We earn a commission for products purchased through some links in this article.
This late-summer gardening job keeps plants healthy and borders filled
Autumn is on the way, bringing with it a whole host of new gardening tasks. One of them is dividing perennials that have formed large clumps underground over the warm summer months, giving you more flowers and your existing plants room to breathe.
'After summer growth, you may find by September that perennial plants like phlox, daylilies and astrantia can become congested,' says Paul Parker from plant and bulb specialists J. Parker's. 'To help thin them out, now is a great time to lift and divide them, which will encourage fresh growth and give them a new lease of life.'
Below, plant specialists tell us how to divide and replant your clump-forming perennials, giving them more room to grow and helping new roots establish before the colder weather arrives. The earlier you do this, the better your chances of the new roots establishing well.
'Established daylilies can be divided every few years when clumps become overcrowded or flowering declines,' says Paul.
Dig up the full root system before gently dividing the individual clumps. You can carefully shake the plant to remove excess soil, laying bare any damaged roots that can be removed.
'Once you have replanted your daylilies, trim the green leaves down to around 20cm in height. It is perfectly normal if your daylilies fail to flower the next year after separating, the flowers should return the following spring.'
Alongside spring, late summer is an ideal time to divide agapanthus. 'Splitting the plant just as flowering ends means that after replanting, the roots have time to settle in while the soil is still warm. This prevents rot, which can arise from dividing too late into autumn,' explains Richard Barker of LBS Horticulture.
Lift the plants from the ground and brush off any loose soil. Then divide the root crown using a sharp spade, making sure that each division has at least two growing points. Replant them as soon as possible and at the same depth as the original plant.
'Keep the soil consistently moist for the first few weeks while roots establish, and the new divisions may only flower lightly in their first year while the plant is focusing its energy on root growth.'
'Hardy geraniums are a good one to split up at this time of year, especially if they’ve started getting a bit too big for their space,' says Jo McGarry, luxury garden design expert at Caragh Nurseries.
She advises lifting the clump with a fork and gently pulling it into smaller pieces, while making sure each division has a few roots and shoots.
'Get them back in the ground at the same depth and give them a good watering. They’ll have plenty of time to settle in before winter and should flower again next summer.'
Read next: What to do with hardy geraniums after flowering for more blooms next year
Has your heuchera started looking a little worse for wear? It might be time to break up its clumps. 'Heucheras can get a bit woody and scruffy as they get older, so dividing them is an easy way to give them a new lease of life,' says Jo.
Lift the plant and separate the clumps into sections, making sure each one has a growing point and roots. Then, pop them back into the ground at the same depth as the original plant, without burying the crown.
'They’ll spend the next few months getting settled, with fresh growth and flowers coming back next year,' Jo adds.
Phlox should also be on your list of plants to divide now. 'It’s important to divide phlox while the soil is workable and before frost is expected to minimise damage and allow the roots a chance to harden before the soil freezes,' explains Paul.
Once you've broken up the clumps, make sure you replant the individual sections immediately after.
'It’s best to prepare by loosening the soil beforehand and watering thoroughly so that the plants can root quickly. You can expect the divided phlox to start flowering next year,' says Paul.
This plant is beloved for its rigorous growth, giving lush foliage in little time. But it also means it can get a bit unruly. 'Hostas can get surprisingly chunky over time, so if yours is starting to take over, it’s a good one to split up,' says Jo.
'Lift the plant with a fork and pull the smaller sections apart by hand, or use a sharp spade if it’s really well established. Make sure each section has some roots and shoots before you replant it. Give everything a good soak afterwards and leave them to settle in ready for next summer.'
This beautiful flowering perennial should also be divided now to give divisions enough time to grow new roots before winter. Use a garden fork to ease your iris out of the ground, taking care not to pierce the underground rhizomes, or plant stems.
'Inspect the clump to identify the firm, healthy sections that have a strong set of leaves and roots. Cut the rhizomes apart with a clean, sharp knife, discarding the old, woody or hollow central rhizome,' says Richard.
Trim the leaves back a few inches to help reduce moisture loss in the new clumps and stop the wind from shaking up the new roots. Replant them in a sunny spot at the same depth they were previously.
'Primrose can be divided before September, as the plant will have several weeks of active root growth before the soil freezes. The divisions also have time to establish, meaning that they are more likely to flower the following spring,' says Richard.
Dig carefully around the clump using a garden fork to keep the roots intact. Shake off excess soil and inspect the roots before easing individual clumps apart.
'The new divisions should be replanted in partial shade at the same depth they were previously, with the growth buds sitting just above the soil,' Richard continues. 'New divisions should be watered deeply after replanting, and keep the soil moist while they are establishing.'
'Heleniums are worth dividing once they’ve finished flowering and started to die back a little,' says Jo. 'Lift the clump with a fork and split it into smaller sections, making sure each one has a good amount of root attached.'
Replant them relatively quickly and water well, giving them the best conditions to establish fresh roots while the soil is still warm. 'They’ve got time to get settled before winter and should give you flowers again next summer,' Jo adds.
'When separating your astrantia, use a spade to gently lift the plant out of the soil. After this, it’s best to separate the root ball by hand to minimise damage and keep as many roots as possible,' advises Paul.
Leave each divided section with healthy roots and at least one growing point, or shoot. 'Astrantia should flower again in the next flowering season. However, don't be alarmed if they do not flower the following year, as smaller plants may still be establishing new roots.'
Do these 3 things if your green tomatoes won't ripen
7 easy vegetables to plant in September
How to grow hardy catmint
September is your last chance to plant these bulbs for Christmas
