where to buy shasta daisies:

5. Asters

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‘Asters can become overcrowded over time, so divide established clumps every few years to help them grow well,’ suggests Chris. ‘September is a good time to separate and replant the healthiest sections.’

‘Good aftercare is just as important as the division itself,’ advises Matt. ‘Keep newly planted sections consistently moist during dry spells while they establish, and once they are settled, give them the nutrients they need to support healthy roots, shoots and flowering. A liquid feed can be an easy way to maintain plants through the growing season without overcomplicating your feeding routine.’

‘For flowering plants, I use Blooms Booster All-Purpose Liquid Plant Feed with Seaweed, from £9.50, from The Relentless Gardener, as part of that approach. It is formulated with higher potassium to support flower growth while keeping nitrogen lower to avoid putting too much emphasis on foliage. It also contains 20% liquid seaweed and can be applied every three to four weeks, making it a simple option for keeping flowering plants fed through the season.'

‘A light mulch of well-rotted garden compost around, but not directly against, the crown can also help retain moisture and improve the soil.’

Where to buy asters:

6. Black-eyed susans

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‘Black-eyed Susans are vigorous spreaders and may need dividing every three to four years to prevent overcrowding,’ says Chris. ‘Split established clumps and replant with plenty of space in flower beds to thrive.’

‘Black-eyed Susans (or rudbeckias) will need dividing every few years,’ advises David. ‘When lifting, be sure to keep as much of the root system intact as possible before separating into smaller sections. Have some well-prepared soil ready to replant them in and water them thoroughly to encourage strong root development.’

Where to buy black-eyed susans:

7. Hardy geraniums

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‘Hardy geraniums can quickly form large clumps, so dig around the plant and lift it from the soil so you can gently tease the roots apart,’ advises David. ‘Sometimes the root ball can be dense, if so, use a sharp spade to divide it into manageable sections.’

‘When dividing plants, choose a cool, cloudy day if possible and avoid dividing during particularly hot or dry weather,’ says Matt. ‘Don’t work on soil that is waterlogged either. If September is exceptionally wet, it’s better to wait until conditions improve or postpone the job until spring.’

‘It’s also worth remembering that not every perennial wants to be divided at the same time. Summer-flowering herbaceous perennials, like salvias, are often good candidates for autumn division, while some spring-flowering plants are better divided after flowering. And if something is still looking fantastic in September, there is no need to disturb it simply because the calendar says so,’ adds Matt.

‘Done at the right time, division is one of the simplest ways to give an ageing perennial a new lease of life. You get a healthier, more vigorous plant, prevent overcrowding and potentially turn one established clump into several – all without spending another penny - so a great budget garden idea.’

Where to buy hardy geraniums:

8. Bearded irises

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‘Bearded irises benefit a lot from regular division as their rhizomes become congested,’ says David. ‘To do this, carefully lift the plant and separate healthy rhizomes, you can discard any damaged or diseased sections. Replant the healthy pieces so that the rhizomes sit close to the soil surface rather than being buried deeply.’

'When dividing any plant, always use a clean, sharp spade or knife to make clean cuts and don’t make divisions too small. Larger, healthy sections will generally establish more quickly and give you a stronger plant the following year,’ adds David.

Where to buy bearded iris:

So now is the time to think about dividing congested perennials to keep them healthy and gain new plants in the process.

Lisa Fazzani
Lisa Fazzani
Deputy Editor

Lisa is Deputy Editor of Style at Home magazine and regularly contributes to sister title Ideal Home. She has written about interiors for more than 25 years and about pretty much every area of the home, from shopping and decorating, crafts and DIY to real home transformations and kitchen and bathroom makeovers. Homes and interiors have always been a passion and she never tires of nosying around gorgeous homes, whether on TV, online, in print or in person.