Reinvigorate perennials and create new plants while the soil is warm and moist, and you’ll reap the rewards next year.
There are plenty of perennials that flower profusely year after year. Some, however, become less vigorous and floriferous as they age, the centre of the plant turning woody and bare, or become so congested that there’s no room for new growth.
Dividing these perennials every three to five years is a way to rejuvenate them, providing an opportunity to remove any old growth and replant the newer, healthy parts of the plant. You can also use the technique to curb those perennials that are overenthusiastic spreaders, keen on taking over the border. And by splitting a plant into sections you’ll be making new plants, which will allow you to tweak the design of your borders or create drifts of your favourite plants for no extra cost.
Autumn or spring?
Garden lore recommends dividing perennials that flower before mid-June in autumn, while those that flower after June should be divided in spring — but this is a rule of thumb and actually you can divide most plants in either season. The caveat to this is if you have heavy clay soil, which retains a lot of moisture. If you do, it’s generally a good idea to do your dividing in spring, otherwise plants will sit in cold, wet soil before they have a chance to develop new roots, and they are likely to rot.
I’ve always favoured spring for dividing plants. It’s the sort of job that suits my energy levels at that time of year. Just as I‘m feeling a spurt of energy, so too is the garden, whereas in autumn I’m busy planting bulbs while contemplating hibernation. In spring, plants are starting back into growth, full of vim and vigour, and the soil is warming up. Normally, April showers also provide sufficient irrigation to set the new plants on their way before summer arrives.
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However, climate change is muddling the seasons and creating fluctuations in our weather. If the past couple of dry springs are anything to go by, April showers are no longer a reliable occurrence. It’s been noticeable that the plants I’ve divided in spring have struggled to become established, and they were the ones that suffered the most during 2022’s extreme heat and drought.
Without doubt, our changing climate is making gardening trickier. It means that gardeners need to be more flexible, to take a few risks and to go with their gut instincts, which is why I’m now doing more dividing in October — I’m making the most of the easy-to-dig soil and the perfect conditions for plants to establish new roots before the cold sets in.
Plants such as pulmonarias, primulas, hostas, hardy geraniums, astrantias, heucheras, nepetas, delphiniums, astilbes and daylilies are all candidates for autumn division. As are herbaceous peonies — it’s a myth that they can’t be divided (or moved). Just make sure you split them once the leaves have fallen in October or November, that each section has at least three buds and that when you replant them you don’t bury them too deeply. Also, the buds should be no more than 3cm-4cm below the surface of the soil, otherwise they won’t produce flowers.
Container plants
The same division rules apply to perennials growing in containers, but bear in mind that the new plants will be more vulnerable to any cold or wet than established pot plants. Moving containers to a sheltered spot, lifting pots off the ground to improve drainage (using either pot feet or some bricks) and wrapping the outside of the pots in hessian during cold spells will all help to protect the plants.
How to divide
Essential equipment includes a tarpaulin or similar, if you don’t want soil all over the place, a spade, two border forks, a pruning saw and some secateurs.
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Dig around the plant so that you can lift out the whole rootball and place this on the tarpaulin. Some plants can be carefully teased into sections, but most will require more effort. Border forks placed back-to-back in the middle of the plant can be used to lever it apart, but I find this technique hard work. A sharp spade, on the other hand, can slice a plant apart quickly — you’re aiming for three or four sections, depending on the size of the plant.
You don’t want the sections to be too small otherwise they won’t have enough oomph to get going. If the plant has particularly congested roots, which is often the case with hostas, the best option is to use a pruning saw or an old bread knife to cut through the rootball. If you have one, a crowbar can be useful for levering particularly stubborn specimens out of the ground.
Discard the middle of a plant if it looks tired and woody, then replant the other sections as soon as possible so that the roots don’t dry out. Make sure that each hole is large enough to accommodate the roots — it’s better to dig a bigger hole or to trim roots that don’t fit with secateurs rather than squishing them in. Improve the planting hole with some peat-free multipurpose compost or garden compost and position the plant so that the crown is level with the surface of the soil, then water in well, even if the soil is moist, as this will help to settle in the soil around the plants.
It sounds like a lot of effort and it can be, but the key is to stagger your dividing so that you’re not doing everything all in one go. Divide some plants this autumn — the plants that need it the most — and wait until next autumn to do any others.
Taproots
Plants that produce long roots like those of a carrot, such as baptisia, verbascums, lupins and eryngiums, are not easily divided because it can be hard to lift them out of the ground without damaging their tapering roots — think how difficult it is to remove a dandelion with the root intact. To divide one into several sections you’ll also need a plant that is old enough to have produced more than one taproot, but this will make it harder to lift out of the ground in one piece, as the roots are likely to go deep into the soil.
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Plants with taproots tend to be fine left to their own devices anyway, so there’s no need to divide them, but if they do get too big for their allotted space, reduce their size by removing chunks from around the edge of the plant. To propagate these plants, sow them from seed or take root cuttings between November and March.
Plants to divide in spring
Spring is the best time to divide ornamental grasses, late bloomers such as asters, achilleas, rudbeckias and any perennials that are tender. Ideally do it as soon as you see signs of new growth emerging from the crown and be aware that unless it’s a wet spring they’ll need regular watering — giving them a thorough soak once a week is better than a dribble every day or so. Mulching around the plants with compost once you’ve watered will help to retain moisture in the soil, giving your new plants the best start.
